Thinking in Aerodynamic Coefficients

Professional aerodynamicists mostly work in terms of coefficients. When they converse with each other, they talk in terms of points, decimals, and percentages. When I started writing about aerodynamics five years ago, I also worked solely in terms of coefficients. Not because I’m qualified to do so, but because I’m a cheap bastard who uses a free tool for racing simulations.

The only way I know to quantify lap time differences in aero setups is to use OptimumLap, and it requires coefficients for inputs. Thus, when I published the results of my aero testing at Watkins Glen in 2019, it was all numbers that were perhaps difficult for the average person to understand. I probably lost readers who don’t think in those terms.

These days I try to report things in pounds of downforce. I usually cite this at 100 mph because I like round numbers, and also because the A2 wind tunnel spreadsheet defaults to 100 mph. I’m still more comfortable talking about downforce in terms of (negative) coefficient of lift, but for the bulk of my audience who measures things in bananas and bald eagles, pounds of downforce is good.

But I also want to convert some of you to using coefficients, and also to using OptimumLap. So this blog post is my take on thinking in coefficients. (I’ll do a how to guide for OptimumLap in the future.)

Coefficient of drag

First let’s talk about the coefficient of drag (cD). Manufactures regularly publish drag numbers and I would guess that most car enthusiasts know that .20s is amazing (EVs), .30s is good (most cars), .40s is OK (SUVs), and .50s is terrible (trucks).

What people may not know is that manufacturer-supplied drag coefficients aren’t super accurate, because they cheat. They may adjust ride height and other things to achieve the best drag coefficient, so don’t take those numbers as gospel. Independent wind tunnel tests are perhaps better, but also not 100% accurate unless it has a rolling floor. Anyway, you can look up the drag coefficients of most cars (Ecomoder has a good list), and so I won’t spend a lot of time on that.

One thing I do want to mention is that you rarely see drag coefficients for cars with open windows. That’s how we track and race our cars in the USA, and it affects cars greatly. Every car is going to behave differently with open windows, so you’ll need to add somewhere between .03-.05 to your cD.

In my Veloster N, opening the front windows increased drag by 15%, and reduced rear downforce by 7%. The faster you go, the more aero balance shifts forward. This makes the car more prone to oversteer at high speed. In short, opening the windows is doubly bad for performance, so keep that in mind, as well as the drag penalty.

In the following table, the cars are all assumed to have closed windows except for the track car, which is a very rough estimate for a car with open windows and some aero bits added. I’ve listed pounds of drag and horsepower consumed by drag, just to give some hard numbers at 62 mph (100 kph). This is a rather low speed, but it comes up again when I examine coefficient of lift.

Type	cD	Lbs @ 62 mph	HP @ 62 mph
EV	0.25	110.72	8.55
Commuter	0.33	127.1	11.29
SUV	0.40	141.43	13.68
Track car	0.50	161.9	17.1
Truck	0.60	182.36	20.52

Coefficient of lift

While most people can wrap their heads around coefficient of drag (cD), they may have never seen values for coefficient of lift (cL). And that’s understandable, because manufactures seldom mention it. But they should! For going around a race track, cL is way, way more important than cD.

That’s because cars lift at speed, and most of that lift is over the rear. As the car goes faster and faster, it loses grip. If you want to go faster around a track, you need to reduce lift, or add downforce, which is essentially the same thing.

Note that OptimumLap inverts the coefficient of lift. So if your car makes downforce, use a positive value; if it makes lift, use a negative value.

So how much lift do most cars have? In the following sections I’ve divided cars into various groups based on their cL. These are general guidelines, to give you an understanding of how much lift or downforce typical cars have.

Low-drag cars (cL .20 )

A car that is streamlined for low drag usually has a lot of lift. The reason for that is the roofline is gradually curved and longer than the bottom of the car, so this creates low pressure above the car, creating lift. As such, most EVs have a lot of lift, but so do some sports cars that lack rear spoilers or wings.

The C6 Corvette below, with a cL of .21, has significantly more lift than most cars.

Typical street cars (cL .15)

Most passenger cars have a cL of around .10 to .20, with the average somewhere around .15. I think I’ve written elsewhere on this site that the average was around .3, but now that I have more data, I see it’s about half that value.

The best sports sedans (BMW M3, Mercedes AMG models) are in the single digits, and some even get close to zero lift, but not quite.

Ideal balance (cL 0.0)

A car with a cL of zero over both axles is ideal; the handling doesn’t change at any speed. To do get that balance often requires a big spoiler or wing, to cancel out the lift, which is mostly in the rear. You won’t see those accoutrements on family sedans, and so this category (with a few exceptions) is for true sports cars.

Cars that have around zero lift include the Audi R8, Koenigsegg CCR, Nissan GTR, and Pagani Zonda F, as well as most Ferraris and Porsches. You don’t really see many sedans that achieve zero lift, but oddly I know of two hatchbacks that do, the Ford Focus RS and Hyundai Veloster N.

Supercars (cL to -0.3)

There aren’t many production cars that have a cL in the negatives, and the ones that do are supercars. Dodge Viper SRT 10, Ford GT, Honda NSX-R, Mercedes MacLaren SLR, and Porsche 997 GT3, to name a few, and also most cars from Lotus.

Lotus Exige S1: cD .43, cL -.24, L/D -.56

The production car with the most downforce (that I have data on) is the Lamborghini Murcielago L670-4 SV.

Murcielago L670-4 SV: cd .38, cL -.29, L/D -.76

Production cars end at around cL -.3, but this is where proper race cars begin. I don’t have numbers for the latest hypercars, like the Aston Martin Valkyrie, but that surely produces race car downforce.

DIY track cars, race cars with spoilers (cL -0.5)

Many DIY track cars with splitters and wings have a cL approaching -0.5. They should be better than that, but are limited by the average builder’s understanding of aerodynamics and average level of execution. Professional race cars with airdams and spoilers also come in around this number.

For example, NASCAR stock cars circa 2002 had a cL of -.46. These cars make look crude, but are highly developed aerodynamic missiles. This is about as good as you can do with an airdam and spoiler.

Another race car with a spoiler was the 1990 RX-7 GTO, which had a cL of -.44. The Mazda looks really slippery, but it doesn’t achieve the same level of aero as a stock car. The interesting thing about the RX-7 was they ran it with both a wing and a spoiler.

My Hyundai Veloster N with the a flat splitter (3″ lip, 4.5″ height) and a big ducktail spoiler tested at a cL of -.43. I guess Hyundai got a few things right, because I was going for the crudest aero I could and somehow landed next to some pro race cars with spoilers.

Winged track cars (cL -.5 to -1.0)

To get better than cL -0.5, you pretty much need a wing or underbody aero. Let’s start with the RX-7 with a spoiler and see what happens when they swapped to a wing. Drag decreased from cD .51 to .48, and lift went from cL -.44 to -.53. That’s a big difference in both directions, and resulted in a 128% increase in aerodynamic efficiency (more on that later). I don’t know if the rules limited wing height, but mounting the wing higher would probably increase downforce even more. In any case, the wing bumped the car above cL -0.5, which is where most race cars with wings reside.

Another car with a similar cL was a GT2 car used in a scientific paper (pdf), which had a cL -0.5. Interestingly, the majority of downforce (85%) was from the rear wing, and it seems like they should have been able get more front downforce to balance that out. But GT2 is a highly regulated class, and so maybe there’s nothing they can do within those rules.

Most Miatas with an airdam, splitter, and wing will also be in this range. The 9 Lives Racing medium downforce kit typifies this build. I don’t trust CFD until it’s been validated in a wind tunnel, but these do seem like reasonable numbers.

My Hyundai Veloster N outfitted with a DIY splitter, canards, and wing also made its way into this category, with a wind-tunnel tested cL of -.68. The wing was rather short at 135cm (53″), and I didn’t optimize the angle or height (I was testing many other things). I also had the splitter at 4.5″ ground clearance, which was too high. But even if I had a chance to lower the splitter and optimize the wing, I’m not sure the car would break the cL -1.0 barrier. That’s serious race car territory.

Serious race cars (cL -1.0 to -2.0)

The average car enthusiast building a track car is going to have a difficult time getting below cL -1.0. To get into this category requires careful attention to details that the average DIY builder doesn’t know or care about.

Still, it can be done, and my fastback Miata with various tricks managed cL -1.2 as tested at Watkins Glen.

Occam’s Racer V1: cD .41, cL -1.2, L/D -2.97

Prototypes (cL -2.0 and less)

Any car that achieves a cL of less than -2.0 is likely a pro-level race car or a time attack build. It’s just very difficult to achieve this level of downforce. Katz cites a generic prototype racer at cL -3.0. The Audi R8 Lemans racer was around cL -2.6. At the top of my list is a Mazda RX-792P that was reported to be around cL -3.8.

Mazda RX-729P: cD .70, cL -3.80, L/D -5.43

Time attack cars don’t race wheel-to-wheel, and so their aerodynamic parts can be wider than the car. I’m quite sure these cars are well under cL -2.0.

Tadeu’s Miata should be in this category.

It’s also possible to achieve a cL of under -2 by sucking the car to the ground with a huge fan. The Gordon Murray T.50 does just that, and has a cL in the -2.2 range.

The following table is based on a car with 2 meters of frontal area (21.53 square feet) and different cL values representing various cars. I chose a speed of 62 mph (100 kph) as OptimumLaps lists this in its vehicle report. It’s also a sensible cornering speed, and about where aero feels like it’s working. (Aero is always working, but this is around the speed where people feel it working.) The downforce value in the table is the cL inverted, which is how you’d put them into OptimumLap.

Type	Downforce	Lbs @ 62 mph
Sedan	-0.15	-31
Sports	0	0
Hypercar	0.25	51
NASCAR	0.45	92
Track	0.8	164
Race	1.3	266
Time attack	2.2	450
Prototype	3	614

Different types of cars and pounds of downforce at 62 mph.

Lift/Drag ratio

I started by discussing drag and then moved onto lift, so now let’s divide lift by drag to get the L/D ratio. The L/D ratio tells you how effective the car is at making downforce, and it’s often referred to as aerodynamic efficiency.

If this is a positive value, then the car makes lift, and the bigger the number, the worse it is. Most street cars and Spec race cars (Spec Miata, SE46, 944 Spec, etc) have a positive L/D ratio. Any car with a negative L/D ratio makes downforce, and the mo negative, the mo better.

In the following sections, I sort cars into rough categories that represent different L/D ratios. I’ll ignore anything that doesn’t make downforce. Fuck those cars.

DIY track cars (-1:1)

Cars with a L/D ratio of negative one to one make as much drag as they do downforce. Most track cars with DIY splitters and wings are somewhere in this range with a cD 0.5 and a cL -0.5. Add canards and it might be cD 0.6 and cL -0.6. Add a dual element wing and it might be cD 0.7 and cL -0.7. You see where I’m going with this, as long as the drag and downforce values are about the same, the car is in this category of mediocre aero efficiency.

My Hyundai Veloster N with a ducktail spoiler, better splitter, and some aero tricks thrown at it was about -1:1.

Veloster, splitter, spoiler, etc: cD .64, cL -.67, L/D -1.05

Race and TT aero (-2:1)

Any car that achieves a L/D ratio of -2:1 or better I’m going to put into the very generalized category of race aero. It’s difficult for a typical home builder to achieve this for a number of reasons. One of which is that in the USA we have to run with the windows down, and this adds drag and reduces rear downforce, making it even harder to achieve aero efficiency. But it can be done if you know what you’re doing, especially if you consult a professional or talk to Kyle Forster.

I don’t have a lot of data on time attack cars, but because they are usually unbound by engine restrictions, they make enormous amounts of power. As such, they can get away with adding downforce and mostly ignoring drag. I would imagine a lot of time attack cars are in this 2:1 category as well.

V8 Miatas don’t need to pay as much attention to drag.

Professional aero (-3:1)

To get a L/D ratio of -3:1 you need to obsess about drag, and choose aero components that return a high L/D ratio. No, I’m not talking about choosing a wing for its efficiency, please stop with that nonsense. I’m talking about choosing components that give the entire vehicle the best aerodynamic efficiency (often this is running wing angle that’s close to stalling).

I don’t think you can get to this level without a lot of research and testing, and by that I mean a combination of CFD, wind tunnel, and track testing. I’m sure there will be a lot of time spent managing and manipulating vortices.

This is the level I’m trying to reach with my race car. I’m purposely not putting an image of a car here, because I’m reserving that for Falconet, after I come back from the wind tunnel and figure out how to squeak over the line with a L/D of -3:1

Prototype aero (-4:1)

It’s difficult to achieve this level of aero efficiency, but it can be done, or even exceeded. In fact, Le Mans prototype cars are limited to L/D of 4.0, so you know that it’s possible to do better than that. But you or me? Nah.

From Corvette C5 Wind Tunnel Test to GLTC Win

Luke McGrew qualified on pole for the first Grid Life Touring Cup (GLTC) race at COTA this year, and then proceeded to win all the races. This isn’t super surprising, because he’s always a front runner. But Grid Life nerfed the flat-tuned cars even more this year. So how the fuck is Luke doing it in a C5 Corvette?

For starters, he’s a hell of a driver. He’s also really smart about the way he sets up his car. For example, he uses a small spoiler rather than a wing. But wait a goddamn minute, everyone knows wings work better than spoilers, right? Well, it depends on the car, and it depends on the rules.

GLTC is a pounds-per-horsepower series that allows some aero for free (small wings and spoilers, undertrays without splitters, hood and fender vents, etc), but penalizes or bans other aero parts. As such, a careful reading of the rules is important, and optimizing to those rules can confer a small advantage.

Luke knows what he’s doing, and part of that is doing the research. In that, he found an old wind tunnel test on a C5 Corvette. After reading that, he asked me to “check his math” so to speak, by running simulations in OptimumLap. After purchasing the wind tunnel report (to get the cL and cD data), I built several versions of his car, ran simulations, and verified his gut feelings were spot on.

No splitter and a spoiler instead of a wing.

There’s more backstory to this story, so let me elaborate.

The wind tunnel report

Back in 2002 a group of SCCA racers took a C5 Corvette to a wind tunnel and published a report on the results. It’s not a very long report, but the story is compelling, and the data speaks for itself. The report is available here for $37. I’m going to review some of what’s in that report, but without any specifics, because the author said not to reprint any of it without permission, and so I won’t.

The group did 26 runs in 10 hours, which is oddly the same number of runs I did at the A2 wind tunnel. They used a much larger wind tunnel at the Canadian National Research Center, in Ottawa, Canada, whuch measures 9 meters square by 24 meters long. This is quite a bit larger than A2 wind tunnel (which is 14 feet wide and 58 feet long), and so the Canadian results should be more accurate.

But how accurate is a wind tunnel compared to the real world? I don’t know. When I posted my wind tunnel data online, some internet pundit, without a shred of empathy or humility, puffed up said I made a major mistake in my report, because the wind tunnel optimizes to a constant Qrh, not V-100mph, so that my data was useless without the Qrh average for each run. I have no idea what that means, but I don’t see anything like a Qrh average column for this wind tunnel report either. And so I guess all this data from Canada is similarly worthless?

Well, I’m not a professional aerodynamicist, I’m a fuggin hack, but I’d have to think the differences from each test run are still important, even if the actual numbers aren’t 100% accurate. So let’s shove all the caveats and internet buffaloes aside and move ahead with what they tested, and the comparative data.

Drag – In the test they tried various things to reduce drag, from taping up the front grill to rounding the B pillar, to putting a hole in the license plate. Some things worked surprisingly well, some had no effect at all.
Rear wing vs spoilers – A couple different wings were tested, and since the baseline car used a spoiler, they included the data for that as well. But isolating the spoiler data is rather difficult.
Wings, end plates, and Gurney flaps – They tested three different end plates on the standard wing, and their results were somewhat similar to mine, which is that end plates are the least important part of the entire aero package.
Splitters – They tested a splitter with a flat undertray and one with diffusers. They call this a Laguna undertray for whatever reason, and I will say the design looks quite good.
Yaw – I didn’t test yaw, but they did, using both + and – 3 degrees for most of the runs, but they also tested higher yaw angles initially before settling on just 3 degrees for the rest of the tests.
Tire life – While tire life isn’t something you test in a wind tunnel, the report concludes with results from the race season, which showed tire life was considerably longer using downforce. This is something I wrote about before, that downforce increases tire life, and their experience was the same.

OptimumLap simulations

With all of this wind tunnel data in hand, I went into OptimumLap and built Luke’s exact car. I started with the basic specifications for a C5 Corvette, but used a 252 horsepower flat-tuned dyno chart instead. Detuning is what allows a Corvette to compete in GLTC, and a result of that is a very flat torque curve. This is recognized as an advantage, and flat-tuned engines are penalized for that. Cars are also penalized for aero.

To see which aero version was fastest, I created nine versions of his car, each with different aero parts. I used the coefficients of lift and drag from the report, and swapped out every version of splitter, wing, spoiler, etc. This may sound easy, but the the table that shows the coefficients has low resolution, which made isolating the individual aero components a little tricky. Anyway, I persevered and had my nine different cars, giving them different weights to match the rules.

Grid Life Touring Cup is a pounds per horsepower series, and penalizes cars for using aero, by making them heavier or less powerful. For example, if you use a spoiler or wing that’s larger than 250 square inches, there is a penalty depending on how large you go. Likewise, a splitter carries a penalty over an airdam, and there are penalties for various combinations of wings with splitters.

Because it’s easier to adjust a car’s weight than its engine tune, I simply changed the weight of each aero build to match the GLTC rules. Thus, the car would weigh between 3213 lbs (free aero) to 3371 lbs (splitter and big wing). Note that these weights may be off by a season, as GLTC again nerfed the flat tunes. And also, don’t take too much into the lap time itself, OptimumLap can’t really predict lap times without a lot of fudging, so this is just comparative data you’re seeing.

I then ran all nine cars around various the race tracks GLTC goes to, to see which would win. The winner wasn’t the same at every race track, but a few builds bubbled up to the top, and some sank to the bottom. The following image shows a speed trace and lap times of five of those builds at COTA. I’m not going to reveal which one was the fastest (that’s between me and Luke), but I will tell you which one was the slowest.

Speed trace of five C5 aero builds for GLTC.

See that black line that has the highest top speed? That’s the OEM aero version, essential a base trim model (BTM) off the showroom floor. It might have a 5-10 mph advantage on the back straight, but it posted a 154.44 lap time, which was the slowest at COTA, and also the slowest at every other track. In the end, cornering speed matters more than top speed.

To get back to what I was saying earlier, Luke uses a spoiler and not a wing. GLTC allows you to use a small wing (less than 250 square inches) for free, and so wouldn’t this be better? Not always, and it’s actually quite close. I go through this investigation in my own wind tunnel report (a $25 bargain), showing that there are times when the wing is faster, but sometimes the spoiler wins.

Falconet #2: Necessary Mistakes

It’s been a little while since I announced that I would be teaming up with Spec13 to Busa-swap my 1994 Miata. I can pretty much assure you that looking back on this years from now, that this was both a necessity and a mistake.

A necessity because the motor allows me to do things I couldn’t have done with any other engine. And this has nothing to do with the power output, ram-air system, 11k redline, or sequential shifting, and everything to do with weight and packaging, and how I can exploit that.

A mistake because I was just about to get out of Miatas entirely, and concentrate on other things in my life. But the availability of this motor swap is creating so many opportunities for radical change that there will be no going back to a standard Miata! This is a one-way trip to the dark side of aerodynamic madness. So, what’s happening?

The engine and transmission are out, and the PPF has been removed. The Spec13 kit doesn’t use the PPF, and so the diff is supported by a rear bracket (I got mine from K-Power, but Spec13 will also offer a version of this).

I bought a V8 Roadsters subframe, which isn’t necessary, but should create more space to mount the engine. Not that the Busa engine needs a lot of room, it’s tiny. What excites me about the tubular subframe is what I can do with all the extra space.

Room for an extractor hood…, and then some.

The area in front of the engine block will be used for a radiator and extractor hood. That won’t take up much room, and I should be able to run the headers in front of the engine, put a hole in the firewall, and make a side exit exhaust.

This is a lot of work, and it’s extra work considering that Alyssa scanned the engine bay and is designing the header pipes right now. The engine sits longitudinally, and the pipes exit like a standard Miata under the car. Well… that’s what the swap kit comes with, but like I said, I want to make a side exit exhaust, and so my headers will likely wrap around the front of the engine.

The reason for that is threefold:

Side exit exhausts are cool looking. I like the idea of a motorcycle muffler set into the door and visible on the passenger side of the car.
A side exit makes it easier to make a flat bottom, because I won’t have to worry about heat management. I may still need to cool the diff with a NACA duct, but that’s a minor thing compared to cooling an exhaust.
With one less item in the transmission tunnel, I can dump air into the empty chamber and make a front diffuser. Huh?

Because the Hayabusa transmission is within the engine case, this leaves the trans tunnel mostly empty. Typically you have a PPF, driveshaft, and exhaust sharing the trans tunnel, but with two out of three of these removed, there’s a lot of air space in the tunnel.

Red arrow shows location of output shaft. With no Miata transmission, there’s so much room for activities!

My plan is to diffuse air from the center of the splitter into the transmission cavity and make downforce in front of it. This is an area that a standard car can’t exploit, and because the diffuser is so far forward, it should create front downforce.

The Toyota GP-One used a front diffuser, but sends air to the sides, rather than into a central tunnel.

The Miata chassis doesn’t allow me to do the same thing, which is why I’m sending some air down the center tunnel. But I’ll also be sending some air to the sides via what I’m calling rocker vents.

Rocker vents

There are many options for fender vents, from expensive laser cut aluminum, to budget eBay plastic, to the cheapest solution – simply cutting a hole in the OE fenders.

Vents on the top of the fender are important, but so are vents behind the front wheel. The budget solution is to make a horizontal cut and then push the fender into the wheel well. Or some people will pull the bottom of the fender away from the body and vent air between the chassis and quarter panel. And finally some people will cut into the quarter panel and install Left Lane Designs fender vents.

All of these are good solutions to venting the front wheels and making the splitter work better. I like the following picture, which is a great example of NA Miata fender venting.

A big vent in the top of the fender and an aggressive cut behind the wheel is about the best you can do on a Miata.

That’s good venting, but it’s not enough for Falconet’s front diffuser, so I’m cutting into the chassis to get even more extraction. My front wheel extractor vents will start about 3” further inside and exit into the bottom half of the doors. Say what?

Cutting into the chassis for fender extractor vents. It’s fucking madness.

I’ve been looking at DTM cars and Japanese GT500 cars (similar chassis), and they all have extreme front vents that exit in the rocker panel. Unfortunately I can’t do that without seriously compromising the unibody. My car has a full cage, but there are no tubes that connect the cage to the shock towers (yet), and so unless I weld those in, I’ll have to limit the amount of venting I can do.

Would love to vent the whole rocker panel, but not going this far.

I’d like to start the vent as near to the center of the car as possible, but I can’t get too aggressive with it, because if I cut into the footwell, I lose space for the dead pedal. I can live without that, but if I go further, I’ll lose space for the clutch pedal as well. So unless I mount the clutch lever on the steering wheel or shifter, there’s a limit to how centrally located the extractor vent can go. (The motorcycle transmission allows clutches upshifts, so this isn’t an insane idea.)

A lot of small exploratory cuts were required to find out where I could remove material.

So for the time being, I’m going to stop the fender vent at the footwell, keeping all of the sheet metal in the cockpit intact. In the image above, that inner wall with the holes in it is the vertical wall that makes the side of the footwell.

I had originally thought I’d cut out the bottom door hinge completely and continue the extractor vent into the rocker panels. But I don’t want to turn my car into a noodle. There’s a vertical support at the hinges which ties into the rocker panel, and so I’m leaving all of that intact. This makes the exit of the duct much shorter and a harder angle than I’d like, but I’m just not feeling brave enough to cut a hinge into the middle of the car and then re-brace it.

This is already a lot of work, and you might be wondering, what’s all of this venting going to be worth in terms of downforce? I honestly don’t know. It’s not just fender venting we’re talking about, it’s a front diffuser.

I will A/B test the rocker vents in a wind tunnel on 6/20, but without spinning tires, I don’t know how accurate it will be. But I can at least compare the rocker vents with other fender vents I’m testing, and see how they compare. I took a bunch of pictures and will do a how-to post in the future.

I can tell you that the idea of the whole aero package occupies my thoughts from the moment I wake up. I’ve drawn and re-drawn several ideas, and I’m not ruling out a front wing molded into the bodywork.

The engine is short enough that I might be able to integrate a front wing into the bodywork.

Instructor Fest 3/2/2024

The Motorsports Safety Foundation (MSF) is an instructor accreditation organization that is trying to standardize HPDE driving instructors nationwide. I have a MSF Level 2 certification, and felt that program (taught by Hooked on Driving, in my case) was great, and I learned a lot. MSF was supposed to have an instructor event this year, but that somehow fell through. Mass Tuning either picked up the ball or already had it rolling, so kudos to them.

Who is Mass Tuning? A bunch of cowboys, that’s who! I’m joking of course, but in the realm of HPDE, they feel like the disruptors. In just a couple years they’ve grown massively to become the HPDE organization with the most events in the northeast. Then they started adding time trials to their HPDE events, and this year they are even starting their own racing series, Northeast GT.

So given their rate of expansion and innovation, it doesn’t surprise me that Mass Tuning is also making waves in HPDE instruction. On March 2nd and 3rd (this coming weekend), they are hosting the first Instructor Fest (register). I believe you have to be a coach or aspiring to be one, but it’s FREE, so get on it!

You can see the full agenda below, but the important part is that I’ll be teaching a class on Saturday, on how to read a speed trace. The session is a little odd, because it’s a hands-on exercise using pens and paper. You won’t learn how to navigate a difficult app (ahem, Race Studio), instead you’ll learn the following things which you can apply to any motorsports data app:

Look at the squiggly lines on a speed trace and know what they mean
Recognize common errors in braking, cornering, and acceleration
Describe “backing up the corner”
Explain why vMin may be the most important data point
Contrast two laps for for time gained or lost

Why data? Because most HPDE organizations don’t have a data coaching program, and don’t have enough right-seat coaches for intermediate drivers. Those drivers tread water in a pool of conflicting information, with little guidance after they’ve been soloed. I’d like to be one of the people that turn that situation around. As part of that, for the past two years I’ve been data coaching for the Niagara Porsche Club of America, and I can pass along my experience so that you can read data as well.

Even if you have no desire to be a data coach in the future, it’s important to understand data. I mean, everyone uses data already, from knowing what gear they are in, to what braking marker they use, and of course their lap time. If we did everything by butt dynos and “you know, I think that felt faster,” we’d get nowhere in life. Objective measures are the only way we can quantify anything, and data is the foundation of that.

So if you’re not doing anything this weekend, and are in the vicinity of Thompson Speedway, register for Instructor Fest. See you there!

Agenda

12:00pm – Lunch Meet & Greet

1:00pm – Instructor’s Meeting

1:15pm – “Survive Your Student” by Nick Fontana (Corner Faster)

2:15pm – “An Introduction to Data Coaching: Reading a Speed Trace” by Mario Korf (Occam’s Racer)

3:00pm – Break, Break, BREAK

3:15pm – “You, Me and the Journey to MSF Level 3” by Eric Kaul (Motorsport Safety Foundation)

4:15pm – “Trends in Causes of Claims & How to be a Safer Instructor” by Steve Katz (OnTrack Insurance)

5:00pm – Dinner & Cash Bar Social

10:00pm – Checkered Flag

Sunday:

8:00am – Breakfast Meet & Greet

9:00am – Instructor’s Meeting

9:15am – “Insurance Concerns for Instructors and Tips to Avoid Incidents with Students” by Ryan Staub (Epic Brokers)

10:15am – “Data Guided Coaching” by Matt Romanowski (TrailBrake.com)

11:00am – Break, Break, BREAK

11:15am – “A Guide to HPDE Instructing” by Matt Leblanc

1:00pm – Lunch Meet & Greet

2:00pm – “Taking Your Instructing Over the Top” by Bill Stevens (BnS Racing Services)

2:45pm – “Communicating with your Student” by Paul Balich

3:00pm – Questions & Answers

4:00pm – Checkered Flag

Miata Wind Tunnel Test Ideas

I wrote this post a while ago, and since then I made it back to the wind tunnel and tested a lot of things. Sadly, Falconet wasn’t ready for that trip, and so instead I took a Miata with the full 9 Lives Racing medium downforce kit. I was able to test everything that’s been modeled in CFD, and the entire 9LR catalog, as well as many other options, such as fastback, hood and fender vents, and various things to reduce drag and add downforce.

Purchase Miata Wind Tunnel Report

I’m going back to the A2 wind tunnel this summer and I’ll test a bunch of stuff on my Hayabusa-swapped Miata, Falconet. Wind tunnel testing is expensive, and it’s an 11-hour drive each way, so I need to be ultra prepared so that I’m not wasting time and money.

As part of that preparation, I want to know what other people are curious about. Do you have some parts to test? Send them to me and I’ll send them back when I’m done. Do you have some ideas you want to test, but can’t implement? Maybe I can cobble something together in time. Please drop me a comment at the bottom or use my contact form to email me, and I’ll do my best to test what’s important to the Miata community.

One of the benefits to wind tunnel testing is the parts don’t have to be race spec, they just need to survive a couple runs. So a lot of parts go on with the minimum number of fasteners and the maximum amount of duct tape. This allows me to do so a lot more fabricating and testing than would normally be possible.

Anyway, here are some things I’ll be testing.

Canards

I’ll be the first to admit I was wrong about canards, and so this is one area where I’ll be spending a lot of ~~time~~ money. Before I tested my Veloster N in the wind tunnel, I thought canards were poseur junk, but after finding out that changing the height by 8″ made a 700% increase in downforce, I realized I knew jack shit about canards.

I’ll test height to find out the optimal position on the lower canard. I’ll also test size, shape, angle, profile (blade vs airfoil), and end treatment (wicker sizes).

Splitter

On my Veloster I tested flat vs curved splitters and found massive gains (150% more downforce) using a splitter that curved upwards at the trailing edge. This is essentially that same thing as using splitter diffusers, but instead of diffusing air into the wheel wells, the air is diffused over the entire width of the car. So I’d like to test this vs a flat splitter with splitter diffusers.

I also added vortex strakes in front of the wheels and this reduced drag quite a bit, but because those strakes were only on one splitter, I didn’t do a proper A/B test. So I’ll test these again on the same diffuser to see how worthwhile that is.

I’ve seen some online conjecture on the drag from splitter rods, and it doesn’t make a lot of sense to me. I’ll double the number of splitter rods (they’ll be fake) and see what happens.

There’s already published data on splitter length, but I might test this if enough people are hungry for that data. Likewise splitter height has been tested and published (but not on a Miata). Just the same, it’s easy enough to put blocks under the tires and test changes to height and rake, and how that affects downforce and drag.

And I might get around to testing an airdam with an undertray and no splitter lip. I think I can get the undertray to make a lot of suction, even without a splitter lip. This test isn’t a high priority for me, because I’m not personally going to set up Falconet like this, but with enough community whining, this test could go higher up the list.

Underbody

When it comes to underbody aero on touring cars (especially Miatas), I’m a confirmed naysayer. But, just like it was with canards, I might find myself eating my own words after this test!

Some of the things I’ll be testing are a flat bottom, a partial flat bottom (trans tunnel exposed), barge boards, and at least one diffuser. Falconet uses a motorcycle transmission, and so that whole transmission tunnel is open. I’ll diffuse some air into that area and see what happens.

Vents

Al at Race Louvers has some of the best wind tunnel data on the web, and I see no reason to duplicate his efforts. But I have some ideas to get more extraction out of the wheel wells, and these haven’t been tested by him yet. I’ll also be testing a hood extractor vent, which is specific to Falconet, but the data may be interesting to others.

Wings

I’ve already tested and published wind tunnel data on five wings, four end plates, and Gurney flaps, but I have a few things still to test.

I have an oddball wing I made with a short 41″ wingspan with 16″ of chord. It seems absurd, but the additional chord was shown to be very efficient in previous testing, with a clear top-speed advantage. I want to try this as a single and dual element.

There are big wings and big wings. This is the latter.

I’ll compare that wing to a Wing Logic, and that in turn to the industry standard 9 Lives Racing wing. Wing Logic appears to be a CH10, which has less camber and thickness than a Be 123-125 (which is about what the 9LR wing measures). If both wings were the same size and had the same Gurney flap, I’m fairly certain the 9LR would outperform Wing Logic. But this isn’t apples to apples, since the latter has more chord and a built-in Gurney flap. Anyway, interesting comparison.

I may also test my MSHD wing as a dual element. It’s designed as a 3D wing, but unlike many, the trailing edge is a single flat line across the span, and so I can add a second element pretty easily. And I kinda want to make a 2D MSHD, this one will be 63.5 x 11 with a built-in 1/2″ Gurney flap.

MSHD 3D 500 sq-in outperformed all other wings in my testing.

Tops

I tested the first version of my fastback at Watkins Glen, and I’d like to correlate the results from real-world track testing to wind tunnel testing. So I’ll bring an OEM roof and trunk with me and I may as well do one run without the top as well. I wish I still had a Chop Top, that would be worth testing again.

The one time I tried my race car’s fastback on my street car, the engine dropped a valve. But notice how narrow it is at the B pillar.

Open windows

Open windows add drag and reduce downforce, and so I’d like to test various things that may help. I’d like to test a wicker or vortex generator on the A pillar, smoothing airflow out the B pillar, using a longitudinal strake along the top of the window, and large NASCAR-style window nets (which are mostly fabric and not a lot of holes).

Mirrors and mirror stalks are another thing that might affect open windows, or downforce in general. By forcing air downwards, it’s possible to move air away from the windows (and wing). Conversely, moving air upwards may add downforce. And how would these trick mirrors compare to OEM mirrors or no mirrors at all? Gotta find out.

And you?

So that’s at least $4000 worth of testing and I haven’t started on your tests yet. What’s keeping you up at night?

The Dumbest Aero Rules? Splitter Rules

As someone who is building a car to fit into multiple racing classes, I need to keep up with the aero rules in different series. It’s important to know not only what is allowed in each class, but how various aero components are weighed vs other performance modifications. I gave a broad overview of several aero rules in Aero Rules… but OMG the Fecking Rules!, but wanted to dive deeper in one area.

Through this journey, I’ve leaned that most rules are written by people who have a Childs understanding of car aerodynamics. That’s not a typo; let me explain.

I used to read the Jack Reacher series of novels by Lee Childs, and in every book the author would make a dumb technical error relating to firearms. I know firearms because I was a nerdy reloader for several years, and in this field, where things can literally blow up in your face, as Jack Reacher would say, “details matter.”

As a character, Jack Reacher understood this, but his creator did not. After the author incorrectly referred to a trio of Thompson submachine guns as “grease guns” I emailed Lee Childs and said that while I enjoyed his writing, he needed a technical editor. I offered to copyedit his next book for free.

Note #1: The M1928A1 “Tommy gun” and M3 “grease gun” fire the same ammunition and perform the same role, but they look nothing alike and even someone who knows nothing about firearms wouldn’t confuse the two. I mean, one of them looks like a tool for squirting out grease, the other one is in gangster movies.

Note #2: Writing the author directly and offering my services may seem like a bold move, but this is exactly how I became a globe trotting motorcycle journalist for Moto-Euro magazine.

I wasn’t surprised when I didn’t receive a reply from Mr Childs. And so I also wasn’t surprised when he fumbled again in a later book referring to to a rifle as a “M14 Garand”. <sigh>

There’s no such thing as a M14 Garand. I own a M1 Garand, and I’ve shot a civilian M14, and while they are similar rifles, they don’t even use the same cartridge. Calling a rifle a “M14 Garand” is as idiotic as saying that Reacher’s new car is a “Mustang Corvette.” Yes, it’s that stupid.

To make matters worse, in the same story Reacher gets a ride from a woman in an enormous pickup truck. Astounded by the size of the truck, Reacher notes that it’s a Honda. Come on now!

Honda doesn’t make an enormous pickup truck, and now I was certain about two things: Childs knows as much about firearms as he does trucks. These are man’s-man topics, as baked into Jack Reacher’s DNA as the fisticuffs he engages in. After these two gaffes I let out a guffaw and could no longer read anything from that charlatan.

Let’s bring this back to aero rules. It’s difficult to write a racing rulebook, it takes the input of specialists with specific knowledge on suspension, tires, safety, engines, and aero. For whatever reason, rulebooks, like the Reacher books, get published with a Childs understanding of aero. And this results in some silly rules.

Of all the nonsensical rules I’ve seen, from banning fastbacks on convertibles, to equating wings with spoilers, to allowing diffusers but not flat bottoms… the most Childs-like rules are the splitter rules.

Flat splitter nonsense

Splitters separate the air above and below the splitter blade. They create downforce via a high pressure zone on top of the splitter blade, and a low pressure zone below it. You know what else creates downforce in the same manner? A wing.

Just like a wing, a splitter creates much more downforce through suction than from pressure. You can vastly improve the performance of a splitter by adding camber. Most people do this by installing splitter diffusers (splitter ramps), which add curvature over a small area in front of the wheel wells. Time attack cars often curve the entire rear of the splitter’s trailing edge upwards, thereby creating a splitter diffuser across the full width of the car. And some cars also curve the front upwards as well.

However you do it, adding curvature creates a Venturi, accelerating air under the lowest part of the splitter. This in turn drops the pressure, resulting in suction and downforce.

Given that this is how a splitter works, it’s surprising how many rulebooks specify that a splitter must be flat (or horizontal, or without curvature). A rule that states that a splitter must be flat is akin to a rule that states that a wing must be flat! I think we can all agree just how well a wing like that would work.

Splitters without side plates

Because splitters behave similarly to a wing, they also benefit from some attention paid to the outer edge. Look at any wing and you’ll see end plates. Look at any pro-level race car with a splitter, and you’ll see various things on the end of the splitter, which I’ll collectively refer to as side plates. These devices trap high pressure air, change the stagnation point, promote extraction, or in other ways improve the splitter’s functionality.

AJ Hartman’s side plates add over 60 lbs of downforce at 100 mph.

If there’s room on the end of your splitter blade, side plates are a no brainer. And yet, how many racing rules state that splitters can’t have anything on the ends of them? Many of them! Just for fun, I’ll pick on the SCCA autocross rules:

Front splitters are allowed but must be installed parallel to the ground… The splitter must be a single plane with the top and bottom surfaces parallel… A front splitter and its associated features shall not function as a diffuser… Splitter fences or longitudinal vertical members that serve to trap air on top of the splitter by preventing it from flowing around the sides of the car are not allowed.

You see the same verbiage from NASA, SCCA road racing, and numerous other club racing rules. Given that the splitter rules don’t allow side plates, I find it surprising they allow wings with end plates. I mean, it’s the same damn thing.

Splitter width and length

Time attack cars don’t race wheel to wheel, and so rear wings and front splitters are often much wider than the car. For example, in the Global Time Attack rules, in the Limited class, you’re allowed to use a splitter that extends 10″ in front and is 14” wider than the body.

On the other hand, most wheel to wheel racing rules limit the span of wings and splitters to body width. This is understandable, as you don’t want aero parts to hit each other on track. However, there’s no standardization on the length of the splitter lip. When you consider how few racing rules mention the chord of a wing, it’s amazing how many rules there are on splitter length. Again, this is the same thing! Here’s a smattering of splitter lengths:

12” Champcar
6″ – NASA ST1-ST4
5″ – SCCA Time Trials Nationals, SCCA GT1, GT2
4″ – NASA ST5
3″ – Grid Life Touring Cup, SCCA STU
2″ – SCCA STO, GT3, Super Touring, T1
0″ – SCCA Street Prepared

I’ll make fun of the SCCA autocross rules one more time, because it’s such low-hanging fruit. Did you see that last item on the list? The SCCA autocross Street Prepared rules allow you to have a splitter, but it can’t stick out past the bumper. Here’s the exact wording:

“A spoiler/splitter may be added to the front of the car below the bumper. It may not extend rearward beyond the front most part of the front wheel well openings, and may not block normal grille or other openings, or obstruct lights. Splitters may not protrude beyond the bumper. “

WTF? You can have a splitter but it can’t protrude beyond the bumper? How is that possible? Perhaps when this rule was written (1973?), all cars had underbite bumpers, but show me a modern car that you can fit a splitter to that doesn’t extend beyond the bumper!

You could fit a splitter to the *Mustang Corvette* on the left, but not on the right.

I don’t want to just pick on SCCA autocross rules, because splitter rules across most car racing rulebooks show a misunderstanding of how splitters work. Whoever is writing these splitter rules could have easily written the following rule for wings: “Cars may use a rear wing, but the wing must be completely flat, installed horizontally to the ground, and with no curvature. Vertical members that serve to separate air above and below the wing are not allowed.”

I’m curious, who was the first rules lawyer that decided to castrate splitter performance? I feel like they have a lot to answer for. Some may argue that cost cutting is the reason, but you can make splitter diffusers and side plates for $10. Heck, you can make a fully curved splitter for free by selecting a warped piece of plywood!

But in the end, I guess it doesn’t matter who started this, because virtually every other rules writer copied and pasted the same absurdity into their rulebooks. That’s on all of them for having a Childs understanding of aerodynamics.

Diffusers Without Flat Bottoms

There are a lot of racing organizations that have rules which allow cars to have a diffuser, but don’t allow a flat bottom. If you don’t understand how a diffuser works, you might think this is some kind of advantage. If you understand how a diffuser works, you might take a hard pass. So how does a diffuser work?

A diffuser helps air to expand both within the chamber, and in the wake of the car. As a result of this, the air in front of the diffuser drops in pressure and increases in velocity. The result is downforce. Contrary to popular belief, the diffuser itself isn’t where downforce is made; downforce is located where the greatest restriction is, in front of the diffuser.

So, if you have a car without a flat bottom, what’s in front of your diffuser? On the minority of cars, the manufacturer has done a good job making everything smooth, and it can almost replicate a full flat bottom. However, on most cars, and certainly my cars, there’s a shit ton of stuff in the way: transmission, differential, exhaust piping, suspension components, fuel tank, hoses, brackets, exposed frame members, etc. When air hits all of those pieces under the car, it creates local flow separations and drag. Accelerating air through that maze of parts doesn’t create downforce, it creates turbulence.

Diffusers help air expand, accelerating the air in front of it. What’s in front of your diffuser?

Clean airflow makes the most downforce, and thus turbulence is the enemy. Let me give you an example using wings, because I have solid data on that. My Miata has a DIY fastback that provides clean airflow to the rear wing, and it makes 390 lbs of downforce. If I use an OEM hardtop, which has more turbulence around the sides of the canopy, the downforce drops to 300 lbs at the same speed. If I then add AirTab vortex generators and thicken the boundary layer over the roof, the wing makes only 216 lbs. And finally, if I remove the top altogether so that the car is a convertible, the wing makes just 120 lbs of downforce. (These figures come from my testing at Watkins Glen report | data).

So while you can make downforce in turbulent conditions, clean airflow is obviously better, and the situation underneath the car is no different than on top. This is why proper race cars with diffusers have a flat bottom or tunnel under the car, so that they can get clean airflow.

But on a car that must to adhere to rules that don’t allow a flat bottom, the area in front of the diffuser is often a total mess. Moreover, because the diffuser has to begin before the rear axle, any downforce you create is only on the rear tires. Unless your diffuser has a better L/D ratio than your wing, why would you do it?

Maybe that’s a difficult question, let me explain first. On a proper racecar with a flat bottom and diffuser, the throat of the diffuser (where the downforce is located) is often ahead of the rear wheels, around the middle of the car. This means that the downforce is created equally over the front and rear tires. Some diffusers extend even further forward, making more front downforce than rear. Typically front downforce is harder to attain than rear downforce, so this is sort of the holy grail of underbody aero, the way I see it.

Conversely, if your car doesn’t have a flat bottom, and your diffuser begins at the rear axle, you are creating downforce over the rear tires only. A car with a rear wing already creates a lot of downforce over the rear tires, and it does so very efficiently. So unless your diffuser is more efficient than your wing, adding more wing is often a better way to make rear downforce (more wing angle, bigger Gurney flap, more planform area, greater coefficient of lift, etc..).

Nature abhors a vacuum

Let’s play the fantasy game where, without a flat floor, you somehow manage to create a low pressure area in front of the diffuser. Fact: the air everywhere around it is at higher pressure. Because nature abhors a vacuum, the air outside wants to invade the area inside, to balance the pressures as it were.

With great suction comes great responsibility, and so defending that low pressure area becomes your life’s work. There are many ways to seal off the area under the car, such as side skirts (barge boards), or by creating vortices on the sides of the car using canards, or under the car using strakes.

Having done all of that, you also have to attend to what’s happening with your wheels and tires. The most significant problem is that as your tires roll forward, they compress the air underneath them, like a supercharger. This tire squirt sends a high pressure jet of air out both sides of the tire, directly in front of your diffuser! You’ll recall that a diffuser only works because it creates low pressure, and so a jet of high pressure air is a significant problem. It’s less important, but air also intrudes through the spokes of your wheels, and lower caketin covers are required to block this air from going under the diffuser as well.

So as you can see, if you want to make downforce using a diffuser without a flat bottom, the odds are not in your favor. The air under the car is likely turbulent to begin with, and mother nature herself is actively working against you. You have to protect the area of suction as best you can, and if everything goes 100% to plan, you’re still only making downforce over just the rear wheels. When all is said and done, rear downforce is usually gained much more efficiently using a wing.

But I have CFD proof!

But wait, you say, I’ve seen CFD that proves that a diffuser works without a flat bottom!

In the Verus Engineering blog they did a neat CFD study called Is a Flat Underbody Necessary for a Rear Diffuser to Function? The data shows that, compared to a car with a dirty bottom, a diffuser without a flat bottom reduced drag by 26.2 lbs and made 10.2 lbs of downforce. Anytime you can reduce drag and gain downforce, you take it, so this looks like a clear win.

But you could also read their data in a completely different way. Note that the flat bottom alone (without a diffuser) made 23% more downforce than the flat bottom with a diffuser, and only gained 3.5% drag in doing so. So based on this CFD data, one could conclude that this diffuser reduced the effectiveness of a flat floor.

Now that’s a pretty strange conclusion, because I would imagine that any diffuser would help a flat bottom work better. It makes me wonder if the CFD model is too simplistic. Let me not be too critical, because Verus and everyone else publishing CFD is doing us all a favor showing us this data. Computers are simply tools, and with a refinement of those tools, we’ll get better and better data. Let’s just keep moving ahead.

Next, I’ll take a look at Kyle Forsters videos. He has two CFD videos examining flat floors and diffusers, using a NC Miata:

Cut bumper vs Diffuser – Kyle’s first video is just a cut bumper vs diffuser. He didn’t test a flat floor, and the muffler got in the way a bit.
Flat Floor vs Diffuser – In the second video, he uses a flat floor and gets different results.

I watched the videos, and made the following notes. (The downforce and drag values are at 180 kph, or about 112 mph.)

A cut bumper added 6 kg downforce and reduced drag by 1 kg.
A diffuser with the muffler in the way made 4 kg downforce and 4kg more drag. This is not as good as the cut bumper.
Improving the diffuser by removing the muffler added 11 kg downforce, but drag remained the same. Based on this, the cut bumper is still better than the diffuser without a flat bottom.
A flat floor with a cut bumper made 49 kg of downforce and reduced drag by 15 kg.
A flat floor with a diffuser made 50 kg of downforce and reduced drag by 18 kg.

Gleaning this data was a bit difficult, because the comparative data is split across two videos. There are inconsistencies as well; In one video he says the diffuser is 3x more effective at creating downforce than a cut bumper (both with flat floors), and in another video the cut bumper data is virtually the same as the diffuser. So, just like with the previous CFD, there are inconsistencies with the tool, or the operator, or the person interpreting it (mea culpa).

I’ve taken Kyle Forster’s course on aerodynamics, and there’s a lot of CFD on underbody aero, but not much without a flat bottom. However, I’ve had some private consultations with Kyle, and in one of those he showed me a hush-hush diffuser from overseas that was designed to work without a flat bottom. He wouldn’t go into details on the design, and when I asked him how many points that was worth, he shut me down quickly, saying that was private information. But know this, it is possible for a car manufacturer and surely an F1 aerodynamicist to make a diffuser work without a flat bottom. The question is, can you or I modify our cars to do that?

My DIY diffuser

I wanted to know if a diffuser without a flat floor was worth it, so I built one and tested it in the A2 wind tunnel. It’s actually quite neat looking and follows basic aerodynamic principles of vertical and lateral diffusion. With it mounted on the car I was like… damn, that’s cool looking!

Obviously the results in this section apply only to this diffuser on this car, and your results will certainly differ. If you are building your own diffuser, you might take some of my build details and do them differently. (Or better yet, do something else with your time.)

The diffuser is as wide as I could make it between the wheels and extends to the rear axle. It has about 3″ of ground clearance at the front, and the leading edge angles up slightly. My thinking here was to make the area of least restriction (where the downforce is located) within the diffuser itself. I thought this was pretty clever, but this might have been a mistake.

Test fitting my diffuser and setting the height.

I added strakes that diverge from the centerline. These are supposed to spin a vortex off the trailing edge, which should help defeat some tire squirt, by sealing off the center compartment. Thus, the outer chambers of the diffuser are mostly sacrificial in nature, allowing the center tunnel to do most of the work.

The strakes are 3″ off the ground, because at least two of the rulebooks I was looking at state that the minimum ground clearance was 3″. Perhaps the diffuser would have worked better if the strakes were closer to the ground, but rules be rules.

Another detail that didn’t work in its favor is that I made a quick-release diffuser that attached to the car via the trailer hitch and two zip ties. On the upside, I was able to remove the diffuser in one minute. On the downside, there’s a longitudinal cross member behind the muffler that blocks air moving rearward above the diffuser. See the image below.

Quick release splitter mounts to the trailer hitch.

Is this significant? I don’t know. I was mostly concerned with air going under the diffuser, and reckoned anything that snuck between the muffler and top of the diffuser wouldn’t be important. If I had to guess, this probably added some drag, but might have also kicked up some air like a spoiler or big Gurney flap. I’ll have to retest this on my Miata in the future, with topside designed so that air can travel cleanly on top and below.

So how did my DIY diffuser work? If you buy my wind tunnel report you’ll get the full story, but the abridged version is the diffuser made about 15 lbs of rear downforce at a cost of about 3.5 lbs of drag (at 100 mph). That’s not a lot of downforce, but a favorable lift/drag ratio. Except that isn’t the whole truth.

The front of the car lost 14.5 lbs of downforce (when you push down on one end of the car, it goes up on the other), and so the net gain was only .5 lbs of downforce for 3.5 lbs of drag. The data shows that the diffuser would make the car slower. Yuck.

The loss of front downforce is normal, but for whatever reason, the ratio is really bad. I tried 16 different rear aero options in the wind tunnel (5 wings, 3 Gurney flaps, 4 end plates, 2 spoilers) and they all added rear downforce at the expense of front downforce. But all of them did so much more favorably than the diffuser. (In fact one spoiler added both front and rear downforce, but that’s a story for another day.) I don’t know why the diffuser lost an equal amount in front, it’s downright puzzling.

Part of the problem may be the wind tunnel itself. The wheels don’t spin, and the boundary layer grows as air moves along the tunnel. I don’t know if this makes things better or worse for the diffuser, but it’s worth noting.

I’m an amateur aerodynamicist, but I’m well studied, and I have good DIY fabrication skills. I generally feel like I know what I’m doing, but I made a diffuser that made my car slower. But this was my first stab at a diffuser, and I can only improve on this. I also didn’t throw all the tricks at it, and I’m sure that side skirts could have helped.

But even if I could have made the diffuser work better, I’m still only making rear downforce, and losing front downforce in the trade. In my mind, the entire game of budget touring car aerodynamics is getting more front downforce, that’s the real limiting factor in an aero package. From all of this I can conclude one thing, which is that a diffuser without a flat bottom is fuggin useless to me; it’s much easier for me to gain rear downforce by adding more wing angle, a larger Gurney flap, or using a larger wing.

Rules and reasons

Given this, why do the people who write racing rules allow a diffuser, but ban flat bottoms? I can think of three reasons:

Cost control – Rule makers are trying to reduce the cost of a full-scale aerodynamics war. Enh, I guess that’s a concern, but compared to engine mods and tires, aero is the cheapest way to get performance, and a flat bottom is dead simple. I mean, it’s flat.
Give the people what they want – Rule makers don’t want to restrict people from bolting on parts that look cool. This one I sort of understand. If someone likes the look of a diffuser, they should be allowed to use one. Even if it makes the car slower. But what if someone likes the look of a flat bottom? I mean, give the people what they want, right?
Copy/paste – Rule makers are lazy or don’t know any better, so they copy and paste someone else’s rules. We’ve all been there.

Given those reasons, it’s understandable that some racing rules allow a diffuser, but not a flat bottom. Which rule sets are we talking about? I’m sure there are many more, but here’s a quick look:

Grid Life – Street Modified
NASA Time Trials – TT4
Ontario Time Attack – All classes
SCCA Time Trials Nationals – Max classes

But… not all cars are created equal. Some manufacturers have done a great job fairing and flattening the underside, and a diffuser will work well on those cars. Whereas on many cars, a diffuser without a flat bottom is going to be totally useless. For parity across different cars, I feel that any rules that allow diffusers should allow a flat bottom. Full stop.

Next stop… flat bottom

Despite failing miserably in the wind tunnel, I think my diffuser design is pretty good, and so obviously the next thing to test is how the diffuser works with a flat bottom. But this time I’ll test it on a proper racecar, not my Veloster. In the Spring, I’ll trailer Falconet down to A2 and test every combination of underbody aero including flat bottom, diffuser, side skirt, etc. And maybe I’ll acquire some diffusers manufactured by various companies and test those. Finally, I’m really curious about side skirts with no other aero, that should be an interesting one to throw into the mix.

If you value this kind of content, and want to support future aerodynamic tests and articles, please buy me a coffee.

Pineview Run 2024

Pineview Run recently released their non-member pricing. Let’s do the math on the various options.

Day Pass

Pineview charges $300 for their day pass, but is it worth it? I think so. I’ve driven or raced 26 tracks so far, and when the asphalt repaving is complete on the track extension, Pineview will rank in my top three favorite tracks. It’s a driver’s track, one that tests your skill rather than how much money you spent on your car. I understand that doesn’t appeal to the majority of people, but for those of us who are students of the game, this is the boss level.

If you’ve driven the track extension already, you know it was very bumpy, and there was a significant dip on the back straight. What happened was, Pineview got shafted by the people doing the asphalt. Core samples around the track showed the thickness was only half of what was contracted for in various locations. That’s being fixed now, and then a final sealer coat will be applied in the spring. And when that’s done, if you’re a driver looking for a driver’s track, a $300 day pass will seem fair.

Compared to other track day organizations, the day pass pricing is right in the middle. On the low end you have Mass Tuning, which goes to a lot of tracks for $249 (including Pineview on 7/27-28). And towards the upper end are clubs like SCDA, who charge $439 for every track they visit (including Pineview on 6/29). And so a day pass is right in the middle of what other organizations charge. (I should mention that when you drive with other groups, you won’t have to use a Raceceiver, and you also won’t need to pay for a checkout ride with an instructor to be in a point-by group.)

But if you’re a bargain hunter simply looking for better deals, there are tracks that have $200-250 days. Off the top of my head, there’s Canaan, Nelson, Waterford, and in Canada you have TMP, Great Bend, and I believe Shannonville has some cheaper days. But these are all several hours away and often mid week.

I haven’t seen Track Night in America’s pricing for 2024, but they should also get you on track for under $200. The main drawbacks being you only get three sessions, and the drivers are generally one step below (the Advanced drivers are actually Intermediates, the Intermediate drivers are simply Novices with faster cars). And there’s a lot of driving to get to each track, and driving home at night.

Season Track Pass

The Season Track Pass costs $2300 plus a $50 day use fee. So if you drive the track 12 days, it’s about $280 per visit. That’s a minor discount on the day pass, but if you’re a track rat and come 20 times, it works out to $165 per visit, and it’s difficult to beat that.

The Season Track Pass gets you in on Fridays and you can also attend the motorcycle events for free, and there’s discount pricing on the Challenge Cup. I don’t think many are going for this deal, but in the future people may look back on this and call it a bargain.

Weekender Track Pass

The best deal in motorsports is the Weekender. At $1200 plus a $50 use fee, you’ll beat the day pass pricing after five visits. If you come a dozen times, you’ve now cut the day use price in half, at just $150 per day. Or if you’re a masochist that can manage 24 track days, you’ll do that for just $100 per day.

The caveat is that you don’t get any other events, so if you want to race in the Challenge Cup, you’ll pay the full $2500. Or if you’re a motorcyclist, you’ll pay extra for Two Wheel Wednesdays and the Evolve GT events. You also won’t get any weekdays.

But for those wanting the most weekend track time for the least coin, is there a better deal in motorsports? No.

Once word gets out on how good the long track is, I think you’ll see Pineview transition to a Monticello model; if you’re not a member, you won’t get in. As a member already, it won’t affect me. But the 2024 Weekender pass might be the swan song for public access.

Before this offer expires, I’m looking squarely at my autocross friends who will gladly pay $60 for 6 runs and do work in between. You’ll get more track time in one day at Pineview than you’ll get in an entire season of autocross. If the competition aspect is the most important thing to you, we’ll start a Pineview leaderboard with SCCA classing. It’s this year or maybe never. Fuggin do it.

Veloster N Driving Modes on Track

The Veloster N has five driving modes: Economy, Normal, Sport, N, and N Custom. Er… I think I have this right, I don’t know, because I only use two.

Eco – Economy mode limits boost pressure, but from what I’ve read online, it’s not any more economical. So I don’t use this.
Normal – I use Normal mode on the street.
Sport – I’ve never tried this, I don’t see the point.
N – I don’t use this, the suspension is too hard, and the pops and bangs of a burble tune annoy this shit out of me.
N Custom – I use this mode on track. I have the eLSD and steering on full, but pretty much everything else is turned off or on the lowest setting. No rev matching, soft suspension, no stability control, and the exhaust as quiet as it’ll go. For some unknown reason, I’ve also had the engine in the lowest setting all year, which should be a nice surprise when I select full power next year (actually, I later found out there’s no difference in peak output).

I’ve driven my VN on track in both wet and dry conditions, and on good and bad tires. Now that I’ve had a chance to look at some comparative track data, I can answer some questions, such as:

Which driving mode is fastest on a wet track?
Which driving mode is fastest on a dry track?
How do all-season tires compare to a proper track tire?

Dry track: Normal vs N Custom

Since I only use the Normal and N Custom modes, I wanted to see how they compared on a dry track. My N Custom mode was 1.9 seconds faster. Woof.

Subjectively, the Normal mode sucks ass on the track. It takes the fun out of driving. The stability control is probably the main culprit, and switching to the N mode (on the fly) resulted in instant smiles from me and a “whoa” from my passenger. The way the car behaves is night and day different in N Custom mode. It’s astonishing.

This immediately changed the way I drove, from trying to maintain a higher vMin, to backing up the corner. I don’t mean to throw jargon at you, so let’s take a look at the data and I’ll explain those terms. I’ve highlighted the the three most significant differences with letters.

Red is Normal mode, Blue is N Custom (nannies off).

This is Turn 2, a right hand turn onto a short straight. You can see that the Red line has about 2 mph higher minimum speed (vMin), but when I get on the gas, the nannies nerf the acceleration and I only get .31 Gs of acceleration. The Blue line has .5 Gs of acceleration, which results in about a .4 second gain. This is entirely down to the car’s intelligent systems getting in the way of my driving.
This is T9, another right hander, this one cresting a hill. By turning off the nannies I can rotate the car, which allows me to get on the gas 95 feet earlier than in Normal mode. We call this “backing up the corner,” and it means that I’ve done my braking and turning earlier in the corner. This allows me to get to full throttle earlier, and gains half a second.
Notice here how the Blue lines are to the left of the Red lines, again this is backing up the corner, and doing that in the final turn is worth almost three quarters of a second as I cross the finish line.

What’s really interesting about this graphic is that I had no idea I was in Normal mode at first. I was driving with a passenger, chatting, and totally forgot to switch modes. Then suddenly I realized I hadn’t changed the driving mode. The Veloster N allows you to do this on the fly, and so part way through the lap I switched modes.

This allowed me to drive the car in a completely different manner. I could immediately feel the difference in the way the car behaved, and was able to extract a higher level of performance, and go 1.9 seconds faster. But I wasn’t aware of how differently I was driving the car until I looked at the data just now.

Wet track: Normal vs N Custom

I also got to try both modes on a cold, wet day. This was the same track with a slightly different configuration to avoid one of the big puddles. It was raining the entire time, and there was a lot of standing water on the track.

In N Custom mode, I spun the wheels a lot on acceleration, and had to short shift to third in a number of places. My best lap was a 1:28.838. In Normal mode, I managed a 1:28.592, but was still getting some wheel spin cresting a hill. So then I left it in third gear and did a 1:26.994. That’s a difference of 1.84 seconds, which is almost exactly the same difference the two modes had in the dry.

So that’s the ticket to going fast in a torrential downpour, put it in Normal mode and drive a gear taller.

The data below isn’t super exciting, the thing to notice is mostly the difference in vMin (or minimum corner speed). I’ve drawn little black arrows to show what I mean. When the nannies are on, I can maintain a higher speed in the middle of the corner, and this results in a faster speed down the next straight. The acceleration curves are about the same in N Custom and Normal, which I find a little surprising, and so I suspect that it’s the stability control more than the traction control that’s helping in the rain.

Red is N Custom, Blue is Normal mode (nannies on).

All-season vs track tires

I have three sets of wheels and tires: 235/40R18 Kumho V730 on Konig Countergram 18×8.5 +43 (42.3 lbs); Pirelli PZ4 235/40R18 on Motegi MR140 18×8.5 +45, (42.0 lbs) and Linglong Crosswind 235/35R19 all-season tires on 19×8 +55 OEM wheels (55.3 lbs).

The only reason I bought the Crosswind tires was because the OEM tires were worn out, and I was looking for the cheapest possible tire I could mount in the winter, while my summer tires are hibernating in a heated basement. I found the Crosswind tires on sale for $65 at Walmart, and expected absolutely nothing from them.

Just the same, I wanted to see what they would do on track, and I’ve been pleasantly supersized. They don’t suck. Because there’s not a lot of grip, they break away gradually, and slides are easy to control. Once sideways, the tires howl like a banshee, which helps you know how much you’re working the tires. Or overworking.

I’m leaning on these $65 tires so hard, they look like they want to come off the bead!

I haven’t tested the Crosswinds back to back with the V730, but I put down a 1:19.1 on the all-seasons, which is about 2 seconds slower than the Kumhos did on a previous occasion. This is surprising, because you’d expect the 200TW to be a lot faster than 400TW. For example, my 1.6 Miata is about 6 seconds faster on RS4s than it is on all-season tires. This all fits in with previous data I hand that shows that FWD cars lose less performance in low-grip situations (such as rain, dirt, snow, or shitty tires).

Red is Crosswing, Blue is V730. Not the same day, and my wife was in the car on the V730, but only a 1.9 second difference.

All said, I’m having Miata-levels of fun on these $65 tires. Had I known this was possible, I wouldn’t have three sets of wheels. With a better all-season tire, like a Michelin Pilot Sport A/S 4, I think you could do everything from daily commuting to track days on one set of wheels. I know a lot of people feel they need track tires for track driving, I’m just not one of them.

Miata Aero Testing Results

This article was originally spread out over several different pages. I’m not sure what I was thinking at the time, but I’ve reorganized this as a single (rather lengthy) article now.

For the full story on how I performed these tests, see Testing Miata Aerodynamics at Watkins Glen. This article is essentially Part 2 of that one, so I can deep dive on the test results of the various aero options.

While datapoints like pounds of downforce at 100 mph, or horsepower consumed, are things we can wrap our heads around, it’s difficult to translate that into the only thing that matters: lap time. Therefore, in the following sections, I also include lap time simulations using OptimumLap.

Testing Miata tops

The first thing I wanted to test was the largest knowledge gap, roofline shape. This meant I had to have different options, that would come on and off quickly, using the same brackets. The four options were an open top, an OEM hard top, a Treasure Coast Chop Top (which should approximate a hard top with the window removed), and a fastback of my own design.

I built the fastback before I had the notion to do this test, or I would have built it differently. The main problem is that I made long brackets along the bottom edge, and this required removing the trunk lid. This meant I didn’t get to test any of the other tops with an OEM trunk lid. Instead, I bolted a plywood cover over the trunk cavity. This new trunk lid is about 3.5” taller at the back than a stock trunk. It’s hard to say exactly what the effect of this was, but it’s likely a reduction in drag and lift, akin to adding a spoiler. So when you look at the data later, note that none of the tops used an OEM trunk.

Open top results

Miatas are meant to go topless, let’s start there and address some burning questions:

What happens when you use a wing with an open top?
How much does an open top affect a wing’s performance?
At autocross speeds, is it better to remove the top or leave it on?

Take a look at the following table, and you can see that an open-top Miata generates about 40% of the downforce as one with an OEM hard top (Total Cl field). Of all the options, this was the worst at creating downforce.

It might be a little confusing that the coefficient of drag (Cd) is better with an open top than with a hard top. This is likely the result of running the tests with the windows open, which turns the hard top cabin into a parachute.

Top	Front Cl	Rear Cl	Total Cl	Cd	L/D %	HP @ 100mph
Open top	-0.20	-0.23	-0.43	0.43	1.01	47.16
Hard top	-0.17	-0.84	-1.01	0.48	2.11	52.78

Let’s plug these numbers into OptimumLap and see what happens. I’ll use three different tracks to represent a range of speeds. These tracks are already in OptimumLap.

Top	Watkins Glen	Waterford Hills	2010 SCCA Nationals
Open top	2:24.02	1:20.97	1:03.88
Hard top	2:22.96	1:19.95	1:03.34

The hard top is worth about one second at both Watkins Glen and Waterford Hills, and just over a half second on the autocross course.

OEM hard top with plywood trunk lid, a concession to the fastback.

For these simulations, the car weight was kept the same. Someone will point out that the top weighs 45 pounds, and that OptimumLap doesn’t factor in the change in center of gravity. Both true. But I can calculate how much weight you’d have to remove from the open top car to match the autocross time of the hard top, and it’s 210 pounds. I’m not sure how high you’d have to place 45 pounds above the car to equal 210 pounds, but it’s probably pretty far up there!

But running an open top car with a wing has two advantages. One, it looks cool. Two, an open top car with this wing beats any top without a wing, every time. That’s kind of jumping ahead in the data, but it’s worth noting.

Let’s get back to the real world and the test at Watkins Glen. Alyssa reported that the car was more difficult to drive with the open top. She had to brake before entering Turn 10, and then had to manage a car that was oversteering badly. With a hard top, she could mash the throttle from the exit of Turn 9 to the exit of Turn 10. That kind of confidence over an 8-9 hour race can mean a lot more than a second per lap.

Chop Top results

Treasure Coast Miata sells their “Chop Top” for budget endurance racing. It’s an economical and lightweight top that does the job of enclosing the roof. This has two benefits: better aero, and you don’t have to wear arm restraints when racing (the car is no longer considered a convertible). I fabricated mounts that attach to the hard top brackets, and with those the total weight of the Chop Top was a scant 7 pounds.

There is a persistent myth in Miatadom, that removing the rear window from a hard top is aerodynamically better. So I put two small Lexan covers on the sides of the Chop Top, closing in the sides. This made the chop very similar to a hard top without a rear window. Let’s add this data to the open top and hard top.

Top	Front cL	Rear cL	Total cL	cD	L/D %	HP @ 100mph
Open top	-0.20	-0.23	-0.43	0.43	1.01	47.16
Chop top	-0.20	-0.33	-0.53	0.45	1.19	49.40
Hard top	-0.17	-0.84	-1.01	0.48	2.11	52.78

As you can see, the chop top allows the wing to work a bit better than an open top, with an increase in downforce. But it’s not as much as you’d think.

However, once you add a wing, the Chop Top performs barely better than an open top. This is interesting, because you’d think airflow over the roof is considerably smoother than an open top. However, it’s what’s happening on the underside of the wing that’s more important, and the Chop Top roof can’t defeat the turbulence coming from the open sides of the cockpit and going beneath the wing.

Chop Top with plywood trunk cover. Note clear lexan and clear gas line, so we can see if the gas is about to overflow.

Next I’ll do the same track simulations, and what I find interesting here is that the Chop Top isn’t really that much different than an open top at any of the tracks. Not enough to really make a difference.

Top	Watkins Glen	Waterford Hills	2010 SCCA Nationals
Open top	2:24.02	1:20.97	1:03.88
Chop top	2:24.04	1:20.82	1:03.79
Hard top	2:22.96	1:19.95	1:03.34

Nevertheless, for those racing with an open top and a wing, the Chop Top is worth a look for a bit of weather protection and not using arm restraints. In addition, we’ve finally dispelled the myth that removing the rear window is more effective. It isn’t. At least when used in conjunction with a wing.

OEM hard top

All along I’ve been citing the data for the OEM hard top without really discussing it. It’s the status quo in racing, looks great, and performs its duty.

In the data, the OEM hard top generated more drag and lift than what I expected from published data. This is likely due to the open windows and wide canopy, which turns the cabin into a parachute. The drag is supposed to be around .38 with closed windows, but we measured over .5. Lift is also supposed to be the high .30-somethings, and we measured .55 (with vortex generators, I don’t have the raw data without).

The hard top with airdam, splitter, and wing made a killer combo: .48 Cd and 1.01 Cl. Those are good numbers. Racing numbers.

Fastback results

The front of my fastback uses the Treasure Coast Chop Top, and the rear fastback section bolts on and slopes back at about 14 degrees. So essentially the roofline is the same as OEM to about the rear window. Starting with the Chop Top made building the fastback fairly easy, and it was also easy to add and remove for this test. (You can see construction photos this and other tops I’ve built at the end of this article.)

An older photo, but the top is the same.

The Chop Top plus fastback weighed 17 pounds less than the stock hard top, and to equalize the two I bolted 8-pound lead weights to the top of the seat belt towers. This was the only time I made adjustments to the weight of the car, and so the open top and Chop Top configurations were a bit lighter.

The fastback significantly reduced drag, and helped the wing create more downforce. Compared to the OEM hardtop, downforce increased 129.7%. Another way of thinking of that is that the fastback turned a 60″ wing into a 78″ wing. Or you could say that the OEM hardtop is so bad that it made a 60” wing behave as a 48” wing….

The large gain in rear downforce was offset by a small loss in front downforce. Essentially, the wing was so effective with the fastback that the front end lifted, changing the height and angle of the splitter, reducing its effectiveness.

Top	Front cL	Rear cL	Total cL	cD	L/D %	HP @ 100mph
Open top	-0.20	-0.23	-0.43	0.43	1.01	47.16
Chop top	-0.20	-0.33	-0.53	0.45	1.19	49.40
Hard top	-0.17	-0.84	-1.01	0.48	2.11	52.78
Fastback	-0.12	-1.09	-1.20	0.41	2.97	44.81

In addition, the fastback reduced drag by 15%. This in itself is pretty surprising, and not only helps top speed, but fuel economy. Combined, the downforce and drag created a lift/drag ratio that was 50% better than the OEM hard top with a wing. Astounding.

Note that the .41 coefficient of drag is actually quite good when you consider that the wing made the most downforce in this configuration, and just as in all of the tests, the windows were open.

But all was not rosy with this setup. Both Anthony and Alyssa commented that the car understeered badly in this configuration, and was boring as shit to drive. Given time, Jeremiah would have changed the mechanical grip by adjusting the front roll couple, by means of spring and/or stabilizer bar. This would have helped balance the vehicle at speed.

Let’s do another simulation in OptimumLap. The fastback gains 1.9 seconds at WGI, and about half that at Waterford, which is pretty spectacular.

Top	Watkins Glen	Waterford Hills	2010 SCCA Nationals
Open top	2:24.02	1:20.97	1:03.88
Chop top	2:24.04	1:20.82	1:03.79
Hard top	2:22.96	1:19.95	1:03.34
Fastback	2:21.06	1:19.02	1:03.12

Vortex generators on an OEM hard top

The shape of the Miata’s canopy is abrupt, and if you look at wind tunnel tests, you can see smoke trails that are turbulent, and then separate, as air moves over the top. Vortex generators (VGs) create a thicker turbulent layer of air, which keeps air from separating completely. This should result in less drag, and may also help interaction with a wing.

Most vortex generators you see are cosmetic fakery and don’t create vortices. I bought the real deal from AirTab. Made of thin plastic, they go on with double-sided tape, just peel and stick. The manufacturer says they should be mounted no closer than 4” apart. I set them at 5” on center, and so that made 9 for the roof.

If you do some research on VGs, there’s good data that they work. They’ve been used on semi trucks, RVs, the underside of race car wings, and many places where flow separation can occur. For cars, take a look at the four-part series on Autospeed, where they tested VGs on a Prius and Insight. Even better, check out Hi-kick Racing’s blog on adding VGs to a Miata. VGs decreased his lap time from 1:02.8 to 1:02.1. Here’s a photo from his site.

Nevertheless, my expectations were low. If vortex generators are the cat’s meow, then every cat would have them, right? As you can see in the table below, VGs made things worse. Total downforce decreased by about 20%, and drag increased substantially. Take a look at HP consumed at 100 mph and you’ll see you have five less ponies at that speed.

Configuration	Front cL	Rear cL	Total cL	cD	L/D %	HP @ 100mph
Hard top, splitter, wing	-0.17	-0.84	-1.01	0.48	2.11	52.78
VGs, splitter, wing	-0.20	-0.61	-0.82	0.52	1.57	57.58

And this is how those values affect lap time in OptimumLap.

Vortex Generators	Watkins Glen	Waterford Hills	2010 SCCA Nationals
Hard top, splitter, wing	2:22.96	1:19.95	1:03.34
VGs, splitter, wing	2:24.32	1:20.42	1:03.54

We placed the VGs at the trailing edge of the hard top, but it’s possible that moving them forward may have helped some. Or perhaps we used too many? I followed the instructions and they were supposed to work.

The double-sided tape was difficult to remove, and so experimentation with the number and location of VGs wasn’t possible. The only lasting impression the VGs made was the adhesive, it was a bitch to remove and so all further tests would use the OEM hard top and VGs combined.

Miata top conclusions

Different tops change airflow over the roof, and this affects how a wing works. It’s unclear how much of this is is based on turbulence, or because of a change in downwash angle as air hits the wing. It’s likely a combination of both, but we didn’t have time to experiment with wing angle and this mystery remains.

In addition, our data revealed a rolling rake angle that changes ~ ½ degree depending on rear wing configuration, and this impacted front downforce and distribution more than expected. By adjusting chassis rake and splitter angle, it’s likely the total downforce and Lift/Drag efficiency would have been higher, and this may have reduced rolling-rake changes as well.

We can make the following general conclusions about using a wing with different tops.

An open top reduces a wing’s effectiveness by about 2.5x. Or, if you thought you were getting 200 lbs of downforce, you’re getting 80.
A Chop Top performs marginally better than an open top.
The OEM hard top is actually quite good with a wing. But don’t remove the rear window. And don’t use vortex generators.
A fastback allows a wing to perform the best, increasing downforce, while also decreasing drag.

9 Lives Racing Big Wang vs cheap dual wing

The low price and availability of aerodynamic car wings are making them more common in crap-can endurance racing. You can buy a cheap extruded aluminum wing on eBay, Amazon, or other online retailers for $50, but are they good for anything?

I broke the piggy bank and purchased a 53” double-decker wing for $75, shipped. This wing is sold under a variety of brand names like BestEquip, Mophorn, Neverland, etc, and I’ll refer to this as the eBay wing, because that’s where I got it.

Immediately upon unboxing, I knew I’d have to make some modifications. Like most budget items from China, it came with trunk mounts too low to allow airflow to go under the wing. I trusted the supplied mystery-metal hardware as far as I could throw them, which was directly in the trash.

The main wing felt light, yet surprisingly rigid. The stiffness is partly from the dual horizontal mounting rails, which allow the wing to be mounted at just about any width. However, the exposed slots and flat underside of the wing can’t be great for keeping airflow attached. In the future, I may add some curvature here.

The upper wing felt like a noodle, and I feared it would vibrate and hit the lower wing at speed. So I riveted on a Gurney flap of 1/4” aluminum angle to stiffen it up. I also added a small stop in the middle of the lower wing to limit downward movement of the upper wing.

The end plates had to go, not only because they were too small, but they didn’t allow the upper wing to pivot into the correct position. According to McBeath in Competition Car Aerodynamics, the upper wing should overlap the lower wing at about 4% of the chord (.3” in this case). In order to accelerate the air, the gap must be larger at the front than at the rear. With those two factors set, the slots in the supplied end plates, which support and locate the upper wing, wouldn’t allow us to pivot the upper wing into a useful angle.

Since I couldn’t get the correct spacing and wing angle with the supplied end plates, I made my own from 12”x10” sheet metal, and drilled new upper wing mounting slots. All done I paid a little over $110 for the wing and modifications, and a couple hours figuring all that out.

The car was set up with -1 degree of negative rake, meaning the back was lower than the front. Ideally it should be the other way around, but we got lost debugging what we thought was a clearance issue with a front sensor and didn’t correct the angle.

I set the lower wing angle to 3 degrees, but because of the chassis rake, the main wing was closer to 2.5 degrees. I set the upper wing to 12 degrees. Measured over the entire chord, this created a total camber of about 14 degrees, which is right in the middle of the values that McBeath cites in Competition Car Aerodynamics. Given more time, I would have experimented with the angle of attack of the upper element, as well as the entire wing. However, not knowing how the wing would perform, I chose middle-of-the-road values from a published text, and figured it was more important to avoid a stall condition than maximize downforce. The wing weighed 7.6 pounds altogether, which is very light.

60″ 9 Lives Big Wang vs 53″ eBay double wing.

The wing I used through all the other testing on at WGI is a 60” 9 Lives Racing “Big Wang”. The standard Miata wing is 64″, but my eventual plan was to end-plate mount this, like a Ferrari F40. I never did get around to that, so the wing is a bit smaller than you’d see on most Miatas. (But as you already saw above, the fastback made it behave as a much larger wing.)

When I took the wing out of the package I was immediately impressed by the sturdy construction. When there are only cockroaches left in the world, there will also be 9LR wings. I made my own 12” x 12” end plates and mounting brackets, and had a local shop weld the mounts underneath. The entire setup was about $500, and weighed in at double the double wing, at 14.4 pounds.

I set the wing angle at 5 degrees, but with chassis rake the actual wing angle measured 4.6 degrees. If you look at the 9 Lives CFD open air data, this is right in the middle of the wing’s working range. I knew the downwash angle would change with every top, and that this would change the effective wing angle. But due to intermittent track closures, I didn’t get a chance to sweep the wing angles, and left it where it was.

For both wings I used the same wing stands, which I cut from aluminum plate. Many racing series limit wings to roof height, so I made sure the highest part of the wing was level with the roof. I bolted the base of the wing stands through the sides of the trunk gutter, and while this seemed strong enough, I added another L-bracket on top of the rear fenders, and this stiffened things up considerably.

Wings compared

So let’s see how these wings compared. In this first chart, we’re looking at front and rear downforce using GPS speed alone. It looks like the eBay wing (red) creates more rear downforce as the 9 Lives Racing wing (blue), and when combined with the front, the total downforce is pretty close.

However, the weather had changed during this run, and we had an 11 mph headwind. After correcting the wind speed detected by the pitot tube, a clearer picture developed. See the bar graph below. Here we can see that the eBay wing generated less downforce than the GPS speed would have us believe. This is why you can’t trust testing using GPS speed alone, and why you hire a guy like Jeremiah.

When you add downforce on one end of the car, you can expect to lose downforce on the other end. This is the natural see-saw effect of pushing down on one end. What’s interesting here is that the 9 Lives Racing wing not only made more rear downforce, but it also had more front downforce. How can this be?

The most likely reason is drag. The eBay wing creates more drag, and this rear-biased force lifts the front end. Whatever the case, front downforce, and thus total downforce, is a lot less with the eBay wing.

Take a look at the following table and you can see the corresponding values for coefficient of lift (which we’ve been familiarizing as “downforce”), and drag. We already saw that the 9 Lives Racing wing practically doubled the total downforce, and here you can see it did that while creating 15% less drag. If you look at the final column in the table, you’ll see that the drag reduction alone equals an extra 8 hp at 100 mph.

Wing	Front cL	Rear cL	Total cL	cD	L/D%	Front Load	HP @ 100mph
9LR	-0.17	-0.84	-1.01	0.48	2.11	16.76%	52.78
eBay	0.03	-0.58	-0.56	0.55	0.90	4.43%	60.79

If we divide the total coefficient of lift by the coefficient of drag, you get the L/D ratio, which tells you how efficient the entire aero package is. Here you can see the 9 Lives Racing wing contributes to a setup that is over 230% more efficient at creating downforce.

The data from testing other tops shows that the 9LR wing changes the coefficient of drag by about .03 across all tops, from open top to fastback. By contrast, the double wing changes the Cd by .10. Yowza, that’s a lot.

One final calculation is the front aero load distribution percentage, which gives you an idea of how much the car will understeer (a low percentage) or oversteer (a high percentage). The low values here indicate that with either wing, the car would understeer badly. This is partially due to the negative rake of the chassis and negative splitter angle (both setup mistakes that should have been corrected before testing). However, even with these setup details corrected, the eBay wing would produce a car that understeers more.

As usual, let’s see what happens in OptimumLap. To spice things up, I’ll also add the data from the 9LR wing with an open top.

9LR vs eBay	Watkins Glen	Waterford Hills	2010 SCCA Nationals
Hard top, 9LR	2:22.96	1:19.95	1:03.34
Hard top, eBay	2:25.71	1:20.99	1:03.81
Open top, 9LR	2:24.02	1:20.97	1:03.88

The 9 Lives Big Wang outperforms the cheap eBay wing in every way. In fact, the 9LR wing with an open top out performs the eBay wing with an OEM hard top on any track that isn’t autocross. 9 Lives Racing is a small, made-in-the-USA business with employees who race cars, and you can feel good about supporting them.

But if you’re racing in 24 Hours of Lemons on a $500 budget, you might find that a cheap wing suits your janky crap-can just fine. The wing could have performed better with more fine tuning, but it’s clearly a case of “you get what you pay for.” If you purchase this wing, you might want to take similar steps that I did to limit movement of the upper wing, optimize the convergent gap and wing angles, and get the wing about 6 feet above your roofline. It is Lemons, after all.

Wing vs no wing

For most of the test I used a 60″ 9 Lives Racing wing, but I wanted to see what would happen without it. The coefficient of drag went down by .03 for all configurations. This is rather interesting, because usually when downforce goes up, so does drag. But this wing had the same drag in all configurations.

When I removed the wing, total downforce took a nosedive, and for the first time in the test, the car generated lift instead of downforce. This was an appropriate time to test the hardtops and see how they did without a wing.

First we threw the Chop Top back on and did a run. Then we attached the rear section, making it into a fastback again. And finally we tested the OEM hardtop. Unfortunately the OEM hard top still had the vortex generators attached, so I’ve made an educated guess on the Total Cl and Cd values below (these are in italics in the table below).

Configuration	Front Cl	Rear Cl	Total Cl	Cd	L/D %	HP @ 100mph
Hard top, VGs, no splitter, with wing	0.18	-0.64	-0.47	0.53	0.88	58.75
Hard top, VGs, no splitter, no wing	0.22	0.38	0.59	0.49	-1.20	54.53
Chop top, no splitter, no wing	0.11	0.20	0.31	0.48	-0.64	52.93
Fastback, no splitter, no wing	0.16	0.18	0.35	0.38	-0.80	42.00
OEM hard top, no splitter, no wing			.50	.45

What’s surprising here is that without a wing, the chop top has the best L/D ratio. This is largely because it creates the least lift. Remember that negative lift values are what we’re looking for (downforce), and the chop top’s .31 Cl has the least lift. The fastback creates more lift than the chop top, but it does so with less drag, and in the end, this is makes a faster car.

Unfortunately we didn’t get data for a bare OEM hardtop (without VGs, wing, or splitter), so we don’t know if it’s the shape of the OEM roof, or the VGs that create so much lift. But the total Cl value of 0.59 is quite a bit worse than either the chop top or fastback. Based on the data we obtained doing the wing tests, a bare OEM hard top should have a CD of about .45. It’s hard to imagine the vortex generators adding more than 10% lift, and that would put the total lift around .50.

Let’s see what happens in OptimumLap when we remove the wing.

Wing or no	Watkins Glen	Waterford Hills	2010 SCCA Nationals
VGs, no splitter, 9LR wing	2:25.65	1:21.12	1:03.89
Hard top, VGs, no splitter, no wing	2:29.14	1:23.14	1:04.91

Yikes, that sucks! The wing is worth 3.5 seconds at WGI and even 2 seconds at a short track like Wateford? Amazing.

Now let’s just compare the different tops without wings.

Tops without wings	Watkins Glen	Waterford Hills	2010 SCCA Nationals
Hard top, VGs, no splitter, no wing	2:29.14	1:23.14	1:04.91
Chop top, no splitter, no wing	2:27.74	1:22.85	1:04.63
Fastback, no splitter, no wing	2:26.51	1:22.42	1:04.63
OEM hard top, no splitter, *no wing*	2:28.17	1:22.87	1:04.80

Here we can see that the fastback is still the fastest configuration, but it’s not a huge difference unless you’re at a high-speed track like WGI. Without a wing, I’d be happy to use a Chop Top at most tracks for the light weight and convenience of strapping in the driver and accessing things like a cool suit, radios, cameras, etc., in the cockpit. And on performance, the Chop Top beats the OEM hardtop, even without factoring in the 38 pound weight difference.

One thing that’s conclusive here is that if the rules allow it, use a wing. This probably even applies to classes like NASA ST6/TT6 that carry a substantial penalty for running a wing.

Airdam and splitter

For most of the test we used a 4″ splitter. This was bolted to a flat undertray, flush with the airdam. I wanted to see what removing the 4″ splitter extension would do. We expected a loss in downforce, but I wasn’t sure if drag would go up or down. You see it both ways online, with CFD data showing that a splitter reduces drag, and the occasional internet expert claiming that drag goes up.

Splitters	Front Cl	Rear Cl	Total Cl	Cd	L/D %	HP @ 100mph
VGs, splitter, wing	-0.20	-0.61	-0.82	0.52	1.57	57.58
VGs, no splitter, wing	0.18	-0.64	-0.47	0.53	0.88	58.75

Score one for the CFD team, the splitter reduced drag slightly. When I removed it, the drag went up from .52. To .53. More importantly, we lost a lot of front-end downforce. Our raw data showed a loss of 69 lbs on the back straight, which calculated to a .38 delta in front coefficient of lift. Let’s see what that’s like in OptimumLap.

Splitters	Watkins Glen	Waterford Hills	2010 SCCA Nationals
VGs, splitter, wing	2:24.32	1:20.42	1:03.54
VGs, no splitter, wing	2:25.65	1:21.12	1:03.89

Obviously, if you’re running just an airdam, and the rules allow it, add the splitter. It’s significant. It’s also worth noting that this was just a plain splitter with no rear curvature or splitter diffusers. Knowing what I know today, the splitter would be twice as effective.

Endurance racing simulations

For endurance racing it’s important to know the amount of energy used per lap, because that determines how far you can go on a tank. You can get this data from OptimumLap simulations. From the energy used, you can determine the length of each driver’s stint, as well as how many laps they can complete in each stint. This can be very important for pit strategy, especially in longer races. Note that simulations are exactly that, and aren’t intended to be exact. But they are useful for making direct comparisons.

The configurations I used for the simulations follow this key. Note that the “B” group has less aero: it uses an airdam, but not a splitter, and no wing.

1a – Open top, wing, airdam, splitter
2a – Chop top, wing, airdam, splitter
2b – Chop Top, no wing, airdam, no splitter
3a – OEM hard top, wing, airdam, splitter
3b – OEM hard top, no wing, airdam, no splitter
4a – Fastback, wing, airdam, splitter
4b – Fastback, no wing, airdam, no splitter

In the table below, the Energy value in the table comes straight from Optimum Lap, and I’ve simply taken a value of 16,500 energy units divided by the energy used to get the number of hours per tank. The Miatas I’ve raced have burned about 7 gallons per hour, so these values aren’t far off.

Config	Cl	Cd	WGI Lap	Energy	Hours per tank	Pit stops	Laps in 8 hours
1a	-0.43	0.43	144.02	9087.53	1.82	4	174.98
2a	-0.53	0.45	144.04	9141.08	1.81	4	174.95
2b	0.31	0.48	147.74	9183.12	1.80	4	170.57
3a	-1.01	0.48	142.96	9223.37	1.79	4	176.27
3b	0.5	0.45	148.17	9078.14	1.82	4	170.07
4a	-1.2	0.41	141.06	9031.23	1.83	4	178.65
4b	0.35	0.38	146.51	8911.28	1.85	3	174.05

Take a look at the first two, this is the open top vs Chop Top, and it’s interesting that they turn almost exactly the same number of laps. The OEM hard top beats those by a lap and change.

But the fastback configuration 4a is the clear winner here, turning 178 laps, two more than the OEM hardtop with the same aero. Make this into a 24 hour race and the fastback wins by seven laps. Notice the Energy column, the fastback with wing is not only turning the fastest laps, but it’s using less gas than any other configuration, save the fastback without the wing.

In the non-aero B-group, the fastback (4b) wins by three laps over its wingless brothers. This is partially because the fastback can take one less pit stop. If I re-run the data with a larger gas tank so that all configurations have the same number of pit stops, then the fastback wins by only two laps.

But once you add a wing, the race is over. In fact the worst combination with a wing (open top) beats the best performing top without a wing (fastback) by a full lap, even though the fastback does one less pit stop.

If you want to check my math, I have a spreadsheet with these values, and for simplicity, I’ve removed a lot of the variables in this table. You have to keep track of things like yellow flags and time taken for each pit stop, which determines the actual driving time per race. The 8-hour race I’ve simulated uses 420 minutes of racing time instead of 480 minutes. Yellow flags at Watkins Glen take longer, and I’ve also subtracted 5 minutes per pit stop.

For shits and grins, the eBay wing again

Let’s see what happens when we use the airdam, splitter, OEM hardtop (without the vortex generators) and the cheap eBay wing. This is configuration 3c in the table below. Look above and compare.

Config	Cl	Cd	WGI Lap	Energy	Hours per tank	Pit stops	Laps in 8 hours
3c	.56	.55	145.71	9393.39	1.76	4	172.95

The eBay wing (3c) loses to every configuration that uses the 9 Lives Racing wing. Yup, even the open top car with a 9LR wing is going to be the hardtop with a cheap wing. However, the eBay wing beats OEM hardtop without a wing (3b) in both a sprint race or an endurance race. So it’s worthwhile running a cheap wing if you have nothing at all.

The biggest difference is the Energy field. Compared to the lowest drag version (4b), the eBay wing uses 5% more gas on every lap. That may or may not be a consequence, depending on how long you’re in the car.

It wouldn’t matter in a sprint race; the eBay wing (3c) would win against the fastback without a wing (4b). But in an endurance race, it would depend on the tank size and driving stint time. If I change the data so that they all take the same number of pit stops, then the eBay wing wins. If I leave the gas tank size as it is, then the fastback without a wing wins.

Wind tunnel testing

I wrote an article called The Dunning-Kruger of Car Aerodynamics, wherein I examine the steps most people go through on their aerodynamic journey. It was lucky that I met Jeremiah online and was able to do these real-world tests, and jump right past the pit of despair.

I’m sort of backsliding on the D-K chart by doing wind tunnel testing now, and who knows, I may slide further backwards towards CFD. But in the future, my ‘Busa swapped fastback Miata, Falconet, will have a full onboard system with strain gauges, pitot tube, and all the works so that I can do the same kind of testing I started with.

Back when I wrote this article, I hadn’t done any wind tunnel testing. Since that time, I’ve tested a Miata in the wind tunnel with the full 9 Lives Racing medium downforce kit, as well as everything in their product catalog. In all, I tested a lot of things:

Splitter diffusers, spill boards, and tire spats.
Canards in various locations and combinations.
Closed windows versus open, plus modifications to reduce drag and turbulence from the open windows, including wickers, mirrors, and venting the rear window in two different locations.
Singular hood vents fender vents.
Brake ducts, NACA ducts.
OEM hardtop with and without a rear window spoiler, versus a CCP fastback.
Blackbird Fabworx spoiler at different angles/heights.
Wings from 9 Lives Racing, Wing Logic, and a couple prototypes.

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