In a previous article I covered Miata fender vents in some detail, including commercially available louvers and DIY solutions. I cited Race Louvers wind tunnel data, which found that after a certain size, more vents don’t equal more downforce. What I didn’t mention was the reason for those diminishing returns.
All of this venting is overkill unless you can reach further inside the wheel well.
Take a look at a Miata unibody with the fenders removed, and you can see the problem. At the top of the fender, vents can reach just about the width of the tire, but the air has to travel all the way up there in order to be extracted. A more direct route is behind the tire, but the chassis is in the way! Moreover, air needs to take a 90-degree turn to make its way out of the wheel well. On a Miata, there’s a sharp lip here where two seams come together, which makes it even harder for air to turn the corner.
Naked Miata shows the crux of the problem.
In order to extract more air, the vents need to reach further inside the wheel well. Ideally the vent should be located low and behind the tire, because that’s the most direct exit route from the trailing edge of the splitter. And while doing that, you’d also design a much smoother exit. The end result would be quite different than the haphazard dumping and venting of air into the wheel well, and more of a purposeful and free-flowing duct from splitter diffuser to behind the wheel. Now these aren’t new ideas; you can see all of that on a McLaren M8 from half a century ago.
McLaren M8 had big vents and smooth exits back in 1971.
A more subtle solution can be seen on the underside of current NASCAR stock cars. Notice the air channels behind the tire that exit at the front of the rocker panel. This allows the cars to look stock-ish, but they perform much better and make a lot of downforce. In fact too much downforce, because NASCAR had to reduce the effectiveness by adding splitter stuffers.
NASCAR underbody aero was so effective they required splitter stuffers (the black horizontal bars in the splitter diffusers) to reduce front downforce.
I don’t know if these vents have a proper name, but I’m going to call them rocker vents based on their location in the rocker panels. They are a very Occam’s Razor solution to extracting air from under the splitter; of the myriad ways you might solve the problem, rocker vents are the simplest. Well, at least from the perspective of air, it’s the most direct route.
I wanted to implement something like the NASCAR rocker vents on Falconet, but hadn’t seen anyone else modify their Miata in such a manner. AJ Hartman did something like this on his Mustang, and said it was very efficient, with a L/D of 14:1.
So I knew it could be done, I just didn’t know how to do this on a Miata. I did a lot of exploratory cutting on the driver’s side, and the pictures show how I did a much cleaner job on the passenger side.
I want to thank Aussie reader/supporter Jim Grant for assistance with this project. He recognized I had no idea what I was doing and explained how cars are put together, and how to get my head out of my ass. Thanks Jim!
Step 1: Measure
My first decision was how large (height and depth) to make the vent. There are a few considerations for height, see the following image with colored lines.
Three logical places to cut.
The white line is the most conservative cut, because it goes just below the curved wheel tub inside the wheel well. Cutting at this height leaves two threaded holes intact, in case you want to add bolt-on fender braces.
The yellow line is about where the mid-body style line is on a NA Miata. Aesthetically, this is probably the best place to cut a NA Miata, but on a NB it wouldn’t matter. This cut goes between the two bolt holes, and so you could still use the upper hole to mount something.
The red line is to the top of the wheel tub. You could go higher than the red line, but most of the air you’re trying to extract is below the splitter, so I’m not sure if a higher cut helps that much. Might look cool tho.
Bring that rocker vent all the way to the top and it might look like a RX-7 GTO. Fucking badass.
In retrospect, if I did this again, I’d cut on the yellow line. Mostly because I think it would look better to have the duct exit on the style line. I don’t know that there’s a lot more air to extract at that height, and it could be that a vent even half this high is just fine (the NASCAR vents are quite low). In any case, I used the white line.
The next decision is how deep to make the cut. I measured and marked a line 9.5” from the inside of the wheel tub. This is where the inner sheet metal is located within the footwell, and so the cockpit remains unmolested.
I removed the white crosshatch area, it starts about 9.5” from the inner wheel well.
The rocker vent would be more effective if I had cut further inside behind the wheels, but then I’d need to weld sheet metal inside the cabin. Going further than that, there’s other unibody structure and cage tubing that would be difficult to work around. If I’d kept going deeper, I’d hit the clutch dead pedal, which could be sacrificed I suppose, but I do use it.
All said, I chose to make the rocker vent only about 3” (75mm) deep. This isn’t a lot of extra volume, but it does allow air to exit at less of an angle.
I used a sharpie to draw a line back to the hinge at my desired height. I’m not going to use the lower hinge (my doors are only half height), but there’s a vertical chassis member here and I wanted to keep that intact. So at this point, I have defined the area I want to remove.
Take a minute to pause and question if you’re really going to do this.
Step 2: Surgery
I used a grinder with a cutoff wheel to cut along the lines. There’s a horizontal shelf inside that has to be removed. It’s spot welded in various places (you can see the dots) and I tried drilling those out and in some cases getting in between with a cold chisel and breaking the welds. This little shelf is a pain in the ass.
Cut an opening so that you can remove the shelf.
I continued to cut out all of the sheet metal and spot welds until I had a rectangular-ish hole like this.
The point of no return. Or, what the fuck have I done?
The interior footwell and exterior wheel tub are two pieces of sheet metal with an air gap between them. I used a cutting wheel to make a slit in this area, and then used a reciprocating saw to cut out the sheet metal. I ended up with a cavity or “smile” between the interior footwell and the fender well.
Cut out all the sheet metal between the footwell and the wheel tub.
To get a smoother exit for air, I used a cut off wheel to cut several straight lines through the bottom sheet metal. I then bent the sheet metal upwards to close that gap.
Slits make it easy to bend the bottom upwards.
I used a hammer to tap these into a more graceful arc.
I should have cleaned off all that gunk with a wire wheel before starting. This needs to be bare metal for welding.
I then cut the front of the rocker panel off at an angle, so that air can make less than a 90-degree exit. Notice the rocker panel is made of a few layers of sheet metal, and plays a significant role in chassis structure and stiffness.
Front of rocker panel cut at an angle.
Next I’ll weld in some sheet metal to cover all this ugliness, and provide a smooth path for air to flow.
Step 3: Welding
Welding Mazda sheet metal kinda sucks. I was once a certified structural welder, but you’d never know looking at the shit job I did on this. I got the best results by spot welding pieces together, and then joining some of those to create stitches. Note that continuous welds are unnecessary, as you can fill the gaps with seam sealer.
First step was to bend in a piece of sheet metal to make a smooth curve, and then cut it to size.
Nice curve.
Next I welded in a horizontal flat that makes the ceiling part of the vent. Otherwise there would be an open cavity that might collect gunk inside. I welded that to the curve I had cut.
The ceiling of the vent.
Next I welded the tabs I cut in the footwell area, that I had bent upwards with a hammer. I ran a cutoff wheel into the overlapping pieces of metal and pulled the little triangles out, then smashed it all flat again. This way the seems were pretty tight and easy to weld inside and out.
Bend tabs up, close the gaps, and weld.
Now the fun part, bending sheet metal into curves and tacking it into place. I used aviation shears to cut sheet metal into various shapes, and then tack welded it all together. This process is more art than science, and satisfying work.
Small pieces of sheet metal can be bent and spot welded to make easy curves.
The next step was to add external bracing to the shotgun panel (the frame member that connects the door hinge area to the shock tower). There are several commercially available fender braces, but I chose to DIY my own and weld it on rather than use bolts.
DIY fender brace and welding spots.
I used 1” square tubing and 1/8” steel spreader plates to make a triangular brace. Weight weenie that I am, I was pleased they only weighed 3 lbs apiece. I then welded the braces in several spots, choosing places where the sheet metal overlapped and was doubled in thickness. I also seam welded the entire front of the chassis for good measure.
Front view shows the 3” (75mm) of extra depth cut inside the wheel well, and rounding underneath and at the exit.
I ordered a new set of fenders, which I’ll cut artfully to expose the rocker vents. In the meantime, I took my existing fenders, which have a large fender cut, and mounted them up to see what this all looks like.
When looking at the rocker vent from the rear, you can see that air will exit much lower and smoother behind the wheel. No more 90- degree bend or having the air exit between the chassis and the quarter panel.
Big vent with a smooth exit will help the splitter make more downforce, and should cool the brakes as well.
Step 4: Finish
The final steps are to fill the gaps between the spot welds with seam sealer (automotive caulk). I have also heard that you can fill the rocker panel and front cavities with expanding foam, which supposedly adds more rigidity. I don’t know that expanding foam from a can (Great Stuff) is appropriate for this, but it would certainly be easy to do.
I’m not going to cover the rocker vents with quarter panels; the area will be entirely exposed. So it needs Bondo, primer, and paint to match the bodywork. I haven’t done all that yet, but it’s just regular bodywork, and I’m the last person you want to watch do that.
When that’s done, I’ll also add some vanes under the car. I’ll take my inspiration from NASCAR again, and guide the air out the new rocker vents with a pair of strakes.
Wind tunnel test?
I’ll add strakes like these red ones.
To find out how much downforce and drag the rocker vents make, I’ll fabricate rigid covers that approximate the shape of the original bodywork. I’ll then remove the covers and A/B test the vents back to back in the wind tunnel.
The A2 wind tunnel has a static floor, and so it’s not as accurate as one with a vacuum to remove the boundary layer, or a rolling floor. So while the numbers won’t be 100% accurate, there will be a useful delta value, and a way to compare the rocker vents to other vents.
Caveats aside, I’ll update this article with those results sometime after 6/20.
Now it’s your turn
If I’ve inspired you to make rocker vents, consider the following:
First, do you really need rocker vents? A splitter alone makes enough downforce to offset a low-angle single-element wing. Add splitter diffusers and vent the fenders, and you may get another 50% more downforce. Add spats, side plates and canards, and you could double the original splitter’s downforce. At this point you’ll almost certainly need maximum wing angle and a Gurney flap, and even then the front aero load distribution may be too high (oversteer in fast corners).
But if you’ve already done all the tricks and still need more front downforce, then the rocker vent is perhaps the next step. But you’ll probably need to add some combination of spoiler, second wing element, and rear diffuser.
On the other hand, maybe you’re not after maximum front downforce and are simply after better efficiency, or maximizing the effectiveness of the undertray. Or perhaps you’re dodging the points taken for using a splitter, but you want similar downforce. That’s a pretty clever use of rocker vents, and I would definitely get on board with any of that.
Next question, do you have a full cage? You’re removing important structure from the unibody and a full cage is arguably a requirement for rocker vents. Without a cage, you’ll need fender braces or other supports that help support the shock tower (no, not a strut tower brace).
As a practical matter, most commercially available Miata fender braces use the two threaded bolt holes in the wheel tub for mounting, and the location of those bolts limits the size of the rocker vent. If you recall the previous image with the horizontal lines, you’ll want to cut on the white line, or lower.
If you make rocker vents, you’ll need a full cage and/or fender braces. Some bolt-in solutions shown.
Final question, if you’re racing, do your rules allow modifying the unibody structure? For any Spec racing series, that answer is certainly no. For other series, it will depend on how they evaluate such modifications.
I’m building Falconet to the NASA ST/TT 3 rules, and changes to the unibody require the “non-production chassis” mod, which is a .4 points penalty to the lbs/hp calculation. If I was building to the NASA ST/TT 4-6 rules, then this modification would be illegal, as the rules state you can’t alter the unibody.
My car also fits into SCCA Time Trials Unlimited 2 class, and this would be legal. But not in any of the Max classes (in fact Falconet isn’t legal in Max for other reasons as well).
I didn’t look up the SCCA road racing rules because it’s a 1000 page rule book. Nor did I look at the 400 page autocross rules, because this is an aero mod, and unlikely to help much at 40 mph. But there’s probably some flavor of unlimited class my car would fit into if I wanted to roadrace with the SCCA or dodge cones in a parking lot. (I don’t.)
Most of the Grid Life Trackbattle classes state that you can’t make modifications to the chassis, and that includes GLTC. But you should be able to get away with rocker vents in Street Mod, Track Mod and the Unlimited classes.
Most endurance racing rules would allow rocker vents, and with their high efficiency, it would be a good idea for AER, 24 Hours of Lemons, Lucky Dog, Northeast GT and WRL. Champcar would assign material points (2 points per square foot of metal), and that might be 4 points total. But a clever team could reuse sheet metal taken from various weight loss trimmings and/or use the lower quarter panels and do this mod for free.
If you aren’t racing or tracking your car, one could argue that rocker vents are a lot of work for Racing Inspired Cosmetic Enhancement (RICE). But if that’s the way you roll, you’ll one up everyone else’s fender vents at the local cars and coffee.
All told, this project cost me maybe $40 bucks in materials (8 feet of 1” steel tubing, welding wire and gas), so if you have more time than money, I say why the fuck not? So now it’s your turn. Let’s see some rocker vents!
If you’d like your car featured in Readers’ Rides, see the information on this page and then let’s get the ball rolling!
The first Readers’ Rides feature is Dylan Pudiak’s 1991 Miata. I’ve watched his car transform from average, to exceptional, back to average, and then to its current status as the northstar build for all normally aspirated BP engines. What began as an engine failure turned into a journey of epic proportions. It didn’t happen over night.
Dylan’s car build is documented in 24 episodes on the Napp Motorsports channel.
A Hero’s Journey
If you don’t know what the Hero’s Journey is, it’s a template for storytelling where a person goes on an adventure, is victorious, and comes home transformed, with knowledge to bestow upon others. Think of stories like Star Wars or The Hobbit. Dylan’s journey follows this template closely, so I thought I’d tell his story like this:
A hero ventures forth from the world of common day into a region of supernatural wonder: fabulous forces are there encountered and a decisive victory is won: the hero comes back from this mysterious adventure with the power to bestow boons on his fellow man.
The Call to Adventure
Our story begins with a Call to Adventure. For Dylan and many other driving enthusiasts, that call was autocross. But on his way home from dodging cones, our hero encountered Supernatural aid in the form of bad luck. Or as many early Miata owners know it, SNC Life.
The World is Yours! Right up until you shear the crank bolt.
When the Miata was released in 1989, the engine had a short-nosed crank (SNC) that could eventually wobble or shear the crank keyway. Half way through 1991, Mazda released long-nose crank engines to fix this problem. Fortunately for us, Dylan had the earlier 1991 Miata, and while driving home from his call to adventure, the crank bolt ejected itself.
The common solution to this problem is to find a later 1.6 engine with a long-nose crank and replace the whole mill. But that would be the end of the story, and a pretty boring one at that. Instead, Dylan recognized that the Threshold Guardians to a faster autocross time were the lack of power from just 1597cc and an open diff with a fragile ring gear. Luckily, these things can be easily remedied with parts from later 1.8 Miatas.
The threshold: from known to unknown
A significant part of the Hero’s Journey is the journey into the unknown, and for Dylan, this begins with swapping in a 1.8 (BP05, 1994-1997). Most people would have chosen a later model VVT engine, but a standard 1.8 was the easy button, because he could keep the 1.6 AFM and ECU, and do an exhintake cam swap as well.
The exhintake cam swap is where you take an exhaust cam, grind off one end, install it on the intake side, and re-time it. The exhaust cam doesn’t have any more lift than the intake cam, but it has more duration. When properly tuned, this can add up to 8 hp, which puts the engine close to what a VVT makes.
For simplicity, Dylan kept the ECU from the 1.6, as well as the air-fuel meter (AFM). The AFM is a flapper type valve that doesn’t flow enough for anything more than a stock motor. People with modified 1.6s often use a RX7 AFM, which instantly gives more power on the top end.
1.8 engine… but what’s that thing in the corner?
I don’t believe I’ve seen anyone swap in a 1.8 engine and keep the 16 AFM; most people splice the wiring to run a 1.8 ECU and much less restrictive MAF sensor, or better yet, go to a standalone ECU. The path less chosen probably cost Dylan a few ponies, and with this combination the car put out 102 hp and 100 ft-lbs on a Land and Sea dyno. These dynos read lower than Dynojets and after correcting by 112%, that’s something like 114 hp at the wheels. This is respectable, but if he’d done all of that with a Megasquirt, he’d be looking at something in the mid 120s.
Dylan lapping his 1.6/1.8 mutt at Toronto Motorsports Park
To put the power to the ground more evenly, Dylan had to address the other Threshold Guardian, the open diff and 6″ ring gear that can break on stock power. He found a 7″ ring gear and Torsen LSD locally from Stefan Napp, which brings us to the other important character in this story.
Han Solo, Gandalf, and Goldilocks
In Dylan’s Hero’s Journey, Stefan Napp plays several important roles, and is simultaneously Helper and Mentor. He’s like Gandalf, Obi-Wan Kenobi, Samwise Gamgee and Han Solo all wrapped into one. Later, he’s also Darth Vader and a drug-pedaling pimp, but that’s getting ahead of the story.
With Stefan’s guidance, Dylan swapped his 1.6/1.8 mongrel, to a junkyard VVT, and then threw the Napp-recipe at it:
150 whp, 127 ft/lbs on 93 octane, as measured on a Land and Sea dyno
All of those modifications resulted in 150 whp on Rick Gifford’s “heartbreaker” Land and Sea dyno. On a Dynojet, that’s probably in the neighborhood of 165 whp. It’s worth pausing a moment to mention Rick, because he’s a Gandalf-level engine tuner with a dyno at his house. He serves as another Mentor character in pretty much all the Dylan and Stefan stories. (Rick also tuned by 1.6 Miata).
So let’s call it around 165 (Dynojet) horsepowers at the wheels. Sounds perfect doesn’t it?
The Napp recipe for Goldilocks.
In fact it was perfect. I drove Dylan’s car at Pineview Run, and it was the Goldilocks of Miata motors: not too much power, not too little, just right. The power felt ideal for the chassis, retaining all of the characteristics of a momentum car, yet providing instant tractable power, and enough oomph to occasionally surprise.
I still don’t know that there’s a more satisfying combination of engine and chassis, it was magical. Stefan’s K24 Miata was there the same day, and I honesty liked Dylan’s more. That’s really saying something.
On a personal note, driving Dylan’s car at Pineview was also the moment I realized I was a complete dumbass. At the time I was being an iconoclast by trying to tune a 1.6 engine, and I thought cams and bolt ons could help my B6-ZE run with BPs. Three corners into driving Dylan’s car I understood exactly what a huge mistake I’d made.
From Goldilocks to Jenna Jameson
Speed is a drug. Napp is a dealer. Dylan saw the huge gains Stefan made, and was naturally tempted to take a bigger hit. Too much is never enough.
So the next stage in Dylan’s journey was to take Goldilocks and hook her on smack, then turn her out onto the street as a common hooker. OK, maybe that’s a little harsh, it’s just a turbo we’re talking about; but it sucks and blows just the same.
Modifications at this stage included the following:
Garret gt2560r turbo
Flyin miata exhaust manifold
Ported turbo exhaust housing
Flyin miata oil cooler
ATI Fluid damper
R8 Ignition Coil Kit
252 whp, 221ft/lbs @14psi (same dyno)
I don’t pay too much attention to turbo Miatas, but it seems like ones putting out mid 200s at the wheels are a dime a fucking dozen. After all the work they’d done to get Goldilocks to dance like a ballerina on swan lake, it seems terribly underwhelming to wind up with a common skank twirling on a brass pole.
Hot. Also a lot of heat.
I didn’t get a chance to drive Dylans turbo at Pineview, but it wouldn’t surprise me if it was slower on that track. But I did get to drive Dylan’s car on the street on the way back from Watkins Glen, and I will say the acceleration was exhilarating. So fucking what.
Part 1 of a 10-part build mistake series.
Transformation and atonement
The next stage in the Hero’s Journey is the revelation that brings our protagonist out of the abyss, transforms him, and sets him on the path to atonement.
What set that off was typical turbo problems: heat. With rose-tinted beer goggles, Dylan looked back on the simplicity, reliability, and ease of driving that Goldilocks NA. And thus the revelation to go back to normally aspirated power. And then go completely overboard, boring the engine to the maximum displacement. And then add cams, shim-under bucket lifters, and ITBs. It has, well, everything:
85.5mm Supertech pistons 11:1 (2L displacement)
Manley H Beam rods with ARP main and rod bolts
ACL Race main and rod bearings
Boundary oil pump
+1mm intake and exhaust valves
Supertech dual valve springs with titanium retainers
The end result still needs final tuning, but it’s likely 200 whp on a Land and Sea (heartbreaker) dyno, and well over 200 on a Dynojet. This is about as far as you can go with a Mazda BP engine, and I want to personally thank Dylan and Stefan for reaching the limit.
Suspension, wheels, brakes
You don’t do all of that work and roll around on OE anything. Dylan did it right yet again:
Ohlins road and track coilovers, corner balanced
Custom extended top hats
Racing Beat front and rear swaybars
Paco strong arm fender braces
4.1 Torsen diff
V8 roadster hubs
Full polybushing kit
Frame rails
1.8 brakes
Hawk HP+ panda
15×9 Konig Countergrams
RS4s 225/45
Chasing the dragon.
Aero
I’m an aero guy, so let’s take a look at how the air moves over and around the car. Starting at the front, Dylan has a SPS air dam, AG Limited front lip and custom undertray. The top is a standard Miata hardtop, but has a spoiler extension on the roofline. For rear aero, Dylan has two trunks, one with an Ikon Style duckbill spoiler, and another with a APR GTC200 wing.
’90s perfection
Judged against modern aero like a 9 Lives Racing medium aero kit, Dylan’s aero isn’t quite up to snuff, and probably has a little more drag and little less downforce. But I honestly wouldn’t change a thing! This is 1990s perfection. Visually, this is the pinnacle of Radwood-era aero, and “fixing” this would be as sacrilegious as fixing Jennifer Connelly’s overbite. For a dual duty car that sees more street than track, changing the aero would be pointless.
You’d grin like that, too.
It’s amazing to look back at this Hero’s Journey and appreciate the Gift of the Goddess. Thank you Dylan Pudiak and thank you Stefan Napp, for showing us the way.
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).
I will not be testing these canards.
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.
Laminating a splitter with a full width diffuser on the trailing edge.
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?
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 with $20K worth of computers. This article is essentially Part 2 of that one, so I can deep dive on the test results of the various aero options.
Summary data.
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.
AirTab vortex generators.
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.
Conclusions
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.
In 2024 I’ll do a full writeup of how to install all the parts and hook it up to an Aim dash (ahem, with a lot of help from Matt Romanowski), but this should bring me full circle back to a system akin to what I started with. With those tech gizmos I’ll be able to get real-world data on downforce and drag whenever I want, essentially making any race track into my own private wind tunnel. Fuck yeah.
My twin brother wrote an article on You Suck at Racing about tire grip, and I’m going to steal some of that content to explore how tire grip and aero are related.
Braking, cornering, accelerating: everything depends on grip. Understanding how rubber tires create and lose grip is therefore fundamental. Let’s start with some theoretical laws of friction.
Amonton’s First Law: The force of friction is directly proportional to the applied load. If this law is true, then a 4000 lb car should stop in the same distance as a 2000 lb car. It weighs twice as much, but it also experiences twice as much friction, so theoretically, the weight of the car doesn’t matter.
Amonton’s Second Law: The force of friction is independent of the area of contact. This means that it doesn’t matter how wide your tires are. Skinny or fat, they have the same amount of grip. And grooves wouldn’t matter either.
Coulomb’s Law of Friction: Friction is independent of velocity. Which means you should have the same amount of grip at all speeds.
Finally, static friction is always greater than kinetic friction.
You might not believe these laws, because you’ve experienced that tires don’t really follow these laws of friction.
If weight doesn’t matter, then why do lightweight cars like Miatas out-handle bigger cars?
If tire width doesn’t matter, then why are wider tires faster? And by the same logic, given the same amount of rubber area, why are slicks faster than a tire with grooves?
If static friction is always greater than sliding friction, why is it faster to have slip angle through a corner?
Four important graphs
In order to understand how tires work, you have to understand the following four graphs. Introductory physics assumes that the coefficient of friction, Mu (μ), is a constant, and that may be true for a block sliding against a table top, but when it comes to tires, μ is not a constant.
Tires generate grip from molecular adhesion, mechanical keying, and hysteresis, and those factors are based on a combination of variables. In each of the graphs below, the coefficient of friction, Mu (μ), changes due to load, temperature, speed, and slip angle:
Graph A shows μ as a function of load (weight). If doubling weight doubled grip, then the line would be flat. But when you double the amount of weight on a tire, there are diminishing returns. When cornering heavily, the outside tires experience more load, and because of that, heavier cars lose more grip than light cars.
Graph B shows μ as a function of temperature. Every tire has an optimum temperature. Both cold and hot tires have less grip than one in the optimal range. If your tires are too wide, they may never get up to optimal temperature, and a narrower tire may heat up more favorably. For this reason, wider isn’t always better.
Tire Rack did a great test where they tested a bunch of wheel and tire widths. The fastest tire wasn’t the widest. And when they went to a wet track, the fastest lap was the narrowest tire.
One thing that contributes to heat is grooves. Squirming tread blocks are a major source of heat. As a result, grooved tires heat up more quickly than slicks. One reason for using slick tires is to spread the load better, but an even more important one is to prevent the rubber from overheating.
Graph C shows μ as a function of speed. The faster the car goes, the less time there is for keying; the ability for rubber to change shape and interact with the road. Under wet conditions, where adhesion no longer applies, grip is highly affected by speed.
Graph D shows μ as a function of slip angle. Every tire has an optimal slip angle. When a tire is twisted, which it always is to some degree, some parts of the contact patch are experiencing static friction while others are kinetic. This mixed state isn’t really addressed by any of the laws of friction, but it doesn’t make this any less real.
Ian made a drawing of what is happening between the road and the surface of your tire, which can help you further understand how tires grip.
Visualizing grip
The following image shows the surface of the road as a jagged green line on the bottom. (If the road surface was perfectly smooth, then the line would be horizontal. But because asphalt has imperfections with peaks and valleys, the road surface is represented as a jagged line.)
Panel A represents a tire (squiggly line) at rest, pressing into the surface of the road (jagged line).
Panel B is what happens when you add load: the rubber goes deeper into the surface, creating more grip. But there’s only so far you can push the rubber in. This is why doubling the load on a tire doesn’t double its grip. Panel B could also be softer rubber or hotter rubber. In both cases, the rubber conforms more easily to the surface, and with more contact, you get more grip.
Panel C shows what happens at high speed. The rubber doesn’t have as much time to change shape, so it doesn’t deliver as much grip from keying.
Panel D shows what happens when a tire overheats. The rubber comes apart, providing less contact with the surface. If the rubber gets hot enough, it may liquify or sublimate, creating a slippery layer between the surfaces.
This visual model isn’t perfect, as it doesn’t give why slip angle matters so much. But hopefully it helps you visualize the interface between your tires and the road, and why some factors add grip, and other factors take it away.
Aero and tires
So that’s how rubber grips the road, but there are other dynamic factors at play here, namely the aerodynamics of the vehicle.
Most cars without aero lift at speed, because the curved surface of the top of the car is longer than the bottom. In other words, cars are shaped like airplane wings, and like wings, they generate lift. The higher the air speed, the less traction there is.
Most cars have a coefficient of lift of around 0.3. Cars with a lot of curvature, like a fastback, have more lift than a three-box sedan or hatchback. By nature of their shape, most cars generate more lift over the rear tires than the front tires.
Cars also generate lift when in yaw, so if your car is pitched slightly sideways in a corner, it has even less traction. Nissan did some tests on this and found there is a fairly linear relationship between yaw angle and lift, and so the more sideways you get, the more the car lifts.
This means as you corner faster and faster, your rear tires have less and less rear grip. You already saw in graph C and panel C that tires have less grip at speed because they have less time for keying. So if you combine the keying losses with the lift and yaw losses, you get a car that’s lost a lot of grip on the rear tires.
And this is why it’s so important for race cars to have spoilers and/or wings.
Aero and lap times
To put some numbers on it, let’s take a look at a few NASA classes at Watkins Glen.
NASA Spec3 is a class for stock-bodied (no aero) E36 BMWs on Toyo RR tires measuring around 14.5 lbs/hp. The Spec3 lap record is 2:13.6.
NASA also has the ST5, class, which is a similar lbs/hp ratio to Spec3, but allows a splitter and wing. The ST5 record is 4.3 seconds faster: 2:09.27.
NASA also has a time trial class, TT5. TT5 and ST5 are the same formula, but the ST5 cars are racing wheel to wheel, whereas the TT5 cars are in a time trial situation with less traffic. Therefore, the TT5 laps are usually faster, but in this case, a surprising two seconds faster: 2:07.202.
If you compare the Spec3 lap record to ST5/TT5, can see that aero (splitter and wing) are worth about 4-6 seconds at Watkins Glen. Let’s call it 5 seconds for simplicity. This isn’t a difference in tire grip, as most cars are on Toyo RRs, but some cars are on Maxxis RC1 for the same lbs/hp (or Hoosiers at a significant penalty to lbs/hp).
Let’s take a look at where the difference is. On the front and back straights, there isn’t a huge difference in top speed, so the cars are pretty similar in lbs/hp. In Turn 7, where aero doesn’t make much difference, the minimum corner speeds (vMin) are pretty similar, and so we’re looking at cars with equal grip, as well. But take a look at Turn 10, the aero cars are going about 10 mph faster!
Lap
Front
Back
T7
T10
Spec3
2:13.6
121
126
62
87
ST5
2:09.3
118
1277
63
96
TT5
2:07.2
123
128
62
98
Now I’m making some pretty big assumptions on driver skill and track conditions being equal. So I’ll do some simulations in OptimumLap, and see if the computer world agrees with the real world.
I’ll start with the Spec3 car using drag and lift values of .44 and 0.3 which is probably in the right area. With these values I get a lap time of 2:13.82 which is pretty close to the Spec3 lap record. I’ll then add a splitter and wing to bring the Cd to .47 and Cl to -0.8 which are pretty fair values for the added aero. Doing only these aero mods on exactly the same car, I get a 2:08.84 lap, which is right in the middle of the 5-second delta we saw in the real world.
So that’s a pretty good verification of aero being worth about 5 seconds. So next I want to take a look at Turn 10 and see how much aero helps here, and if there’s really a 10 mph delta.
Turn 10 Watkins Glen
For the OptimumLap simulations, I’ll use four cars instead of two cars to get more granularity. I’ve given them the same tires with 1.2g of lateral grip, but different aero packages.
No aero – This is the Spec3 car with a coefficient of lift of 0.30, and is represented by the red line.
Zero lift – This car has some minor aero like a spoiler, which cancels out lift, and so the Cl is zero. This is represented by the orange line.
Mild aero – This is the kind of lift and drag you’d see from a spoiler and airdam done professionally (NASCAR level). This is represented by the blue line.
Good aero – This is a car with a splitter and wing, and is represented by the green line.
I’ll examine the grip in the middle of Turn 10, at the 16000′ mark from the start/finish line. This is not quite the minimum speed in the corner, but shows a high lateral load and is as good a place as any to look at G forces. There are a lot of spikes in the graph (like you’d see in Aim Solo data), so imagine it’s more of a smooth arc.
Lateral Gs in Turn 10 on the same tire.
Starting at the bottom of the graph, notice that the car represented by the red line is pulling only 1.14 Gs. Recall that I gave all the tires the same 1.2g of grip, but because of aerodynamic lift, they are losing traction at speed. This is a normal situation for a street car or spec racer with no aero. What the simulation doesn’t show is that most of the grip is lost on the rear tires.
The orange line is a car with a spoiler, which mostly cancels out lift. Lateral Gs are very close to the the static 1.2g value.
The blue line has more than 1.29g grip because tire load is increased with downforce.
The green line is even more dramatic, with 1.4G grip. This is significantly more grip than the car with no aero.
You might be wondering how an increase in lateral Gs plays out in speed through the corner, which is the next graph. I’ve chosen the same 16000′ spot on track to measure the speed, and you can see it’s a difference of almost 10 mph between fastest and slowest.
Turn 10 is a very fast corner, and you’d see a smaller delta on a slower corner, but this is still pretty remarkable. By increasing the load on the tires, tire grip went from 1.14 to 1.4 Gs, and the car with good aero went about 10 mph faster in the middle of the corner.
Downforce and tire wear
Most people imagine that aerodynamic downforce will make your tires wear out faster. More grip = more wear, right? No. Oddly, downforce makes tires last longer.
Tires wear by abrasion; from sliding or spinning. Have you ever flat spotted a tire? Then you know that sliding a tire can wear it out in a couple seconds! Aero increases the load on the tire, giving you more grip, which makes it less likely to spin or lock up.
Aerodynamic downforce also loads the tires more equally. When cornering, the outside tires get loaded more, as a normal byproduct of mechanical grip. However, aero loads are based only on air, and is balanced across the car, left to right, helping to balance the car better.
One could even imagine an active aero device that would split the wing in half and only load the inside tires. Or rudders or vanes that help the car turn using air alone, and use the tires even less.
But let’s jump back from fantasy land… in reality, you get more aerodynamic downforce from rear aero than front aero, and this helps a lot in braking zones, increasing rear load and rear grip. The same is true in acceleration, and more rear grip reduces tire wear (on rear wheel drive cars).
Another way aero increases tire life is when you drive under the limit of the tire. For example, take Turn 10 at Watkins Glen again. Miatas can usually go through at full speed or with a slight lift on entry, and that’s because there isn’t much of a straight between Turn 9 and Turn 10 and Miatas are dog slow. If you’re going through at 85 mph without aero and on the limit of traction, you’ll go through it with aero at the same speed, but well under the limit, and you’ll wear the tires less.
The uphill esses are another place where a Miata is flat out while cornering. You can’t ever reach the limit of lateral grip because the car can’t accelerate fast enough to get there. So you slip less and use less tire.
Now this isn’t going to be true at every race track, most of the time when you have a higher limit, you fuggin use all of it. But sometimes there’s a corner or two where aero now puts you under the limit, and in a long endurance race, this can be the difference between changing tires mid race, or simply saving money. The point being, aerodynamic downforce can make your tires last longer if it keeps your car from sliding.
A good real-world example is the 2000-2002 Corvette SCCA cars, which went from a 315 rear tire in 2000 to a 275 tire in 2001. To increase grip, a rear wing was added in 2001, but it wasn’t enough and tires would only last about 4 laps before starting to go off. After optimizing the aero in a wind tunnel to create more downforce, the same tires in 2002 would last an entire race.
Aerodynamic balance
At this point I’ve only looked at how aerodynamic downforce affects grip and longevity due to increased tire load. But there are other aero factors at play that are important.
Earlier in this way too long blog post, I by mentioned that cars without aero lose rear grip from lift and yaw. In truth, the car loses both front and rear grip, but it loses more rear grip. As a consequence, as the car goes faster, it transitions more and more to oversteer. Most people find this an unsettling situation.
Personally, I don’t mind if a car oversteers at low speed, in fact, I like it. But if it does that at high speed, it scares the shit out of me. Ideally, I like a car that rotates easily at low speed and then transitions to understeer at high speed. This is ridiculously easy to do: add a big wing, and then tune the amount of understeer by adding or removing wing angle.
This is the magic of dynamically balancing the load on your tires using downforce. It’s so easy, and it’s so tunable. However, rear wings and end plates also increase stability by increasing the static margin.
Static margin is the distance between the center of gravity and the center of pressure. Anything that adds rear drag increases the static margin, sort of like streamers on a kite tail. In addition, horizontal areas on the back off your car, like big end plates, shark fins, or even bodywork like a hatchback or station wagon, increases the static margin through sideways resistance to air.
A greater static margin makes the car harder to turn, but also makes the car more stable. When a car goes over the limit of grip, the driver must make steering corrections. Cars that have a higher static margin require fewer steering corrections to bring the car back into line, which is easier on the driver. This also ends up being easier on the tires, and can make the car faster, if a bit less exciting to drive.
Conclusions
Tire grip is arguably the most important factor on a race car. Understanding how tires make grip is therefore one of the most important things a racing driver needs to know. Aerodynamic downforce can greatly influence the balance and grip of the tires at different speeds, and can be used for tuning the car’s handling and ultimately make the car turn faster lap times, and/or stay on track for a longer duration.
If you’re serious about downforce, use a wing; it can generate more downforce, and is more efficient than a spoiler. It begs the question, why would anyone want a spoiler?
Spoilers are usually cheaper than wings.
Some racing rules don’t allow wings, but allow spoilers.
A small spoiler can reduce both drag and lift.
Wings are often gaudy on a street car, but spoilers almost always make a car look cool. Not only my opinion, but NASCAR fans as well.
I’m going to build an adjustable-height 70-degree spoiler so I can find out what’s ideal on a Miata. But before that it’s worth looking at the existing literature and products.
How a spoiler works
Cars are basically shaped like airfoils, and as air moves over them, it creates lift. The faster the car goes, the more lift and instability is generated. A spoiler, as the name implies, “spoils” the airflow coming over the top of the car, fooling the air into behaving as if the car has a different profile. This cancels some lift, and often reduces drag as well.
How a spoiler works.
A spoiler also concentrates high pressure air on the rear deck lid. Pressure is akin to weight, and so this adds downforce to the rear of the car.
A spoiler also moves the center of pressure rearwards, and like a streamer on a kite, this promotes stability.
Spoiler height
How high should a spoiler be? Let’s take a look at what the pundits say. In Race Car Aerodynamics, Katz shows two different graphs for spoilers. The first is based on spoiler height alone, at a fixed angle of 20 degrees from vertical, or what I’d call 70 degrees.
I’ve put some pencil marks on the graph and drawn some conclusions.
A low spoiler about 1″ tall reduces drag the most. It also adds a bit of downforce. From a drag and downforce perspective, it’s a win-win!
A 3″ spoiler doesn’t add any drag, and doubles the downforce of the low spoiler. In other words, you get something for nothing!
A taller spoiler adds downforce and drag, but downforce increases more rapidly than drag. The gift that keeps on giving!
So no matter what height spoiler you chose, it has a benefit. Based on theory alone, we should all have low spoilers on our street cars, and taller spoilers on our race cars (rules permitting).
Spoiler angle
Katz includes another graph on spoiler angle, this time using a fixed-height spoiler. Confusingly, this time the angle is measured from horizontal, not vertical, and the 70-degree angle from the previous graph isn’t included.
Some observations of this data:
Drag increases fairly linearly with angle.
Lift-drag ratio seems best at a very shallow angle, but this may simply be the low overall height of the spoiler. Also note that L/D ratio is at best 3:1, whereas a wing can be 12:1 or more, which is why you use a wing if you’re serious about downforce. (If you look at pressure plots of wings, you’ll notice they have about 3x more suction under the wing than pressure on top. Spoilers only make downforce from the top side, and so that’s why they are a lot less efficient.)
Increasing spoiler angle to 60-degrees or more increases downforce, but at a diminishing return.
Spoiler height and angle combined
Next I’ll look at my other favorite reference, Competition Car Aerodynamics. McBeath cites CFD work done on NASCAR spoilers, in which they changed both the spoiler height and angle. Now we’re getting somewhere.
I’ll use the above results to compare spoilers of different lengths and angles that result in a similar total height above the deck. Which in turn allows me to figure out the most efficient spoiler angle.
160mm spoiler, 20 degree angle, 54.7mm total height
80mm spoiler, 40 degree angle, 51.4mm total height
60mm spoiler, 60 degree angle, 52mm total height
It’s a bit difficult to see in this graph, but a 60mm spoiler set at 60-degrees is slightly better than a 160mm spoiler set at 20 degrees, even though the longer spoiler is a little bit taller. In other words, a higher angle works better. But it’s only by a small amount.
Based on Katz and McBeath, here is my simplified conclusion: The total height of the spoiler is all that matters.
NASCAR spoilers
NASCAR used rear wings for a short period of time and then switched back to spoilers. Not because they could get better performance from a spoiler, but because the series is always looking for ways to make racing both closer and safer, and the wing did neither. In addition, the fans didn’t like the look of a wing. To be fair, the CoT wing was hideous, see for yourself.
Yuck.
So we can’t look to NASCAR for the most effective spoiler design, because we know their priorities lie in close racing rather than outright speed. But it’s worth noting a few things about NASCAR spoilers.
NASCAR probably knows more about spoiler design than any other race series, and they still don’t settle on one design. In fact, the regulations change almost yearly. Looking only at the height, in 2016 it was 3.5″, in 2017 2.375″, and in 2019 8″.
Some years the spoilers were adjustable for angle, some years they were fixed, and there have been different heights, widths, and shapes throughout the years.
NASCAR uses the spoiler to balance not only the overall aero package, but as a way to balance the performance between different cars, and at different tracks.
When NASCAR reverted from rear wings to spoilers, they set the spoiler angle at 70 degrees. In 2019 the fixed angle remains 70 degrees. Interesting.
The 2019 spoiler is flat across the top, but different shapes have come and gone.
Curvy, almost bat-wing style.Convex top edge.
The size and shape of Miata spoilers
So now that we’ve looked at spoiler theories and real-world examples from NASCAR, let’s get down to what matters: Miata spoilers.
Miatas have a roofline that is peaked in the middle, and you might imagine that the ideal spoiler shape has a matching convex arc to it. Although like all things aerodynamic, this could be totally false, and maybe the sides should be taller.
The rear edge of the trunk is curved and so a curved spoiler would look more natural, and could be an easier DIY project as well. Also, a curved spoiler would be more rigid than flat. However, some race series say that the spoiler must be flat, with no curvature. Booo!
There’s no reason to “spoil” the air coming along the sides of the car, and so a spoiler much wider than the rear canopy seems like a waste. Although the exposed spoiler ends are probably adding downforce. Albeit not very efficiently, and at probably a different angle than is ideal for spoiling the roofline shape.
Miata products
This IKON spoiler is an attractive design, with a convex top edge and curved profile. It would be neat to see something like this with a flat extension that’s adjustable for height.
The Rocket Bunny spoiler is flatter across the top, taller, and with a steeper angle. I’d guess it’s slightly more effective than the Icon, but it has a tacked-on look that doesn’t really appeal to me.
And then there’s this JSP spoiler that looks like a wing, but isn’t (air isn’t going to flow under it, hence not a wing). The shape follows the curvature of the sides and roof, and this may be an efficient design. But meh to the looks.
Of course all of these spoilers have a fixed height and angle, so there’s no way to adjust the aerodynamic balance. On the other hand, the Blackbird Fabworx spoiler is large and adjustable for angle. I’m also not a huge fan of the way this one looks, but the beauty lies in the function.
Spoiler done right.
DIY spoiler, testing height
I made my own spoiler, it’s about 3.5″ tall and has some curvature to it that follows the trunk shape. It’s made of plywood and fiberglass, and there are 6mm T-nuts so I can add an extension.
With the low spoiler (without any extension), I ran very consistent 1:22s at Pineview Run. And by consistent, I mean 1:22.03, 1:22.05, 1:22.07, and in my second run, 1:21.99, 1:21.99 and 1:21.93. This was a hot day, and if I compare the times to previous ones, the track was definitely slower than normal.
With a 3.5″ extension (total 7″ height), my lap times were less consistent, most of them around 1:21.5, but my fast lap was a 1:21.03, almost a full second faster. But that one was an outlier, and if I average the five fastest laps, the taller spoiler was about .55 seconds faster than the lower spoiler.
The following table is an average of four back-t0-back runs, two with the spoiler extension, and two without. I’ve averaged the top six fastest laps.
Configuration
Avg Lap
Simulated
HP
Lbs
Cg
Cd
Cl
Low Spoiler
1:22.0
1:21.11
112
2400
1.00g
.44
-0.25
Tall Spoiler
1:21.45
1:20.63
112
2400
1.00g
.45
+0.20
I added .01 to the Cd as a guess, but drag isn’t that consequential anyway. I came about the Cl figure by changing that value in OptimumLap until I got the .55 delta in lap time. It seems absurd to think a spoiler can make a .45 swing in Cl, but that’s what the simulation says. Interestingly, this is also the value cited for a 8″ tall spoiler in MacBeath’s Competition Car Downforce.
In Race Car Aerodynamics, Katz cites several examples of spoilers, but none that go as high as 7″. In his examples, the relationship between height and coefficient of lift is nearly linear, and from 0″ to 4″ there’s a change of about .4 in Cl. So if I extrapolate those values from a 3.5″ spoiler to 7″, I’d only expect to see a change of .4 Cl, which is again pretty close to the test result.
Whatever the case, a 7″ tall spoiler works on a Miata. Now I have to make a taller one and test that.
I have two Miatas, and even my faster one is slow. Adding an airdam, splitter, side skirts, wing, and all the other aero bits add weight, and some of them increase drag as well. The last thing you want to do to an underpowered car is add more weight and drag, right? Maybe.
Drag reduction matters most when accelerating on a straight, but pretty much everywhere else downforce is preferable to drag reduction. Even still, there are times when drag reduction is more important, such as in an endurance racing strategy where you want to do one less pit stop. It also seems logical that at a high-speed track you’d want to skew your aero package towards top speed and reduced drag, especially in an underpowered car.
Or so you’d think. But like most things aerodynamic, what seems obvious could be completely wrong. So let’s examine downforce vs drag on a very fast racetrack that is dominated by long straights and top speed.
I did a motorcycle track day at Portland International Raceway, and I’ll describe it like this: it has a really long and boring front straight, a couple corners, another long and slightly less boring back straight, and a couple corners. If ever there was a track where you’d want to reduce downforce and optimize for less drag, this is probably it.
In Race Car Aerodynamics, the author Joseph Katz calculated lap times for a generic prototype race car at Portland International Raceway factoring in grip and drag. Take a look at the track layout in the chart inset, it’s like I said. The full results are in SAE Paper 920349, but this is what I make of it.
At this track, you might think that you should set your aero for the least possible drag, thereby attaining the highest top speed. But that actually sets the worst lap time, some 6 seconds off the pace. Or you might think to optimize for the highest L/D ratio, and with that you’d at least be within the same second as the fastest cars.
But somewhere in the neighborhood of maximum downforce, that’s where the fastest lap time was. On any other track I’d guess that maximizing downforce is the right thing to do, but I’ve raced down the front straight on this track (which is nearly a mile long), and the results are surprising.
This is a calculation, albeit a very sophisticated one, and it’s based on a race car with a lot of power that can overcome drag. Still, it makes me wonder if we should chase all the downforce we can and not worry about drag reduction at all. Miatas are all about cornering anyway, and we’re used to getting passed on the straights!
In 1993, the Mazda Miata had a coefficient of drag of .38, and the RX-7 had a Cd of .29. Same manufacturer, same year, both two-door sports cars, and yet the RX-7’s had 20% less drag.
There’s nothing magical about the RX-7 shape, and if you compare its Cd to new cars, it’s only so-so. In The Most Aerodynamic Cars You Can Buy Right Now there are many cars with Cds from .27 down to .22, and a unicorn at .189. (Follow along in this Aero Timeline and see how Mercedes has incrementally improved their aero from high .4s down to .24 complete with wind-tunnel smoke trails.)
But let’s stick with the same year and manufacturer, and see what would happen if you could magically put a RX-7 body on a Miata, and what that would do for performance and fuel economy.
Calculating top speed
To calculate top speed, I’ll use the RSR Bonneville Aero-Horsepower & Drag Loss Calculator. I’ll enter data for a 1993 Miata, with frontal area of 18 sq feet and a Cd of .38. Miatas of that vintage had about 116 crank hp, and if I multiply by .82 to simulate driveline losses, that’s about 95 hp at the rear wheels. (You can argue driveline losses, I’m using figures from Competition Car Aerodynamics.)
First I want to calculate top speed, so I’ll throw some numbers into the calculator until the Horsepower Needed field reads 95. Turns out that 116 mph is the top speed.
Now I’ll drop a RX-7 body on the Miata, and drop the Cd to .29. The top speed goes up by 10 mph to 126 mph. Wow!
However, top speed is rarely important, so I’ll plug in some more common values. I’ll use 60 mph to represent the exit of a corner, and 90 mph to represent a faster section of track. How much power is required to go that fast, and how much power remains?
Cd
HP to go 60 MPH
HP Remains
HP to go 90 MPH
HP Remains
.38
16.5
78.5
46.8
48.2
.29
13.8
81.2
37.8
57.2
At 60 mph, the low-drag RX-7 body has an additional 2.7 hp available over the standard bodywork. Meh. At 90 mph, there’s an additional 9 hp available from the sleek RX-7 body. Wow! Obviously, the faster you go, the more important drag becomes.
Simulating lap time
Drag is obviously important, but more important is lift (downforce). We need the numbers for both drag and lift in order to calculate a lap. I don’t have any published numbers for lift on a Miata, but the Hancha group did CFD testing and I’ll use their lift value of 0.27. In Race Car Aerodynamics (p. 19) Katz lists the RX7 at .24 lift, and AutoSpeeds article on Aero Testing even breaks that down into front lift vs rear. Let’s plug these values into OptimumLap and simulate lap times at my local track, Watkins Glen International.
Miata
RX7 Miata
Delta
Drag
.38
.29
.09
Lift
.27
.24
.03
Lap
2:34.93
2:33.38
1.55
1.5 seconds!
So a Miata with a RX-7 body would go over 1.5 seconds faster than a stock Miata. Some people would give their left nut for a second-and-a-half per lap. I’m betting that with windows open, which is how I’ve always raced, the RX-7 advantage would be even higher. This because the Miata hardtop is quite wide, and acts as a parachute, especially when in yaw.
Fuel economy and race strategy
In sprint racing, fuel economy is meaningless, but in endurance racing, it can be important. Especially if longer stints will allow you to do one fewer pit stop during the race, or if your car is right on the cusp of doing the maximum allowed stint. OptimumLap shows a 2.5% decrease in fuel economy using the RX7 body. That doesn’t seem like much, but it can be a big difference.
Let’s use my race Miata as an example. It burns about 7 gallons per hour, and with its 12.7-gallon gas tank, it can go about 1:50 before the tank runs out. This is not a problem in AER where stints are 90-minutes long. But in Champcar or Lucky Dog, stints are two hours long, and I end up doing an extra stop each day. In cases like this, 2.5% fuel economy can be a huge deal.
So not only is the RX7-bodied Miata going 1.5 seconds faster per lap, it’s doing that while burning 2.5% less fuel. If I calculate the total number of laps per stint, the driver in the stock Miata can do 42.6 laps per stint. The driver in the RX-7-bodied Miata can do 44.1 laps.
Imagine if Mazda made a RX-7-bodied Miata, without the design compromises of a convertible top. It would be sleeker, lighter, more rigid… and probably fall flat in sales. Ah well, it would have been great on track!
Occam's Racer 