Category: aerodynamics

I recently attended my first ever autocross with my twin brother, Ian. He blogged about his experience, and it’s a fairly accurate read from my perspective as well. We are endurance racers first, track people second, and probably would have never done an autocross, but we started discussing doing One Lap of America, and that event has a few autocrosses mixed in. So we figured we’d better learn how to dodge cones.

Miata Classing

My first stop was to investigate which class my Miata would go in, so I went to the rules and dove headfirst into the (OMG) 380-page rulebook.

First I looked at the “Street” classes, and while those allow me to change anti-roll bars, tires, and shocks, I can’t upgrade springs. Huh. Who upgrades shocks and not springs, is that even a thing? I’ve definitely seen people put lowering or stiffer springs on stock shocks, but not the other way around. Anyway, I’d also need 6″ wide wheels for this class, and I don’t have anything near that.

So next is “Street Touring” and that looks good except I replaced the 6” ring gear with a 7” ring gear and a Torsen. The 6″ ring gear can break, even under stock power, and so most people upgrade to a 7″ ring gear. And when you do that, may as well do a Torsen at the same time. That’s what sensible people do, but not STS people, so I’m out of that class.

But there’s another “Street Touring Roadster” class for Z3, Z4, S2000, ND, Boxster S, and other convertibles. This class also allows 1994+ Miatas if you feel like being outclassed. Mine is a 1993, and so not technically legal, but maybe they won’t notice. However, I have shackle-style motor mounts which are slightly lighter than stock, and that kicks me out of the class. Fuck, that’s a pretty specific rule!

M’kay, next is the “Street Prepared” class, where my car would go into CSP. I can have a splitter (yay), any wheel, any DOT tire, and even a spoiler. But not a wing, and no cams, and no decking the head over .01″. Well sheesh, my head is decked .035″ and I have a mild cam. So CSP won’t have me either.

Next is the “Street Modified” class, which allows a rear wing (yay), any tire, canards, and a splitter…. But the splitter can’t have any end plates or fences. FML. NA Miatas have cute little bumpers and the tires stick out, and so I have little fences in front of them to divert air around my tires. Bodywork that covers the front tires is standard on virtually every fucking car made in the past 20 years, but those little tire fences put me out of SSM.

OK, so next is the “Prepared” class, and I guess DP is where I go? Splitter end plates OK! Spoiler or wing OK! Cams and decking the head OK! Fastback? Doesn’t say…. That word doesn’t appear once in 380 pages of rulebook. I have a lot of different tops and I could certainly use an OEM hardtop (or remove the top), but it worth noting that I can’t go in this class with my fastback or shooting brake tops. Best to be prepared and see what’s left.

Finally I get to the “Modified” class, and SSM looks like the only class where you can significantly change the roofline shape. The rules say I can change the bodywork as long as “the shape of the body must be recognizable.” So that would put me in a class with basically unlimited cars. Great. For any car with an aftermarket fastback, or this track-bred 1993 Miata that’s on equal terms with a stock NB2, I’m racing against the fastest cars in the paddock.

Finally, I say fuck it, I’ll leave my Miata at home and race my wife’s daily driver, a bone stock 2018 Honda Civic.

2018 Honda Civic

This is a great daily driver, I get 48 mpg on the highway, and 40 mpg in mixed driving. It’s a 6-speed manual with a turbo that’s tuned for torque, and it weighs around 2800 lbs. What’s not to love?

The stability control, for one. I’ve tracked the Civic at Pineview and Lime Rock, and it overheats the brakes like crazy. I think this is the stability control, which Honda calls VSA. It must apply the brakes constantly trying to keep the car balanced. My front and rear rotors are at 800 degrees with mild tracking.

Turning off the VSA requires the following ridiculous procedure.

1. Start the car.
2. Make sure the electric parking brake is off.
3. Push traction control button off and then on again.
4. Press and hold the brake pedal. 
5. Turn traction control off and then on again. 
6. Release brake pedal.
7. Turn electric parking brake back on. 
8. Turn traction control off and then on again. 
9. Press and hold the brake pedal.
10. Turn traction control off and then on again.

If you get it all right, the traction control light flashes on the dash. If you get any part of that process wrong, nothing happens, and you have to start that whole thing over again. If you turn the car off in between runs, of course the VSA goes back on again.

Until recently, Jenna’s Civic had 200 treadwear N Fera Sur4G tires on all four corners. But I took two tires to PittRace as backup tires for a minivan I was racing, and we decided to use them at one point during the weekend. And they delaminated. So now the Civic has Sur4Gs only on the front and 7oo treadwear all-season tires on the rear. This should make the handling… interesting.

The Event

The autocross was held at nearby Seneca Army Depot. In the 1980s, the Depot was in the news a lot because they stored nuclear weapons there, and as a result there were a lot of protests and demonstrations. Quite possibly related, this area has an abundance of albino deer, and until recently, you could even book a tour to go see them. Apparently they even glow in the dark! (OK, I made that part up.)

As an event location, it’s pretty awesome. It’s a flat airfield, with a mix of blacktop and concrete, with a few bumps to give it more character. It’s not a parking lot rectangle, but shaped like a lowercase “q” (although the locals call it a “p” for some reason).

We got our cars teched and classed, and both Ian and I were in H Street (HS). I found out Clayton’s Miata was put into a class called “Extreme Street B”. This wasn’t in the rulebook, but apparently different regions have their own classes, and if I knew that I guess I might have brought my Miata after all. FWIW, the XSB class is for track-spec cars with things like aero and whatnot, on 200 TW tires. There are two classes for different weights, but engines are basically open, so my 1.6 Miata would go up against turboed and V8 swapped Miatas. So while not a class that has a lot of parity, the rules are simple, and my car would fit in just fine.

After tech we lined up for a track walk, which was marginally useful, but because I’m used to seeing the road from inside a windshield, I can’t say the track walk helped that much. I had a lot of questions like, “why are there three sets of cones there” and “why is that cone tipped over” and such, and the track walk was good for learning the lexicon.

One curiosity is that they don’t allow novices to drive the course ahead of time. Not even at 5 mph. For a HPDE track day, the first session, or at least the first laps, are under a full-course yellow. This helps people who have never been to the track know which way the corners go. Autocross is so focused on the competition aspect that they don’t want anyone to get a glimpse of the course ahead of time. Not even a novice that’s never been to an event before. As if novices are going to threaten the competition? (Nevermind the fact that someone had to set up the cones and test drive it at least once.) Ergo, the track walk was as much as any of us novices would experience before racing. This is so fucking dumb I’m not going to put any more words to it.

But they don’t leave the novices out to dry, there’s coaching and ride alongs all day. I had a coach in my car in just about every session, and it was very helpful. All of them were super nice, very knowledgable, and passionate about the sport. I didn’t do any ride alongs because I was feeling a bit queasy, and I sometimes get seasick as a passenger.

The Racing

Autocross is fun! It requires fast learning, and it’s always enjoyable to learn a new skill. Dodging cones requires reading. Literally. Instead of words, it’s a lexicon of cones and shapes, and when you’re literate, it’s probably even more fun than when you’re learning to read.

I wasn’t very literate, it took me a long time to read the course. It’s a vision-based skill, and I just wasn’t processing the information very quickly. In my six runs, I didn’t hit any cones, but I also failed to read the course correctly and flat out missed some turns entirely. I only fully completed one run, and that’s because my coach was telling me where to turn.

On the plus side, when I finally got the VSA turned off, the Civic was a hoot to drive. I got oversteer on some sweepers and was able to drive the car more or less how I wanted. After three runs lasting a bit over a minute each, we began the first of two long working shifts.

The Working

In between the sessions, you have to work. And this means standing out in the heat, trying to avoid getting hit by a car, and then running out to go put a cone back in place before the next car comes through and tries not to hit you.

I have $100k into my knees, with metal plates and screws holding the bones together. My doctor says I’ll need two total knee replacements in the near future, and when that happens depends entirely on how much I abuse my knees. So I quit skateboarding, and for the same reason I sure as shit am not running anywhere. I’ll walk briskly to go set up a cone, but if you fucking yell at me for not running to put a cone up, I’m still not running. I have bigger problems than keeping your event on time.

Speaking of which, there were a few timing errors and re-runs, and that made the day drag on a bit longer. But kudos to the organizers, they did their best, and mistakes like this always happen. The entire event was really well run.

The Lyme

While sitting next to my cones I was chatting with another fellow, and he just got a phone call saying that he was positive for Lyme. I told him that happened to me last year, and TBH, I was feeling a bit Lyme-y myself that day. Lyme disease feels a bit like being drunk, but not in the good way, more like you had one too many. You’re confused, don’t put things together quickly, and your balance and vision are off. These are not great symptoms to have when trying to drive anywhere, much less autocross.

Welp… I got my test results back and I have Lyme again. I knew something was up. I’m usually a fast study, but I was driving like shit. And I needed some kind of excuse anyway.

As I write this I’ve just used the last dose of a 21-day prescription of doxy, and I’m feeling no better. I can’t drive, I can’t always think straight. I’m going for more tests and to see two different Lyme specialists. Apparently I can still write, and so I’m doing that instead of driving. But goddamnit I had so many plans this summer and now I’m not racing bikes or cars. Fuuuuuuuuck!

Looking Back, Looking Ahead

Autocross is the most popular form of motorsports competition for a reason. You don’t need any special car prep. You’re not going to hurt your car, or yourself. Every other form of motorsports competition is way more expensive. When I do a track event at PittRace I always see more people in the parking lot doing autocross than I do on track. It makes a lot of sense to a lot of people.

Looking back in time, I can definitely see a point in my life where autocross would have been relevant. In my dirtbag 20s, with more time than money, I would have loved autocross. It’s cheap if you’re a student, and if you also consider it an all-day sporting event. And sitting around with your friends talking about cars is fun.

But as someone who is in their mid-50s, with more money than time, no. Paying $60 for an eight-hour day with 7 minutes of track time is not worth it. Honestly, if it was free I wouldn’t do it again. Would I do it without the work requirement? Maybe. It’s still sacrificing a whole day for a very limited amount of track time, but autocross is a lot of fun, and a skill that I would like to acquire.

In the end, I get why people are passionate about autocross, and while I highly doubt it, maybe I’ll be one of those people one day. I love motorsports, and I love competition. It’s a great group of people, with a perfect venue, and it’s nearby. Wait, did I just talk myself into another event, or is that the Lyme-brain again? Hm.

I live near two very different tracks, Watkins Glen International and Pineview Run. WGI is the fastest track in North America, and Pineview might be the slowest. When I run racing simulations in OptimumLap, Watkins Glen is one of the few tracks where drag matters. On any other track I can increase drag quite a bit and it’s not a big factor. Conversely, at Pineview Run, drag is almost inconsequential, and the same would be true on an autocross course.

I’ll run a quick simulation and show you what I mean. Here’s a Miata at three local tracks, Watkins Glen International (WGI), Pineview Run (PV), and New York Safety Track (NYST) using drag coefficients of .45 and .495 (110%). This is a lot of drag. For reference, adding 10% drag would be like adding a wing, four mirrors, and putting your hand out the window.

	WGI .45	WGI .495	PV .45	PV .495	NYST .45	NYST .495
Lap time	140.58	141.34	73.85	73.89	98.76	99
Max speed	120.61	117.88	75.22	74.96	106.8	105.49

At WGI, adding 10% drag is a difference is .76 seconds and almost 3 mph top speed. That’s the difference between winning and losing. Conversely, at Pineview, it’s only .04 seconds and not worth talking about. I put NYST in just to show what an average track is like, and here the added drag is only a quarter of a second.

So drag can be pretty significant at high speed, but when optimizing for low speed aerodynamics, let’s ignore drag and go for as much downforce as possible.

Aspect Ratio

Wings make most of their downforce in the middle of the wing; the further you move from the center, the less downforce the wing makes. This is because the ends of the wing create vortexes that destroy lift.

Therefore, for a given area, a longer wing makes more lift than a shorter wing, simply because the ends are further from the center. You may have heard of a high aspect ratio wing; that’s a wing that is very long and thin, such as on a sailplane, or glider. Theoretically, you might think that would be a good wing on a racecar.

Well, not necessarily. For one, racecar wings (and most airplanes these days) use endplates, which keep the upper and lower pressure zones from creating the lift-destroying vortexes. So you don’t need to have a really long wing to avoid losing lift. Just use a big enough end plate.

Another reason a high aspect ratio wing would be difficult for cars is construction. Building a really long wing with a narrow chord is going to be a noodle. The stiffness of a wing is the thickness cubed multiplied by the chord, and so a high aspect ratio wing would be fragile or at least very flexible.

Another reason a high-aspect ratio wing isn’t ideal is because the mass is high up and away from center. Mass centralization is important for handling, and a high-aspect ratio wing is the antithesis of that.

Finally, probably the most significant reason a high-aspect ratio wing won’t work well at low speed is it would generate low Reynolds numbers. What?

Reynolds Number

I’m not going to go into a deep explanation of what a Reynolds (Re) number is, but I think of it as a resistance number. At low speed, air doesn’t have much resistance; you can put your hand out the window as if it were a wing, and at 5 mph change the angle of attack freely. Do that at 60 mph and you’ll feel a lot of drag. At that speed, air seems to have the same resistance as moving your hand through water.

Likewise, a smaller hand will have less resistance out the window than a larger hand. So you can probably tell that the Re number depends on the chord of the wing (the size of your hand), and the density of air (which is basically the speed you’re moving).

Wing chord	MPH	Reynolds Number
4″ (.102m)	62	187k
9″ (.23m)	62	420k

In the table above, you can see that the 4″ wing has a Re number of just under 200k. The 9″ wing has a Re approaching 500k. OK, so how is this significant?

Look at the image below, which shows a Selig S1223 wing at three different Re numbers. The lines show the coefficient of lift divided by the coefficient of drag (Cl/Cd), which you can simply think of as how efficient the wing is at making lift. The higher the Re number, the more efficient this wing is. The the green line is 200k, the purple line is 500k, and the gold line represents a Re of 1 million. The Selig wing is designed for high lift at low Re, but at 200k, the wing just isn’t working that well. At 500k, it’s about 130% better, and at 1M, it’s almost twice as efficient. Note that a 4″ chord high-aspect ratio wing doesn’t get to 1M Re until a very unrealistic 333 mph.

All of this means that for a wing to be useful at low speed, it needs a really large chord. For argument sake, I’m calling this 16″ or more. Such a wing would have a Re of 1M at a much more realistic 87 mph. Another advantage of a large-chord wing is that flow separation is delayed at higher Reynolds numbers, which means you can use a low-aspect ratio wing at a higher angle of attack. And if you grok what I’m saying about the Reynolds number, you understand that you can also use more wing angle at higher speed.

Wing Shape

The Airfoil Tools website has a pretty good search function, you can look for wings of different profiles and sort on factors like thickness, camber, and even Reynolds number. I started by searching for wings that had the highest lift/drag ratio (Cl/Cd) at 500k Re, using this search. That returned a lot of results, some of them rather strange. Next I looked at the Cl vs Alpha graph, and many of them peak around 1.5.

But what I’m specifically looking for are high lift wings. I don’t want a wing that’s the most efficient at low Re, I want one that has the highest lift at low Re. So I changed the search parameters to use a minimum thickness of 10% and minimum camber of 8% (both values that should drive up lift) to find wings that have a Cl of 2:1 or greater. (Note that single wings have peak downforce at about 12 degrees, but double wings and low Re numbers can use a thicker wings for more angle.)

That search returned four pages of results, and after culling those, the ones I’ll use for this investigation are the Church Hollinger Ch10, Eppler 420, and Selig S1223. (There are other potentially good ones I didn’t look into, such as FX 74-Cl5-140 MOD, FX 72-MS-150B, GOE 525, etc.)

Airfoil Tools allows you to compare wings by clicking Add to comparison. So let’s do that for each wing. You get a view like so, with the corresponding data.

Turbulence

Wings work best when they have laminar flow. Conversely, when the air is turbulent, it increases drag and decreases lift. Airfoil Tools allows you to simulate turbulence using the Ncrit value. An Ncrit of 9 is like a wind tunnel, or about what you’d experience on a race track, on a still day, with no other cars in front of you. In other words, pretty unrealistic.

To simulate what your wing is doing behind your car’s canopy, during a race, possibly in the wake of another car, you need to use a Ncrit value of 5 or less. Unfortunately, Airfoil Tools doesn’t give you all the Ncrit values, and depending on which wing you choose, 5 might be as low as you can go.

Head to Head on Reynolds

I’ll use these three wings and then compare the various graphs at a Ncrit value of 5, starting with Re 200k, then 500k, and finally 1 million. In the graphs below, Cl is lift, Cd is drag, and Alpha is the angle of attack.

Re 200k

A Reynolds number of 200k is absurdly low, and for our theoretical wing of 16″ chord represents a speed of less than 20 mph. Still, this is useful to look at because most people don’t have a 16″ wing, and so this represents how these wing shapes would perform at a more standard 9″ chord and around 30 mph.

At 200k, the S1223 wing is clearly ahead of the CH10 and E420 in both Cl and Cl/Cd, which are the most important factors. (Sorry about the colors, I can’t choose them.)

Re 500k

A 16″ wing at 41 mph would have a Re of about 500k. Pineview Run’s average minimum cornering speed is around 43 mph, so this is a close approximation for that track, or for an autocross.

The S1223 wing is winning on Cl and Cl/Cd. This is a good wing for low speed applications. The E420 is starting to diverge from the CH10, showing a bit more promise at higher wing angle. Where the CH10 wing shines is in lower drag, but we don’t care about that for this application.

Re 1 Million

At this point the 16″ wing is traveling at 87 mph, which is not what I’d call low-speed cornering, but it’s worth looking to see how the wings perform at higher Reynolds.

Conclusions

• S1223 looks like the low-speed wing of choice. It has the highest lift, but also the highest drag by a fair amount. This might also be a good all-purpose wing on a powerful car that can overcome drag, but not one of my underpowered Miatas.
• CH10 looks like it would be a great low-drag wing for a momentum car. It’s very efficient and seems tolerant of wing angle. This looks like the best wing for a “set it and forget it” mentality. The 9 Lives Racing wing profile is a lot like the CH10, but 9LR has more camber. Camber is good, it makes more downforce. This is one of those situations where I look at the 9 Lives Racing wing and say to myself, why the fuck am I doing anything with wings?
• E420 splits the difference and seems like a great choice for an all-purpose wing. It’s thicker than the other wings, and would probably be a better choice for a double wing setup, where that can be exploited for more wing angle and downforce.

What Next?

AeroDesign in Australia makes a 400mm (15 3/4″) chord single-element wing. It’s made of carbon fibre and has an integrated Gurney flap. The dimensions and wing profile look perfect, but at $3500 AUD ($2600 US dollars), out of my price range.

In Part 2 of this series I’m going build one of these wings, and test it at a low-speed venue. But while the S1223 is a clear winner, I’m not exactly sure how I can build it strong enough, it’s just so damn thin at the end. I’ve only built a few things in carbon, and I don’t have an autoclave, so this will be an interesting challenge in my usual materials.

I’m not 100% stuck on this profile, and have played with the plotter to increase thickness. The S1223 also comes in a Richard T. LaSalle modified version, which is slightly thicker and has more camber. This wing generates even higher lift, and is more efficient at 12+ degrees angle of attack.

If you look at the Cl/Cd vs Alpha graph, you can see the lines cross at about 12 degrees. This is where the RTL wing really takes off.

Wind tunnel testing

I finally had an opportunity to test my large-chord, small wingspan S1223 wing in a wind tunnel, as both a single wing and as a dual wing. The results were not what I expected. I go over all of the details in my Miata Wind Tunnel Report, which is available for $35.

In the wind tunnel report I test several wings, as well as the following:

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