Gridlife Touring Cup Wings and Aero

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.

Cheap wing with modified bottom profile.

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.

fx72150b-il at 120% thickness, -5 degrees, upside down, and reversed.

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.

Reynolds numbers 200k and 500k with Ncrit 5.

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.

250 sq-in wing doesn’t look half bad on Justin’s car.

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 wing623 sq-in wing
50 mph11 lbs28 lbs
62 mph17 lbs42 lbs
82 mph30 lbs73 lbs
Downforce at cornering speeds

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. I know this totally contradicts your aeronautics courses, but they were talking about flying, and we are never getting off the ground.

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.

Simple spoiler might work better

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.)
Vehicle1. No aero2. GLTC wing3. GLTC wing, splitter4. 9LR wing5. 9LR wing, splitter6. 9LR wing, splitter, detune
Lap time1:37.351:37.871:36.321:36.361:35.931:36.03
Min Speed47.9648.1948.5248.6348.9449
Max Speed121.89121.31120.63119118.35117.15
Max Lat G1.251.291.341.371.421.43
Max Accel0.40.410.410.420.420.42
Max Decel-1.41-1.49-1.59-1.63-1.7-1.71
Lbs246324632537253726112463
Hp197197197197197186
Cd0.450.460.450.490.480.48
Cl00.20.50.60.90.9
Aero Efficiency00.431.11.221.861.86
Simulations

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.

Mid-Ohio speed trace.

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.

That’s fast company, nice job Justin!

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.

CarWHPWeightLbs/HpCdClLap
HT stock170210412.3750.450.399.33
HT stock (2251#)182225112.3750.450.399.06
HT stock (2601#)210260112.3750.450.398.59
HT spoiler170210412.3750.5-0.1598.09
HT spoiler, splitter165210412.750.493-0.4597.29
HT 250-wing170210412.3750.46098.34
HT 250-wing, splitter165210412.750.45-0.397.49
HT 701-wing165210412.750.49-0.696.76
HT 701-wing, splitter160210013.1250.483-0.996.00
FB170210412.3750.380.3498.92
FB, spoiler170210412.3750.43-0.0797.89
FB, spoiler, splitter165210412.750.423-0.3797.06
FB, 250-wing170210412.3750.39-0.1597.35
FB, 250-wing, splitter165210412.750.38-0.47596.40
Various configurations using hardtop (HT) and fastback (FB).
miatahubs.com

One thought on “Gridlife Touring Cup Wings and Aero”

  1. Forget aspect ratio for a moment, it’s less important for car wings because we use end plates. Instead, look up wing profiles and how they compare at low Reynolds numbers. As the Re number goes up, the efficiency of the wing goes up. Wings don’t work well at low Reynolds numbers.

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