Category: aerodynamics

Justin Lee of Miatahubs recently asked me what I would do for a Gridlife Touring Cup (GLTC) wing that measures no more than 250 square inches. This sent me down a rabbit hole of possibilities, and to consider all their aero rules and options.

GLTC Wing Rules

Racing rules change all the time, and aero rules even more, but let’s take a look at the current rules.

• A rear aero device, oem or aftermarket is allowed. If the surface area is above 250in2 up to 701in2 (chord × length) the adjustment must be taken.
• Only one aerodynamic element such as a wing or spoiler is allowed. The only exception to this is OEM hatchback spoilers which are utilized to mount wing brackets, are not considered an aero device. For example, a dual element wing is illegal, a single element wing with a trunk mounted spoiler is illegal, a single element wing mounted to the oem hatchback spoiler is legal.
• Entire assembly (including endplates, and wing mounts) may not extend more than 5” past the most rearward part of the rear bumper when looking from above. There are no height restrictions.
• Each wing endplate is unrestricted, however, should not exist in space that causes potential contact to cars during close quarters racing. Wings causing potential or actual contact problems will be asked to be removed.
• Gurney flaps are allowed. Active aero is not allowed.

In the first bullet it says, “the adjustment must be taken.” The adjustment is a 3% hit to the lbs/hp ratio. GLTC is based on 12.5 lbs/hp, and if your car is 2500 lbs and 200 hp, then a wing costs you 6 hp or 75 lbs. That’s not a very large penalty for aero, and if I was racing GLTC, my first instinct would be to use a 700 square inch wing. That’s 9″ x 77″ or 12″ x 58″, and fairly large.

You can also run a wing that’s under 250 square inches for free. Well that’s interesting. I don’t know why they chose 250 square inches, perhaps some sedans with wings (Integra-R, WRX STi, etc) measure in under that size? In any case, that’s a pretty small wing and one wonders if it will do anything. Let’s find out.

250 sq-in Wing Options

I have a 53.1″ wing that measures 4.7″ chord, which calculates to 249 square inches. It just barely squeaks under 250, perfect. Originally the wing was flat on the bottom, but I modified the bottom so it’s curved.

It now looks like a bit like a Wortman FX72 airfoil or a GOE-525. I’ll explore that in Airfoil tools and set realistic Reynolds numbers (realistic car speeds) and see how it performs.

Notice first in the Cl v Alpha (angle of attack) chart, the wing makes the most downforce at around 10 degrees. There isn’t much difference in Cl (downforce) between the 200k and 500k Re lines, and so the wing is generating downforce, despite the low speed.

Below that chart in the Cd v Alpha, you can see that the drag really spikes up after 10 degrees. Lift and drag are combined in the Cl/Cd v Alpha, and you can see the wing is most efficient at around 5 degrees, but I’d go for more downforce and aim for 8 degrees. Downwash from roofline shape would come into play here, and so I’d probably set the wing to about 3 degrees and then play with the AOA from there.

Notice also in the Cl/Cd v Alpha chart the difference between the 200k and 500k Reynolds numbers. At low speed (or small chord), the wing is about 60% as efficient. If I included 1 million Re, the wing would be about 2x more efficient than 200k. In other words, a bigger wing would be more efficient.

All in all, this is a good wing profile for low Re, and there aren’t many wings that have a max Cl above 2.0 at 500k. A custom wing built specifically for this purpose would have more chord and camber, but wouldn’t give that much more downforce at these speeds. Anyway, for a $50 wing modified with a pine baseboard glued to the bottom, this is not so bad.

To optimize the wing I made oversized end plates, added a 1/4″ Gurney flap (5% of the chord) to both the wing and the end plates. I sent the wing to Justin for testing. He sent me back a pic, and it looks good, but I wonder how it’ll work? He’s racing GLTC this weekend, and I’ll look at the data after the race and report back.

Calculating Downforce

I previously wrote a post on a simplified way to calculate downforce, and I’ll use that same calculation here: (mph*1.47)^2 * .00119 * sq-ft.

I won’t make you go through that, so I’ve done the calculations and listed downforce for three speeds: 50 mph, 62 mph and 82 mph. These speeds are specific to Mid-Ohio, where Gridlife will be racing.

• 50 mph is about the min speed through the slow turns: T2, T5, and T12.
• 62 mph is the average minimum cornering speed of all corners combined.
• 82 mph is the min speed in T1, the fastest corner.

Let’s see how the no-points 250 sq-in wing stacks up against Justin’s 67″ 9LR wing, in downforce.

MPH	250 sq-in wing	623 sq-in wing
50 mph	11 lbs	28 lbs
62 mph	17 lbs	42 lbs
82 mph	30 lbs	73 lbs

The standard 67″ 9LR wing is making almost the same downforce at 50 mph as the little wing does at max cornering speed. In addition, the larger wing would be more efficient at these speeds because of the larger chord, which brings the Reynolds number up to a more useful range. Before you write me and say that I have this backwards, please go look at data for wings at low Re. At low speed, a larger chord is more efficient.

Downforce increases grip relative to the weight of the car, so a lighter car gets more out of a 250 sq-in wing than a heavier car. Justin’s car is pretty light, and I wouldn’t even bother with a small wing like this on something 3000 pounds or heavier.

But a free wing is free, and it adds a not insignificant 30 lbs of downforce in T1, which is about 1.2% more grip on a car that weighs 2500 lbs. That could be useful, but through the slower corners, the downforce is negligible, while causing drag on every straight. Still, T1 is important, and extra rear grip would be useful when braking from high speed into China Beach.

Spoiler

Another free option is a 250 square inch spoiler. I wrote a whole article on Miata Spoilers, but to recap it, spoilers are effective at three things.

• Changing the shape of air as it passes over the car. This can reduce drag and cancel lift.
• Creating a high-pressure zone on the rear deck lid. Pressure is akin to weight, and that means downforce.
• Moving the center of pressure rearward, which usually makes a car more stable.

I’m a fan of spoilers, I like the way they look, I have data that prove they work, and they are cheap and easy to DIY. If you made one 8″ tall by 41″ long, with a semi-circular shape on each end, that would be 250.26 square inches. Shape the bottom center portion so it follows the cambered trunk line and it would be less than 250″. It could be made adjustable for angle, but for simplicity, I’d set it at 70 degrees.

A spoiler of this size should have a coefficient of lift around -.45, which is pretty significant. (This is according data from MacBeath’s book, Competition Car Downforce, and also corroborated in my spoiler testing.) In the 250 sq-in size, I’m guessing a free spoiler might be faster than a free wing.

You might be thinking, wings are more efficient than spoilers, how could a spoiler be faster? Because wings are designed to fly fast, and that’s when they are efficient. At low speeds, or with a small chord (which is the same thing), wings have more drag, less lift, and in this size, might be slower. I tested a 9 Lives wing vs a 7” spoiler at Pineview Run, which is a very low speed track. The spoiler was .5 seconds faster in redundant tests. I’ll have to test both the 250 wing and spoiler at a faster track, just to close this loop.

Splitter

Gridlife allows a splitter up to 3″ in front of the bumper. This is a bit on the small side, especially considering the 701 sq-in wing allowance. Most racing rules allow a longer splitter, but whatever. I tested my car with a 4″ splitter and it dropped Cd by .01 (a splitter is less drag) and increased downforce by .38 Cl (win, win). So I’ll estimate that a 3″ splitter will be worth .007 drag reduction and a .30 delta to downforce. I’ll use this later in simulations.

The GLTC splitter rules are pretty well written, and say that if you have a bumper or undertray or anything that acts as a splitter, they’re going to call it a splitter. This is a recent rules addendum: “Any upper horizontal surface that is non-OEM and exposed to airflow will be classified as a splitter regardless of protrusion from or location in the front fascia. Example: Flat section floor in front of radiator.”

Simulations

I’ll run some simulations based on GLTC rules. For the cars with aero, I’ll penalize 3% for a wing, and another 3% for a splitter. That penalty can occur through reducing power or adding weight. If you read my previous article on the subject, then you know adding weight is theoretically faster, but I’ll do one simulation with the same weight and detune the car for less power.

Here are the 6 different builds:

1. No aero: This is the baseline build at 12.5 lbs/hp. NC Miata, 6 speed, 4.1.final drive. I made the Cl about zero to simply calculations on my end, but in reality there would be some positive lift.(Cl 0.001, Cd .45, 197 hp, 2463 lbs.)
2. GLTC wing: This is the 250 sq-in wing only. It’s free aero, it should go faster. I’m guessing on the drag and lift values. (Cl .2, Cd .46, 197 hp, 2463 lbs. )
3. GLTC wing, splitter: Add a 3″ splitter to the above. Car pushes a bit already, so this should balance the car better. Notice the increase in weight to offset the 3% aero penalty. (Cl .5, Cd .453, 197 hp, 2537 lbs.)
4. 67″ 9LR wing: Remove the wing and splitter, replace with Justin’s 9LR wing. I can’t simulate aero balance, but the car might push in this configuration. (Cl .6, Cd .49, 197 hp, 2537 lbs.)
5. 67″ 9LR wing, splitter: Now add a splitter and get the car to be more neutral. Bigger weight penalty running two aero items. (Cl .9, Cd .483, 197 hp, 2611 lbs.)
6. 67″ 9LR wing, splitter, detuned: This is the same configuration as above, but instead of adding weight, I’ve removed power. So the car weighs the same as configuration 1, but has 11 hp less. It’s the same power/weight ratio as #5. (Cl .9, Cd .483, 186 hp, 2463 lbs.)

Vehicle	1. No aero	2. GLTC wing	3. GLTC wing, splitter	4. 9LR wing	5. 9LR wing, splitter	6. 9LR wing, splitter, detune
Lap time	1:37.35	1:37.87	1:36.32	1:36.36	1:35.93	1:36.03
Min Speed	47.96	48.19	48.52	48.63	48.94	49
Max Speed	121.89	121.31	120.63	119	118.35	117.15
Max Lat G	1.25	1.29	1.34	1.37	1.42	1.43
Max Accel	0.4	0.41	0.41	0.42	0.42	0.42
Max Decel	-1.41	-1.49	-1.59	-1.63	-1.7	-1.71
Lbs	2463	2463	2537	2537	2611	2463
Hp	197	197	197	197	197	186
Cd	0.45	0.46	0.45	0.49	0.48	0.48
Cl	0	0.2	0.5	0.6	0.9	0.9
Aero Efficiency	0	0.43	1.1	1.22	1.86	1.86

The fastest configuration is #5, the big wing and splitter, with added weight. It’s .1 seconds faster than #6, which has the same power/weight ratio, but is lighter. And that’s expected, because you need power to overcome drag, and that power results in a 1.2 mph top speed advantage. Notice, however, that the lighter car has slightly more grip, 1.46g vs 1.45g, and so OptimumLap figures that part correctly.

In addition, Gridlife allows the 2611 lb car to use a wider tire than the 2463 lb car, and so the heavier car would have wider rubber as well. Finally, if you win a GLTC race they add 150 lbs to your car. That’s a larger penalty to a lighter car than a heavier one. If you can exploit the rules, lighter is not always faster!

Configuration #3 is interesting, with the splitter and GLTC-250 wing, it beats the no-aero car by over 1 second. In the real world, it might have good aero balance and be a good choice overall. Hard to say without real-world testing tho. Oh hey, Justin did that.

Results

Justin tested both configurations of wings at Gridlife, and you can watch the Gridlife live broadcast and find him. He had to add 75 lbs of ballast to use the bigger wing to make the race weight, and so this is a great real-world A/B test. Justin sent me the data, and I’ve marked the points of interest on the speed trace. The 250″ wing is red, the 9LR wing is blue, and I’m using three laps from each to normalize the data. Let’s jump into it.

Exhibit A – This is Turn 1, a fast left hander. The bigger wing has more downforce and grip, which translates into a higher corner speed, and that pays dividends on the next straight. Despite having more drag, the bigger wing is faster all the way down to the Keyhole.

Exhibit B – Turn 2 is a slow and long right hander. The small wing is making about 11 lbs of downforce here (probably less considering the low Reynolds number), which is negligible. The 9 Lives wing is making 2.5 times more downforce, and while this is only about 1.1% more grip, it’s significant because of the long straight coming up.

Exhibit C – You’d think the smaller wing would have a higher top speed, and on average, it does. The difference is just over 1 mph, 124.83 vs 123.56 mph.

Exhibit D – Through the esses, the area under the curve is greater with the bigger wing, meaning more grip. If you look at the time graph on the bottom, the esses are a big chunk of the time difference.

Exhibit E – The entry of Turn 9 requires confidence, and Justin is able to trail brake better through here. Notice the hockey stick shape is the same for blue and red, but the blue is done at significantly higher speed. That extra confidence in corner entry means a higher min speed, and he’s faster through Thunder Valley.

Exhibit F – In low-speed corners, the blue and red lines are close together. The faster the corner, the more they diverge. Simply put, the higher the speed, the more the big wing is at an advantage. You can see this clearly in T1, and again here in T11.

Exhibit G – The final corner is a low speed carousel where the little wing can’t do shit, and the big wing can. If you ever think to yourself 1% more grip is nothing, think again.

Exhibit H – The table in the upper left corner shows peak values for speeds. I should really do one of these for lateral Gs, because that’s where the real difference lies, but the results are so obvious I’m not going to add another graph.

Final Conclusions

Real-world testing confirms that a large wing with a 3% penalty to lbs/hp is faster than a smaller wing in the “free” 250 sq-in size. Simulations predicted that the big wing would be about a half second faster, while in the real-world, it was a difference of about 1.25 seconds. Justin reported that the bigger wing gave him MUCH more confidence, and that’s probably where the extra time came from.

Certainly there’s some noise in the data, but Justin is a really consistent driver. I looked at 11 laps from each wing and calculated theoretical best laps from a four-sector map, and the data was virtually the same. So while the numbers might be off a bit here and there, the trend is clear.

Is there a legitimate usecase for a smaller wing? Possibly. There’s been some chatter online postulating that a 250 sq-in wing on a very light car, or perhaps using a fastback, would somehow be better. Those things would add more grip, but think about it for a minute… a lighter car, or a Miata with a fastback would get even more out of a larger wing! If anything, the gap would increase and show that the larger wing was even better than the small wing.

It’s also worth noting that we’re comparing a 250 sq in wing with a 67″ 9LR wing which is only 87% of the area of the 701 square inches allowed in the rules. If we were comparing the rules limits of 250 vs 701, you’d see an even bigger gap.

However, there’s something compelling about getting something for nothing. If you don’t have aero now, and you can’t/won’t add ballast or detune your car, then a free spoiler or wing is going to be faster than nothing. With a clean sheet of paper I’d choose a different wing profile optimized for high lift at low Reynolds numbers (Selig 1223 RTL), make it 40″ x 6.25″ to maximize chord at 250 sq-in, add oversized end plates with Gurney flaps and slats and what not, get the wing high up and centered over the rear wheels as much as possible, and then brace myself for disappointment.

As a side note, it’s kind of neat to see that simulations predicted a 1:36.36 lap time, and Justin did a 1:36.872. OptimumLap can’t factor in things like elevation changes and camber, so I usually have to create a “fudge factor” by baselining off Spec Miata lap times and then I change the surface grip in the simulator to get more accurate predictions. With the Mid Ohio map, this wasn’t necessary.

Appendix

I ran several more simulations on a lightweight NB Miata to see what happens to power, weight, and aero using variables I didn’t use before. New variables include stock aero, weight changes, spoiler, and fastback.

One interesting matchup is that if you use the standard hardtop, a spoiler beats the small wing by a quarter second. If you use a fastback, the small wing beats the spoiler by three-quarters of a second.

Car	WHP	Weight	Lbs/Hp	Cd	Cl	Lap
HT stock	170	2104	12.375	0.45	0.3	99.33
HT stock (2251#)	182	2251	12.375	0.45	0.3	99.06
HT stock (2601#)	210	2601	12.375	0.45	0.3	98.59
HT spoiler	170	2104	12.375	0.5	-0.15	98.09
HT spoiler, splitter	165	2104	12.75	0.493	-0.45	97.29
HT 250-wing	170	2104	12.375	0.46	0	98.34
HT 250-wing, splitter	165	2104	12.75	0.45	-0.3	97.49
HT 701-wing	165	2104	12.75	0.49	-0.6	96.76
HT 701-wing, splitter	160	2100	13.125	0.483	-0.9	96.00
FB	170	2104	12.375	0.38	0.34	98.92
FB, spoiler	170	2104	12.375	0.43	-0.07	97.89
FB, spoiler, splitter	165	2104	12.75	0.423	-0.37	97.06
FB, 250-wing	170	2104	12.375	0.39	-0.15	97.35
FB, 250-wing, splitter	165	2104	12.75	0.38	-0.475	96.40

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.