There’s a scene in The Road Warrior where Max hits a switch on his shifter and his supercharger spins up, giving him an extra boost of speed. I’ve always wanted something like that for my Miata. Or rather, I want something more effective than turning off the A/C.

The aerodynamic equivalent of that is a DRS system, like on Formula 1 cars. In F1, the upper wing of a dual-element wing can pivot into two positions, one for downforce, and one for low drag. It works, and makes F1 racing a lot more fun to watch. But you don’t see active aero much in other racing series, which is a shame, because racing improves the breed.

Since active aero is largely banned in racing, most of the advancements are happening on street cars. The first street car with active aero was way back in 1986, the exotic and unattainable Porsche 959. But just two years later you could get an active spoiler on a pedestrian Volkswagen Corrado.

And two years after that, the Mitsubishi 3000GT had active aero on both ends! A Russian magazine did some testing and reported that front lift reduced by half, and rear lift became actual downforce. This reduced top speed by 5 mph, which is an acceptable trade.

Those cars are now considered vintage, and on modern supercars like the Pagani Huayra, active aero is much more sophisticated.

Given unlimited resources and time, I’d develop a system that changes around the track based on GPS points: On the straights the aero would settle into the position of least drag; In braking zones the aero devices would move 90-degrees to the wind and behave like a parachute; In corners, the aero would pivot into the position of max downforce. I’d also have rudders and vanes on the sides of the car to help bank it into turns, loading up the tires differentially and using air resistance for turning.

But bringing this back to reality, I’d start with something simple, like the VW Corrado spoiler. I’ve got a pair of Miata pop-up headlight motors on a shelf that I’m saving for just such a project. I’d hook them up to the headlight toggle switch on the dash, which would give me a high downforce setting I’d use for most of the track, and a low-drag DRS setting for straight line speed.

Before I embark on that journey, I’m going to look at a few aftermarket active aero systems, and then run some simulations to see how much active aero is worth.

NINTE Lifting Spoiler

I’d never heard of NINTE before writing this article, but apparently they make active aero for many sedans. You can tell from the shape and the integrated 3rd brake light that this is not a wing – they really don’t care what the underside is doing.

The spoiler rises up automatically when speed reaches 60 km/h, and descends automatically after 10 seconds when the speed drop down to 30 km/h. You can also control it manually.

In general, I’m a fan of spoilers on street cars, because they reduce drag and lift, and cars look better with them. Aesthetically, I like simple spoilers and this one is not. But at $850, it’s not outrageously expensive for something that could be fun and different.

Active Miata Wing

Carbon Miata sells an active wing for Miatas. It’s a three-position system for DRS, normal, and airbrake. The motors and levers are exposed and the entire thing has a DIY look about it. Normally I’d dig that, but if I was shelling out $3k, I’d want a more professional look. I think they could have hid the motors on the underside of the trunk lid or something.

The airbrake mode is interesting, it tilts the wing to 90 degrees and while I believe it would be effective, it also scares the shit out of me. With the wing in airbrake position you’d lose rear downforce, but you’d move the center of pressure rearward and get a lot of drag to boot. I wonder how much stress the system can take, and if it’s designed for fast racetracks or just autocross.

The carbon wing looks quite nice, and even if you never used the airbrake mode, the standard setting and DRS would be useful.

TRK1 Active Aero Smart Wing

If you have $3500 bucks lying around, you might want to try the SPT 1 Smart Wing. The wing comes in different sizes from 58″-70″, and has five downforce settings, low-drag, three downforce levels, and airbrake. The downforce levels can either be preset or programmable.

If you opt for the presets, you get three downforce levels: 10 mph, 50 mph, and 80 mph, plus the low drag and airbrake settings. If you buy the CPU version, you can program the wing to change angle at any three speeds you want.

For example, you might set the wing for low downforce at speeds up to 40 mph, to help the car rotate in slow corners, then transition to a maximum downforce setting up to your fastest corners, say 80 mph, and then start dropping wing angle at 100 mph, and go into full DRS mode at 120 mph.

You set the airbrake based on longitudinal Gs. Now I imagine this one would be a little tricky to set up, because just lifting off the gas will give you around .3 G of deceleration, and there are times when you want to lift and brush the brakes slightly, but still retain maximum downforce and grip. In such a situation, you wouldn’t want the wing to pivot into airbrake mode. So maybe after some experimentation you’d find that around .6 or .7 G translates to straight line braking, and that’s when you’d activate the airbrake.

It all sounds very high tech and fascinating, and one wonders what it would be worth in a lap time. The manufacturer claims that their single-wing DRS system was 10 mph faster than a fixed wing when tested on a Shelby GT350 at COTA. I’m not going to call total bullshit on them, but if F1 cars get only 7 mph out of a double-wing DRS system, it does sound a little far fetched.

OptimumLap simulations

This is Occam’s Racer, where we do simulations and pretend it’s meaningful. In this make-believe world, I’ll do the simulations at Watkins Glen. The straights are long enough to use active aero, and in the real world, the track is only 25 miles from me, so I might be able to test this for realz one day.

The first time I ran the simulations, I cheated: I simply took a wing and reduced its drag value, reckoning that this was the same thing as active aero. It’s just less drag, right? Wrong. I immediately saw my error because the active aero wing had a lower top speed than the low-drag wing. The reason is, the car weighs less in when the wing is in a low-drag setting. Wings make downforce, which is the same as weight. So I had to scrap everything and start over.

To properly simulate active aero, you have to take into account not only drag reduction, but weight reduction. To do this required increasing mechanical grip of the low drag wing, such that it matched the exit speed of the high downforce wing. And then I had to determine the amount of time from corner exit to the braking zone between the active wing and the low drag wing. Good fucking god that was a lot of work!

And not to get too far ahead of myself, but the values were tiny, and difficult to see in the data. However, in doing these simulations, I found out something useful, which is that you can easily zoom in or out in OptimumLap. If you draw a rectangle on any of the charts starting in the top left and drag to the bottom right, it zooms in. Do the reverse and it zooms out.

Using this method, I was able to zoom into the time-distance graphs and get the exact amount of time it takes to cover any amount of distance on track. By subtracting starting and ending points, I then got the delta for how much faster the low-drag setting was. Ugh, tho.

Single element wing

On a Miata, you typically run a single element wing at 3-5 degrees angle of attack. Air comes down the roofline at an angle, which puts the middle of the wing at around 10 degrees. Most wings will stall at more than 10 degrees, and so you don’t want to set the wing with much more angle than that. A wing set like this adds 3 points of drag, so if your Miata has a Cd of .45 without a wing, it’ll be .48 with a wing.

In the low-drag wing configuration, the car would have a Cd of around .465. This is really the best you can do on a 2D wing, as the ends of the wings are always 5-7 degrees offset from the middle of the car, and a properly set up wing doesn’t have a lot of drag anyway.

Watkins Glen has eight potential DRS zones: the front straight, the back straight, and the short straights between the corners. In those DRS zones, I’d have to push a button to lower the wing after exiting each corner and then push the button again to raise the wing before the start of every braking zone. Sixteen button pushes would take some concentration, and I might find my lap times were worse using DRS.

Instead I’m going to say that there are three DRS zones: front straight, back straight, and between turns 7 and 8. I’ve added 10 lbs to the car with active aero, to account for motor and levers.

I’ll simulate going around the track three times. Once with the wing fixed in the low drag position, another time with the wing fixed in the standard downforce position, and then using active aero in the three DRS zones. Here’s the lap times:

• The slowest configuration would be the wing set to the lowest drag setting. This corresponds to the wing set for maximum efficiency, which is why choosing a wing or angle of attack based on its efficiency is meaningless.
• Setting the wing to the standard 3-5 degrees angle of attack makes the L/D ratio of the entire vehicle the most efficient. This is faster than the low-drag setting by almost 3 seconds.
• Finally, the active aero wing would be about 0.36 seconds faster than a fixed wing. Now wait a goddamn minute… that’s it? Yep.

Single element wings are efficient at producing downforce and do so with very little drag. In fact, a wing has about the same amount of drag as your two side mirrors combined. Thus, active aero on a single element wing is about as effective for drag reduction as if you moved one of your mirrors inside the car on the straights.

Given that, is active aero worthwhile? For causal lapping at a track day, notsomuch. In a racing situation, sure.

At Watkins Glen, the back straight is where drag reduction makes up the most time. From the exit of Turn 2 to the braking zone of the Inner Loop is about 3000 feet. Setting the wing into the low-drag position makes the car go 1.5 mph faster, and gains 0.17 seconds per lap over a fixed wing.

But a better way to visualize that is that DRS gains about 25 feet on the car with a fixed wing. So with two evenly matched cars racing close together on the back straight, the car with active aero should be able to pass the car with a fixed wing.

And that’s pretty much how it happens in Formula 1. The trailing car activates DRS and makes a pass on one car at the very end of a straight. Speaking of F1, wouldn’t a dual element wing be a better usecase for active aero?

Dual element wing

Dual-element wings aren’t as aerodynamically efficient as single wings. As you add elements, you gain downforce, but you gain drag at a higher rate. However, because the wing contributes a very small amount to the overall drag of the car, the aerodynamic efficiency of the vehicle is better with a dual wing. As I’ve simulated it, the aerodynamic efficiency of the single-wing car is 2.08, and the dual wing is 2.98.

The first simulation I’ll run is a fixed single wing versus a fixed double wing. Despite a 2 mph slower speed on the back straight, the double wing is about 1.5 seconds faster than single. It makes up time in every corner and keeps that speed onto the shorter straights.

So now I’ll make the upper wing active and it goes .60 seconds faster than a fixed dual wing. Here’s the lap times:

That’s not too shabby, and most of the gain is on the back straight. Activating DRS on the run from T2 to the inner loop is worth .35 seconds alone. This is a gain of over 50 feet of track on a car with a fixed dual wing. So as you can see, active aero makes more sense for a dual-element wing than a single wing.

So how come active aero isn’t as effective as we see on TV? Mostly because F1 cars are so damn fast. Drag force doubles with the square of speed, so there’s four times as much drag at 200 mph than 100 mph. At autocross speeds, drag is inconsequential, but at high speed it’s a game changer. Even so, on a F1 car, DRS is only worth about 6-7 mph. So it stands to reason that a dual-element DRS wing might be worth only 1.5-2 mph on our cars.

New York Safety Track

Watkins Glen is a long, fast track with a higher top speed than any other track I’ve been on, and represents a DRS best-case scenario. I also simulated what would happen at New York Safety Track, because it has shorter straights and a lower top speed, and is more similar to the average road course. I also coach at NYST several times a year, so if I build a DRS system, I’ll be able to test it in the real world as well.

NYST has just one DRS zone, the 1400′ front straight that they use as an airstrip for small planes. I only simulated the dual-element DRS wing, as it was the most effective.

If I activated the DRS system at the exit of T18, I’d gain .058 seconds to the Start/Finish line, and from there to the braking zone for T1, I’d get another 0.062 seconds. All told, DRS would be just 0.12 seconds faster than a fixed dual wing. In terms of top speed, it’s a difference of less than 1 mph. That’s enough to make up only one car length against an evenly matched fixed wing car, and would be of dubious benefit.

Conclusions

Active aero is typically banned at the club racing level, and after doing this theoretical investigation, I’m inclined to agree with that restriction. Active aero adds weight and complexity, and mechanical things fail mechanically; I can only imagine the jank that me and other DIY pioneers would litter the track with.

Now if you’re racing in 24-hours of Lemons, go ahead and make an active aero system and see if it’ll pass tech. Or build an anti-aero device that makes a jack-in-the-box pop out of your roof. But for serious racing, you’ll achieve more by reducing drag anywhere else on your car.

Think about it: if you want to reduce the weight of your car, you don’t look at all your carbon fiber components and try to lighten them. You go after the parts that weigh the most.

Wings are already designed for optimum performance and low drag. Reducing drag on what is already the most aerodynamic part of the car, is as silly as trying to lighten what is already made out of carbon fiber. Your hardtop, cooling system, mirrors, wheels, rear surfaces, and everything else on your car are better places to concentrate on drag reduction.

For HPDEs, there’s even less to be gained with DRS than in racing. Moreover, I ran these simulations at a very fast track with three long DRS zones. Most race track have fewer and shorter straights, and DRS would be worth only a tenth of a second.

Just the same, I’m pretty sure I could build a robust DRS system. And I have those headlight motors just sitting there on a shelf. And it’s still over half a second at WGI that I’m not going to get anywhere else….

Addendum: Spoiler

The first simulations I ran were with spoilers, but once I realized that I’d made incorrect assumptions, I didn’t bother recalculating the lap times to the same level of detail as I did with the wings. However, someone might be curious about the effectiveness of spoilers and this data is close enough.

• A low spoiler makes the car about a second faster than a car with no rear aero.
• A tall spoiler is about a second faster than a low spoiler.
• An active spoiler is .28 seconds faster than a tall spoiler.

If you read my previous posts on 3D wings, you’ll know that a 3D wing designed for the Miata’s roofline shape is marginally better than a 2D wing. I’ve included the 3D wing data and a car with no rear aero in the summary data below.

In this article I’m going to compare spoilers and wings, from cars that have used both, to the effects of trailing car aerodynamics, to when you’d choose one vs the other (or both) based on different rulesets.

Note that I wrote this article before my recent tests in the wind tunnel, where I was able to test a Blackbird spoiler at three different heights, versus wings from 9 Lives Racing, Wing Logic, and others. The test data and conclusions are in my Miata Wind Tunnel report, along with many, many other tests, which you can purchase here.

First things first: some people get confused about the difference between a spoiler and a wing. Air goes over the top of a spoiler; Air goes on top of and underneath a wing.

This is a spoiler.

This is a wing.

This is a wing, but it’s mounted so close to the trunk that it behaves like a spoiler.

Key differences

If you look at the aerodynamic efficiency of a spoiler, most aerodynamic texts show they are around a 3:1 lift to drag ratio. So if a spoiler creates 30 pound of downforce, it’s also creating 10 lbs of drag. In my own wind tunnel testing, I’ve seen spoilers range 2:1 to 11.5:1, depending on the shape of the car and the height of the spoiler.

Wings typically have higher lift/drag ratios, and depending on the shape of the car, can range from 3:1 to 24:1. But around 8:1 is a normal range. Wings are more efficient because unlike a spoiler, air goes under the wing, and it’s the underside that’s doing most of the work. So if the top of the wing is generating 20 pounds of downforce via pressure, the bottom side is generating 60 pounds underneath due to suction, at the same 10 lbs of drag. That’s a simplification to illustrate a point, the actual numbers depend on wing angle, airfoil shape, etc., but just know that the low-pressure region under the wing is what’s important.

The low pressure area is often about the same height as the chord. Meaning, if you have a wing with a 10″ chord, you don’t want to mount it any closer than 10″ to the decklid, or the wing loses performance. Now this is only a guideline, because cars with diffusers might want to mount the wing lower to extract more from the diffuser.

I’ve seen a lot of poorly mounted wings, mostly due to people thinking the top of the wing does the work. Another common error is too much wing angle, from not taking the roofline downwash angle into account. And then there are the low-performance wings that are largely cosmetic; pretty much any wing with a rounded trailing edge is a piece of shit.

As a practical matter, wings are more expensive and complex than spoilers. Initially you need to figure out the height and setback distance to extract the most performance, and then you might have to compromise with trunk access. Then there’s the question of Gurney flap size, after which you’ll probably mess with wing angle ad infinitum. Some people enjoy that kind of thing (guilty), but wings are not ideal for the set-it-and-forget-it crowd.

Spoilers on the other hand are dead simple. They are cheaper, lighter, and easier to mount than a wing. Small spoilers (less than an inch) are great for street cars, as they reduce drag and add downforce for free. But for racing, you want more downforce than a small spoiler can give you.

So yeah, let’s talk racing. What are the effects of drag and lift when using a spoiler versus a wing, and what happens when following a car with one or the other? Let’s start this investigation by looking at some cars that had both spoilers and wings on the same body.

Spoiler vs wing on a Mazda RX-7

A good apples-to-apples comparison is the 1990 RX-7 IMSA GTO race car. The svelte body kit included a small splitter, airdam, and spoiler. I’m still trying to figure out what the B-pillar vent was for, but those sexy extractors at the front wheel arches that blend seamlessly into the side skirts…. so fucking hot. But I digress.

The car originally came with a spoiler, and had a coefficient of drag (Cd) of .51 and a coefficient of lift (Cl) of -.44, for an aerodynamic efficiency of .86. Later the spoiler was replaced by a wing, resulting in a Cd of .48 and a Cl of -.53, for an aerodynamic efficiency 1.10.

In comparative terms, the wing produced 6% less drag and 17% more downforce, for a 128% improvement in aerodynamic efficiency. OK, but what does that mean for a lap time? Let’s find out.

If you follow my blog you know I like to quantify things in OptimumLap. So I’ll build the exact same car, and then change the aero values for drag and lift. The IMSA car had a four-rotor Wankel that put out 600 hp, which is a bit unrealistic for most of us, so I’ll also run another simulation with the engine detuned to 200 hp. I’ll simulate them at my home track of Watkins Glen, and see what happens. (Note that I chose 200TW generic values for tire grip, so the lap times aren’t meant to represent real-world lap times. The important part is the delta in lap time, by changing the aero.)

With the 600 hp engine, the wing was 1.08 seconds faster than the spoiler. With the 200 hp engine, the wing was .89 seconds faster than the spoiler. So on average, the wing version is about a second faster than a spoiler.

But I think there’s more that could have been done with the wing. If you look at the following pic, you can see the wing is the full body width of the car, which is the maximum width in a lot of wheel-to-wheel racing rules, but the wing is mounted quite low, and would make more downforce if it were higher. There are reasons to run a wing this low, but that has to do with extracting more from a rear diffuser, which this car doesn’t have.

Borrowing CFD from the JKF Aero course I took, putting the wing higher would result in .043 more downforce and .005 less drag. If I re-run the simulations with those values, the powerful car drops another .46 seconds, and the 200 hp car goes .29 seconds faster. Adding that all together, the wing is faster than the spoiler by 1.54 seconds with the 600 hp engine, and 1.18 seconds faster with the 200 hp engine.

What would you give to be 1.2-1.5 seconds faster than your competitors? I’d give my left nut for that. (But I’m already fixed down there, so they are merely decorative at this point anyway.)

OK, so on this car, you clearly want a wing and not a spoiler. Which is precisely what the IMSA team did, and they dominated. So are there any other cars that had both wings and spoilers on the same body?

Spoiler vs wing in NASCAR

If you think the aerodynamic package on a NASCAR racer is crude, you’d be wrong. The bodywork is highly developed and there are numerous aerodynamic tricks. One source cites drag and lift numbers of Cd .39 and Cl -.46. If you compare that to the IMSA RX-7, you can see that the stock car has a lot less drag, and the downforce value is between the spoiler and wing versions. Put both bodies on the same chassis and a NASCAR stock car would go faster than the RX-7 with a wing. You can read about that here.

So if NASCAR stock cars are sophisticated aerodynamic missiles, why don’t they use wings instead of spoilers? Well, for a brief period of time, 93 races to be exact, they did. This was in an era where the car was dubbed the “Car of Tomorrow,” and amongst many other changes, there was a rear wing.

The airfoil NASCAR chose was for low drag and high speed, and they mounted it low on the trunk. NASCAR is primarily concerned with the spectacle of close racing, so performance was not their driving factor.

So why did NASCAR get rid of the wing and go back to the spoiler? Safety and aesthetics, mostly. The safety issue was this: cars that spun 180 degrees went into the air and flipped upside down. Spinning at 180 mph isn’t something that happens to most of us, but it happens a lot in NASCAR. Also, fans hated the look of the wing and demanded the spoiler back.

There’s also the fact that the racing was better with the spoiler than the wing. Fans want drafting, slingshot passes, and trains of cars moving through the field. There were some good races in the COT wing era, but spoilers made for closer racing than wings.

Trailing car aerodynamics

Racing history is full dominance, where one car is so technically superior that it lines up in pole position every time and is never headed during a race. While those eras are memorable, they are boring to watch. Close racing is much more fun, and so rules are changed all the time to control costs, especially aerodynamic costs, and achieve parity.

If you follow Formula 1 rules, you’ll know there was a big rewrite in 2022. Just like NASCAR rule changes, the purpose was closer racing. The main problem was the “dirty” wake created by the lead car, such that trailing cars lost downforce, and even with DRS, they had a difficult time passing the lead car. Among the changes for 2022 were a rear wing designed to push the aerodynamic wake up and over the car following behind. The front wing, body, and wheels also had wake deflectors and other gizmos so that the trailing car didn’t lose as much downforce. All of this was so that cars could draft each other better.

(If you are reading between the lines here, then you know it’s possible to create an aerodynamic package that makes it difficult for other people to follow you. If I had any concrete information on how to do that, I sure as shit would not be posting this publicly. Am I developing such a package on my race car? Maybe.)

Drafting

Whether you’re talking about bicycles or NASCAR, racing on oval tracks is a game of drafting. The leading vehicle punches a hole in the air, creating a low pressure wake behind it. The person behind the leader can get in that wake and gain straight line speed. How effective is drafting? At the 2023 IMSA race at Daytona, MX-5 Cup cars were about 4-5 seconds faster per lap when they were drafting, rather than driving on their own.

As drafting relates to spoilers vs wings, take a look at the CFD below, comparing spoiler (top) to wing (bottom). Notice how the wake of the spoiler is both higher and longer than with the wing. If we’re racing against each other, and your car has a spoiler, thank you – I’m all up on your ass.

When is a spoiler better?

Wings have more downforce and less drag than a spoiler, and if your racing rules allow one or the other, you’d chose a wing every time. So are the any instances where a spoiler is better than a wing? Kinda.

Racing spectacle

From the spectators perspective, less aero is better. Wings don’t work as well in turbulent air, and so the trailing car loses downforce and stability. This makes passing more difficult, the cars spread out more over time, and it’s just not as fun. Watch NASCAR, Spec Miata, or really any non-aero series and you’ll see more drafting, with closer and better racing.

If I was designing a racing series from scratch, I would absolutely spec a spoiler over a wing. The Superspec Cup series in California (nee Supermiata) does this, and for some reason it hasn’t caught fire like everything else in California. Maybe another series (ahem, Grid Life 18:1 enduro class) could adopt their aero rules? Spoilers are cheap, they make street cars look like race cars, and are safer, as they cancel out the rear-biased lift generated by virtually all street cars.

Street

Beauty is subjective and so this either applies to your or not, but I don’t like the look of most wings on street cars. OEM wings are typically cosmetic and don’t do shit. Big wings look gaudy, and invite too much attention from cops and wannabe racers. Small wings are stupid and useless. The only wings I like the look of on street cars seem to be on Porsche 911s. Well, I like the 9 Lives Street Wang a lot, but it reminds me of a P-car whale tail, so that’s the same damn thing.

On the other hand, I like the looks of a spoiler on pretty much any street car. A low spoiler (less than 1″) reduces drag and lift, so a car will handle better and get higher the car milage than the same car with or without a wing. For a street car that gets occasional track use, a taller spoiler is appropriate. Get one that is adjustable for height/angle and you have the best of both worlds.

Autocross

SCCA national autocross aero rules were written by people who were afraid of or didn’t understand aero, and so they don’t allow wings until you get to the Modified category. Once you get to that category you’re allowed a dual-element wing with 8 square feet of area, which is absurdly large.

On a Miata, this would be a 12″ chord main wing and 6″ upper wing, which is about twice the amount of area you’d see on a typical track Miata. A car set up thusly would have so much rear-aero balance it would have criminal amounts of understeer on a race track.

Now those are the national rules, and at the regional level there’s an Xtreme Street category for track cars with wings. However, the rules allow the same ridiculous 8 square-foot wing as the Modified category.

There are no national or regional rules that have concessions for sane people who want to do both track driving and autocross with normal sized wings. So if you aren’t building a car specifically for the parking lot grand prix, you might be better off racing in the Prepared category. In which you are allowed an absurdly tall 10” spoiler.

I just don’t get SCCA autocross rules, it’s as if they’ve never seen a wing or spoiler? Who drives around with a 8 foot wing or a 10” spoiler? The hilarious thing is they allow a gigantic wing, but then restrict what you can do on the front. You can’t even put an end plate or fence on your splitter.

Anyway, for casual autocross, I’d wager a spoiler is better than a normal-sized wing. Not only because SCCA autocross rules are fucking stupid, but because a spoiler might actually be faster around your mall parking lot. I tested a spoiler and wing at Pineview Run, and I found the spoiler was half a second faster than the wing in A-B-A testing. Pineview Run is a tight and twisty track, with many fast changes of direction; it’s a lot like autocross.

Why was the spoiler faster? Because the wing added 14 lbs, at roof height, at the far end of the car, and this creates a higher center of gravity and more polar inertia. If you’re unsure of what that is, take a broom and hold it out in front of you and “slalom” around your house. Now pull the broom in tight to your chest and do the same thing.

Mass centralization is important for handling, and when you have weight high up at the far end of the car, it makes it more difficult to change direction. So even if the wing was performing statically better (more downforce and less drag than the spoiler), it was slower than the spoiler.

Grid Life Touring Cup

GLTC rules allow a 250 square-inch wing or spoiler for free. Justin Lee and I tested a 250 sq-in wing versus a 9 Lives Racing wing, and it was clear that the larger wing was faster, even if it did require a 3% lbs/hp penalty. Moreover, the 9 Lives wing wasn’t the full 701 sq-in size that the rules allow, or the smaller wing would have fared even worse.

One of the reasons for that is that a 250 sq-in wing has a very small chord. For the most part you can ignore Reynolds numbers (which you can think of as low speed, small chord, or both), but most airfoils don’t perform well at low Reynolds. The following image shows the airfoil efficiency of the CH10 airfoil at different angles of attack, with 200k (brown) and 500k RE (blue). This graph is essentially the difference in efficiency between a 250 sq-in wing and 625 sq-in wing at the same speed (100 kph, 62 mph).

I tested a 250 sq-in wing vs a 250 sq-in spoiler in a wind tunnel, and it was an even match. A 250 sq-in spoiler has more surface area because it’s using not just the blade, but the entire decklid to aggregate pressure (downforce). Consequently, the spoiler should have a much larger Reynolds number, which would be less affected by low-speed aero losses.

However, as we already saw, a spoiler creates a larger and higher trailing wake, making it easier for cars to follow you. So while a 250 sq-in spoiler might turn a better lap time than a small wing, the wing might actually be better for racing. I don’t know the answer to this question, and I’m unlikely to test it.

Why? Because regardless of which “free” option you choose, a spoiler or small wing, a bigger wing and a 3% penalty to lbs/hp ratio has already been proven superior, so I don’t know why anyone racing in GLTC would consider a small spoiler or wing to begin with. Just to be contrary? Or because you like losing?

Convertible

If you drop the top on a convertible, it destroys a wing’s performance. If you were making 100 lbs of downforce with the top up, you’ll be at 40 lbs with the top down. That’s not conjecture, that’s hard evidence.

So, if you have a Miata or other convertible, and you’re dedicated to the drop-top, a spoiler is probably better. This isn’t total guesswork on my part, but conclusions drawn by Kyle Forster in the video Do Rear Wings Work on Convertible Race Cars. With the top down, the spoiler lost less performance than the wing, not just because there’s less performance to begin with, but a spoiler just doesn’t seem to be affected by turbulence as much as a wing.

Spoiler and wing together?

So what about using a spoiler and a wing at the same time, is this a Nuts and gum, together at last situation, or the best of both worlds?

Most club racing and time attack rules don’t allow you to use both a spoiler and a wing, you have to choose one or the other. And you’d choose the wing, natch. But if the rules allow it, or if you have a HPDE car that doesn’t have to conform to a ruleset, then using both a wing and a spoiler is like peanut butter and jelly.

A spoiler helps extract more out of the wing in a similar manner as a second wing element. Air kicking upwards can activate the trailing edge of the wing, which in turn can allow you to run more wing angle without separation.

I tested a wing with a spoiler and without in the wind tunnel, and the spoiler added downforce and drag, as you’d expect. But the resulting L/D ratio was good enough that you’d want to use it on anything but a high speed oval.

The spoiler will also raise the height of the rear wake, which pushes the rear of the car down. If you have a rear diffuser, that upwards airflow will help extract more from the diffuser as well. Win-win, hallelujah, and let’s see more of that on something other than a Noble M12.

Author’s choice

After all this investigation, you might wonder what the author uses on his two Miatas.

• My street car has a Galvez spoiler. It’s a neat design that mounts easily, but I felt it could be both taller and more rigid. I made a new spoiler blade that is taller and narrower, and matches the profile of the roofline more closely.
• My race car has a DIY spoiler and a 41×16 S1223 wing mounted via the end plates. I have rivnuts in the trunk lid so that I can quickly swap between a small 1” lip or a more aggressive 4” kicker. Or I can run it without a spoiler when the rules call for that.

I recently completed Race Car Aerodynamics: the Definitive Course, by Kyle Forster. This online course consists of 10 hours of videos, in which Kyle lectures you on aerodynamic fundamentals, and provides case studies of real-world examples. Kyle uses a whiteboard to explain theory, and switches to computation fluid dynamics (CFD) to show specific examples of touring cars and open wheelers.

Because single-seaters (open wheelers) are often quite different than touring cars, Kyle separates much of the content into specific sections for each. I found the single-seater content interesting, but it’s highly unlikely I can put any of it into practice.

The touring cars Kyle examined in CFD were a time-attack Porsche 944, a Mustang, and a new Supra. The 944 was especially interesting, as it had two Venturi tunnels and a huge diffuser. No Miatas, but that’s OK, he has some YouTube content already with Miatas, which I’ll get to in a different post.

I don’t have a lot of experience with video-based learning, and I was initially skeptical, but all in all, I was extremely happy with the course. About half way through the course, I thought to myself, there’s no way this course is only 10 hours of videos! So I opened up a spreadsheet and summed up all the lessons, and indeed, it’s 10 hours.

The course probably took me over 40 hours though, because I’m an obsessive note taker, there are tests (knowledge checks), and hands-on learning with tools to try out (Java Foil, OptimumLap, Race Studio 2), and spreadsheet calculators to mess with.

You can also ask Kyle questions from within in the course, and he answers them in the sidebar. This is a great resource, because you can see the questions other students ask, and some of it is very illuminating. For example, I wanted to know about Front/Rear aero balance, and how that’s different for rear-wheel drive and front-wheel drive cars. No, I’m not going to tell you the answer.

That’s another thing I learned in this course, which is to hold your cards close to your chest. Knowledge is free on Facebook, YouTube, etc, and you get exactly the value that you paid for it. I used to correct people online when they made silly aero mistakes or lead others down the same path. Now I just bite my tongue and/or message them privately.

Some difficulties

The last time I took a math class was in 1985, and I still have nightmares about unfinished homework assignments. There isn’t much math in this course, but it’s on the cusp of what I’m comfortable with. You can skip over the math as long as you understand the principles behind it.

Another minor difficulty was that it’s hard to follow the CFD at first. Kyle often cycles quickly forward and backwards through the pressure plots, and it takes a while before you understand which way he’s going. The pressure plots themselves are an aerodynamic LSD trip, complete with all the vivid colors, confusion, and eventual revelations you’d expect from dropping acid. I get it now, but it took some getting used to.

Kyle is obviously passionate about his work, and sometimes that comes out in a cursor that moves a little too quickly. His computer arrow is small and white, and it can be difficult to pick out at times when he’s moving it on the screen to show a particular area of interest.

But even if there were some difficulties, it’s nothing I couldn’t handle, and by the end of the course I knew what to look for.

The cheat codes

If you simply want pragmatic advice, like how long and low your splitter should be, where to mount your wing, how to optimize airflow through your engine, etc, you can jump ahead to Key Development Areas. In this section Kyle follows airflow from the front of the car to the rear, providing you with all the aerodynamic solutions for your touring car or open-wheel single seater.

Honestly, I don’t want anyone I’m racing against to take this part of the course. This section has all the cheat codes for the game of aero, and if everyone knows this stuff, then the playing field is level. And I can’t stand that kind of parity, I want an advantage!

But since most of you cheap bastards won’t pony up a thousand dollars, I figure the secrets are pretty safe. The course goes on sale occasionally for 30% off, which is how this particular cheap bastard afforded it.

Consultations

I’ve also done six hours of video consultations with Kyle, some of this as a fly on the wall, and some of this on my own car. The way it goes is you send Kyle details on your car, and he analyzes your full aero kit. He points out the good and bad, and what you can do better based on your ruleset.

Kyle charges $175 per hour for video consultation, which is a downright bargain considering he was a Formula 1 engineer. For the best in-depth analysis you’ll need to get your car laser scanned, and then he can do CFD. I don’t know where to do scanning, and I’m not sure I ever will bother with that because I’m not very serious about winning. But if you are, that’s the second step.

If you’re interested in getting a consultation, then taking the course is the first step, it will save you a lot of time in the long run. This way you can get all of the fundamentals out of the way and start optimizing a car that’s done 90% right.

Conclusion

This course was the best money I spent in 2022. I don’t think that will be the case for everyone, but I’m an armchair aero nerd, and it was exactly, precisely what I wanted. I came into this course knowing a thing or two about aero, and all of that background knowledge definitely helped me get more out of this course. But I think the average person with a keen desire to learn could jump right in without any prerequisites.

As I look back on what I learned in this course, and look ahead to the practical ways of putting it to use, I’m super excited about working on version 2.0 of my Miata’s aerodynamics package. Fucking hell, I’m positively giddy about it.

Grid Life is coming to my home track of Watkins Glen International, April 28-30 2023. This is the first time Grid Life will come to this iconic track, and I hope to race in one of the events.

Unfortunately my Miata doesn’t have anywhere near the 12.5:1 lbs/hp limit of Grid Life Touring Cup, and I’m not an A-class driver besides. The end result is I wouldn’t be competitive. Still, I thought it would be fun to run some simulations and see my car’s potential. There have been a few rule changes to balance out engine performance, and I thought this would be a good opportunity to smoke test those as well.

For these simulations I’ll use five different Miatas. Each has an identical aero kit that results in .48 Cd and -1.0 Cl (values I’ve measured on my car), with standard values for air density and rolling resistance. I’ve spec’d the same tires at 1.2g of lateral grip, and 1.1g of longitudinal grip.

Per GLTC rules, all the builds take a 4% penalty to weight because they run a splitter and large wing, but being Miatas, they get some back by having no ABS (-2%) and running wheels 16″ or smaller (-1.5%). The end result is that all the builds take a .5% weight penalty to the standard 12.5 lbs/hp ratio.

Now here’s where it gets interesting: each Miata has a different engine, and this changes the final lbs/hp ratio. Per the latest rules, small-bore engines less than 1999cc get a 1.5% break and 1999-2500cc get a 1% break. Turbos are penalized 1.5%, K24 swaps are a 2% penalty, a 5-liter is a 4.5% penalty, and so on.

I’ll do five simulations based on different engines. Note that I went out and created each engine in OptimumLap using dyno charts I found online, and so the torque curves should be pretty accurate.

• N/A – 170 hp, 2103.75 lbs. This represents a well tuned 1.8 BP. NB2s would have more torque, as I used the dyno graph from my NA8 and bumped it by 141%.
• Turbo – 200 hp, 2512.5 lbs. I pulled this dyno from someone on Miata Turbo. The 1.5% penalty for using a turbo is offset by the 1.5% benefit for under 1999cc.
• Ecotec – 192 hp, 2388 lbs. I used the dyno chart from an Ecotec swap I found somewhere.
• K24 Z3 – 200 hp, 2562.5 lbs. I used the dyno from KMiata’s Z3 blog post. I could have used the A2, but then I’d have to detune it more. Notice the power output is the same as the turbo, the 50 lb weight penalty comes from the K-swap.
• 5.0 Mustang – 220 hp, 2887.5 lbs. Fox Mustang old school V8 Miata.

It’s worth noting that four of the cars came in at less than 2725 lbs, and would be limited to a maximum 245mm average width tire. The 5.o Mustang swap is slightly heavier, and thus could run a 255 tire. To simulate that, I’ve given it a very small bump in grip (from 1.2g to 1.212g).

Before we look at the lap times, understand that I’m not trying to predict an accurate lap. OptimumLap is a tool that’s good at predicting the differences in changes you make to your car, but can’t factor in weight distribution, elevation, camber, surface friction, or other variables. So how did the different engines shake out? Take a look at the speed trace:

• 2:09.11 – Turbo (orange, like 949 Racing)
• 2:09.20 – Ecotec (green, cuz eco)
• 2:09.39 – N/A (red, because mine is)
• 2:09.82 – 5.0 Mustang (blue, for USA int’l racing color)
• 2:09.85 – K24 Z3 (purple, for Miata royalty)

The first thing you might notice is that the red car (N/A) is considerably slower on the back straight. However, because aero works better on lighter cars (the percentage gain in grip is higher), the red car has higher min speeds in the fast corners. I thought the N/A would be last, but it’s right in the middle.

The fastest builds are the turbo and Ecotec, but there’s not much between them. The Ecotec has a great powerband, but the little turbo has a fatter torque curve and wins overall. The slowest cars are the K24 and Mustang swaps, about half a second adrift. These engines are penalized the most, so it’s not surprising.

But what is surprising is how close the times are on such an extreme speed track. I’d say the rules parity is very good right now. Naturally, there are some caveats here, because OptimumLap is a single-point mass calculator, and can’t factor in elevation changes or camber.

And that’s a problem because all but one of the corners at Watkins Glen has favorable banking. If the simulated lap times seem slow, that’s why. If I adjust the grip by 110% and re-run the simulations, two seconds disappears. Notice that the K24 and 5-liter swap places, but the other cars remain where in the same order:

GLTC cars have yet to race at WGI, and so I don’t know if 2:09 or 2:07 is more accurate, that’s not really the point of this experiment anyway. I’ve seen some NASA TT5 cars do 2:08-2:09, so that’s in the same ballpark, though.

Now it’s time to see how my Miata (black in the following speed trace) would do against these cars. While the engine is down on power, my car has better aero than the average Miata. On the back straight I’d give up 7 mph, but over a lap it would be only about a second slower. Not that bad, actually.

Grid Life will be at several tracks this year, including Gingerman, Mid-O, Lime Rock, and Laguna Seca. I have those track maps in OptimumLap, so I ran those simulations as well. The parity across the classes is pretty good, and the order stays mostly the same. However, there is one surprise winner.

One final comment is that these simulations were all done on Miatas with aero, and aero favors light weight. But air has resistance, and it takes power to overcome that. A heavy, torquey car with maximum tire width and a lot of mechanical grip exploits that imbalance. I don’t have the data for such cars so I can’t run those simulations, but if you look at the race results, that’s also a winning formula. Therein lies the success of GLTC, a diverse selection of cars and evolving parity in the rules.

I’m really looking forward to April 28-30, although I’m not sure if that will be as a participant or spectator. The weather probably won’t be great, but it’s going to be a heck of a party.

If you’ve been reading this blog, then you know that aero works. Aero rules over power, over weight, and (almost) over tires. If the competition regulations allow it, do it. But then you look into the aero rules for different racing organizations and you’re like, what the fuck have I gotten myself into?

There’s no standardization. Every racing organization has their own set of aero rules, and if you want to race in more than one series, it’s confusing to keep track of. I thought it would be a good idea to review the aero rules in different series and compare how they treat aero. (Note that many rules change yearly and these will certainly go out of date.)

Briefly

Here’s how various time-trial and racings series treat aero. I’ve listed the number of pages in their rulebook, a brief summary, and an overall rating of how they treat aero.

All Good

Let’s start with the bracket-based endurance racing leagues: American Endurance Racing (AER), 24 Hours of Lemons, Lucky Dog Racing (LDR), and TREC. These series don’t have any rules for aero (aside from the parts being attached safely). The racing organizers place you in a class where they think you belong. This is great for people who like to get creative with aero.

Champcar

Champcar’s aero rules appear to be more based on simplicity and retaining legacy rules than on balancing performance. And that’s OK, nobody likes a complicated rulebook, or rules that change every year. (Ahem, radiators.)

The legacy rules state that any bodywork changes or unlisted aero devices are assessed at 1 point per 12″ square of material used, rather than any performance benefit. This point system is based on using wood as your material of choice (not kidding), and you pay 2x points for metal and 3x for plastic.

Based on this point system, putting a lexan rear window on your car can cost you 20 points or more. So a lot of people use no rear window at all. Really. Go to a Champcar race and count the number of rear windows.

If you reuse parts of your car, you aren’t always assessed points. And so it’s possible to build a Frankenstein fastback out of OEM seats, soft top vinyl, and other parts you yarded off a parts car for zero points. That’s kind of cool in a way because it controls costs, but does nothing to balance the performance of the car. It makes for odd looking cars, as well.

As it pertains to rooflines, it would be simpler if Champcar were to say that any roofline changes or rear widows cost 10 points. Not only is that simple and speeds up tech, it follows the same formula as their more recent aero rules. An airdam, splitter, wing, center pan, or diffuser, are each a flat 10 points.

Champcar gives you all of 12” for splitter length, wing height, and the set-back distance for wing and diffuser. They have an excellent graphic.

COM

Corvette Owners of Massachusetts (COM) is an old club that has evolved a very elaborate and fair set of time trial and racing rules. Their rules include an extensive list of aero modifications and point values.

• Splitters are 1 pt for less than 3.5″, but 2 points for 3.5″ and over.
• Wings range from 1-3 points depending on height and width.
• A modified top on a convertible is 1 point.
• Less useful things like vortex generators and canards, are also 1 point.
• Underbody aero is interesting: 1 point for side skirts, and 2 pts each for underbody aero and diffuser.

If you think the aero rules are specific, wait until you get into wheels and tires! Thankfully, COM has an Excel-based class calculator. All said, COM allows a lot of aero and balances performance fairly across pretty much everything you can think of.

EMRA

Like COM, Eastern Motor Racing Association (EMRA) is another New England based historic club does HPDE, time trials, and wheel-to-wheel racing. Their philosophy is “run what you brung,” so if your car is safe, they’ll let you race with them. If you have modifications that aren’t in the rulebook, they’ll figure out how to class your car. This is so much cooler than the restrictive rulesets that, for example, put you automatically in an Unlimited class because you have a fastback (Gridlife, NASA, SCCA, etc.), or simply don’t allow you to race because you have a double element wing (or whatever).

EMRA’s balance of performance is done with a point system; you take a point for every performance modification, aero or otherwise. I had a hand in rewriting EMRA’s rulebook, and so the aero rules are very particular.

• A spoiler is 1 point, a wing is 2 points, a dual-element wing is 3.
• A splitter is 1 point, or 2 points if it’s longer than 4″.
• VGs, canards, side skirts, and other misc stuff are all 1 point or none.
• Hood vents don’t cost anything for cooling, but if you are using the to make downforce (you have a non-OE undertray), then they cost 1 point.

Global Time Attack

GTA has the following classes: Enthusiast, Street, Limited, Unlimited, International Unlimited, and ProComp. For all classes, “vehicles must have a silhouette that is largely faithful to the original with the general body shape and outline remaining largely true to the original body.” This is pretty loose wording, and invites rules lawyering, but at least there’s some wiggle room.

Like Grid Life (examined later), they allow a certain number of “significant aero”devices. At the front, a splitter and canards are each considered significant aero. At the rear, a wing, spoiler, and diffuser are each considered a piece of significant aero.

GTA Enthusiast

The Enthusiast class allows one significant aero device at the front, and one at the rear.

Front air dams, lips, splitters and diffusers may not extend more than 3″ beyond the bodywork in any direction as viewed from above. An aftermarket splitter may only go rearward to the front edge of the front wheel opening. This is more restrictive than any other rule set, which typically say to the front axle. Or instead of the splitter you can use up to four canards that project up to 5″. But I would question your sanity on that choice.

Wings can be body width, roof height, and set back up to 3″. End plates and swan necks can also be 3″ taller than the roofline. Although hatchbacks are allowed to put the wing 10” above the roof. For your one significant piece of rear aero, you could use a spoiler instead of a wing. Or you could use a diffuser. But I would question your sanity.

No barge boards or flat underbody, and you can’t vent the front fenders.

GTA Street

This class builds on the Enthusiast class and allows another 2″. They call this the “Street” class, but would you drive on the street with a splitter that extends 5″ outside the body of the car? I wouldn’t. I wouldn’t even track a car like that.

• So that means canards are up to 7″ and splitters are up to 5″ wider than the car, and can now extend rearward to the center line of the front wheel.
• Diffusers are also up to 5″ aft of the bumper.
• The wing rules are mostly the same as Enthusiast except hatchbacks can now mount the wing 14″ above the roof.
• You can vent the front fenders. Also, barge boards can go to the pinch weld and 3″ outside the body.
• Still no flat bottoms or active aero.

GTA Limited

Most of the rules are similar to Enthusiast and Street, but you get another few inches here and there. Rear wings are still a single element, but can now be 10″ above the roof (18″ for hatchbacks) and you can use a hella wide splitter and wing. You can (finally) use a flat bottom, and with that a diffuser that extends 10″ behind the bumper.

A car like this is so purposefully built that it’s no longer streetable or trackable. From above, it looks more ready for flight than it does for driving.

GTA Unlimited and Pro/Comp

The Unlimited and International Unlimited rules have no regulations for wing, splitter, underbody, and diffuser. The only restriction I found was that you can lower the roofline of a sedan by 3″ maximum. Except convertibles can do away with the windshield frame altogether.

The Pro/Comp class is truly unlimited.

Grid Life

Grid Life has seven time trial classes. The most restrictive classes are Street, Street GT and Sundae Cup, which allow factory aero only. Most also allows any lip. I’m not sure what lip means, it’s not really defined anywhere, except that there’s no horizontal component. Is an airdam a lip?

ClubTR allows minor bodywork changes for airflow (vents). You can use a splitter up 3″ long, and a single-element wing up to 701 square inches that extends no further than 5″ past the bumper. Side skirts are allowed provided they are no wider than the car. These rules are similar to GLTC, but ClubTR doesn’t allow you to make changes to the OEM roofline shape (no fastbacks). Please, rulebook people, allow convertibles to use fastbacks in ClubTR. Club TR could then be a direct feeder series to GLTC, with the same aero rules.

Street Modified is where things get all GTA, with aero well outside of the dimensions of the bodywork.

• You can run two significant aero modifications at the rear. Their example is a wing and a diffuser, but maybe you could use wing and a spoiler? Doesn’t say. In any case, wings must be single element, roof height (4″ exception for hatchbacks), and no wider than the car. I feel like they should make dual element wings count as two significant modifications to the rear, rather than ban them, but it’s not my rules.
• You can run one significant aero modification at the front. A splitter can be up to 5″ wider than the bodywork. Or you can use 4 canards, sticking out up to 5″.
• You can use a diffuser as one of your significant rear aero devices, but not a flat bottom. This is peculiar, since a diffuser without a flat bottom is almost worthless, and some cars now come with a flat bottom. I wonder at the reasoning here.

Next is the Track Modified class, and it reads mostly the same as Street Modified. The difference is you can now run two significant aero devices at the front, and a dual-element wing is OK. Still no flat bottoms, still no active aero.

The Unlimited and Super Unlimited classes have no specifications for aero, so I guess this is where flat-bottom cars go, regardless of any other modifications.

Grid Life Touring Cup

This is the wheel-to-wheel class for Grid Life, and the aero restrictions are similar to Club TR, with aero kept within the lines of the bodywork for less chance of contact. Let’s start at the front of the car and move back:

• Vertical airdams are free. Modified bumpers are OK as long as they don’t have canards or anything molded in.
• Splitters are a 3% penalty to your lbs/hp ratio, and are limited to 3″, which is on the small side.
• Hoods and venting are unrestricted.
• Tire spats are OK as long as they follow the shape of the bumper and aren’t any wider then the tires.
• You can add one fender vent or louver above each tire with an area 45 square inches or less. Or you can cut the bottom of the fender behind the wheel and push it inward.
• Side skirts are allowed as long as they don’t stick out past the tires, and are no wider than 5″ total.
• There are three wing options: A 250 square inch wing (or spoiler) is free. This is such an interesting topic I’ve got a whole post on this subject. You can add a single element wing up to 499 square inches for a 2% penalty. Or a 701 square inches for a 3% penalty. You can mount the wing as high as you want, but no further rearward than 5″ past the bumper. End plates can be any size. If you use both a splitter and a wing it’s only 3% total for the medium wing and 4% total for the bigger wing.
• There are no restrictions for roofline shape, so convertibles can use a fastback, shooting brake, or whatever. Hallelujah!

Other aerodynamic elements or additions that may increase downforce or reduce drag such as winglets, dive planes, canards, diffusers, vortex generators, flat floors, tunnels, wheel arch covers, tail extensions, etc. are not allowed.

Grid Life is also getting into endurance racing, with three classes. One is basically Sundae Cup, one is 18:1 power to weight but no aero, and one is GLTC. I don’t see why they don’t allow aero in the underclasses, since they already have a formula for it. My guess is they are trying to attract B-spec and Spec Miata racers who are afraid of other cars that have aero. Lame.

NASA ST/TT

NASA ST rules are for wheel-to-wheel racing, and TT rules are for time trials, but as it relates to aero, they are the same. This is a lbs/hp-based series, and like GLTC, you take a penalty to the lbs/hp ratio for some aero devices.

In the rules, if the part isn’t listed, you can’t do it. And that means if your car has canards, diffuser, side skirts, or anything else not listed, you have to remove those parts to run with them.

Overall I’d say the rules are very fair, but why not list a point value for side skirts and other things that make your car illegal? Allow people to “run what they brung” and penalize them for it, rather than exclude them.

For example, they recently changed the height of hood vents to something like 3/8″ max, and this was after everyone already had hood vents in their car, and most of them were taller than this. Rather than force everyone to buy new hood vents, just say that vents taller than 3/8″ incur an extra .1 or .2 penalty. This at least allows competitors to add a little ballast and use their existing hardware for the rest of the year.

NASA ST5 and ST6 (TT5/TT6)

In ST6 you can change the front fascia, and by this I’m assuming an airdam is OK, but the rules don’t explicitly say “airdam”. In ST5 a vertical airdam is allowed with +/- 5 degrees variance. It’s confusing they specifically mention airdam in ST5 but not ST6, because the description of “front fascia” in ST6 sure sounds like an airdam. You can’t use a splitter in ST6, but you can add up to a 4″ splitter in ST5 for -.5 lbs/hp.

For a penalty of -.4 lbs/hp, you can change the roofline shape of a convertible, but the top can’t extend past the forward edge of the trunk. So, no fastbacks, but you could get creative here. You can also cut away part of the rear bumper cover, as long as you don’t cut into the bumper.

In ST6 you can add a wing or spoiler at a hefty 1 lbs/hp penalty. In ST5 the wing is free, but is limited to body width, roof height, 12″ set-back distance, and end plates no larger than 144 square inches.

NASA ST1 to ST4

In ST1-4 you can change the roofline shape for -.3 lbs/hp instead of -.4. It’s difficult to tell if this only relates to convertibles, because the rulebook says that, but the official calculator does not. The roof must end before the trunk begins, and it needs a sealed rear window.

The wing can be 8″ above the roofline, but no wider than body width. The splitter is now free, and can be 4” long, but otherwise the rest of the aero rules are the same restrictive ones as ST5-6.

Ontario Time Attack

Ontario Time Attack (OTA) treats aero similarly to COM and EMRA, meaning fairly. Their rules have different points for splitter length, wing height, hood and fender vents, etc., and all of it makes sense and seems pretty good. I won’t list all the rules here, but if you’re within driving distance, I think it’s worth exploring this series. I had planned to do that for the past two years, but COVID put the kibosh on that.

SCCA Autocross

In the overview table, I gave this series only one star, mostly because they are making a big deal out of nothing. Unless you’re using really big parts, then aero just isn’t that useful at 40 mph. Also, when I tried to class my car, it was an exercise in frustration. Anyway, some classes allow aero, some not.

SCCA Time Trials

The aero rules are pretty simple. In the Sport and Tuner categories, you can’t do much, and in the Max and Unlimited categories, you can.

Sport and Tuner Categories

OEM bodywork only. You can add or remove parts provided another model was equipped that way from the factory in the USA. You can add parts from the manufacturer’s accessory catalog, provided it was for highway use, and not sold through a manufacturer’s performance catalog (no Ford Racing, HPD, Mazdaspeed, Mopar Performance, Mugen, NISMO, SPT, TRD, etc.).

Factory wings and spoilers may not be adjustable and can’t have end plates, so cars with active aero, like a Mini Coupe, Audi TT, or Porsche can’t play here. Likewise, the wing rules disqualify the Cayman GT4, Camaro ZL1 1LE, etc.

The Tuner category is similar to the Sport category, but is aimed at NA Miatas. OK, not really, but check it out: you can use a hardtop of the same shape using alternate materials, modify pop-up headlights, and add cosmetic bodywork items that don’t have any aero benefit (mudflaps, R-package rear lip, luggage rack, etc).

Max and Unlimited Categories

The Max and Unlimited rules allow you to change body panels within reason. The wording is bit grey for me, so I emailed for clarification on fastbacks. John Krolewicz replied: “The ‘dream street car built or bought’ guideline is pretty wide open, so body panels can be things like a Mugen top for the S2000 or the Miata fastbacks. Now… with that being said, we do want the car to maintain a recognizable factory silhouette. If the allowance was used to create an S2000 that looks suspiciously like an Acura ARX-05, we might have an issue!” OK, so a fastback is in, but my LMP bodywork is out, got it.

• Splitters can extend 5″ in front of bodywork (plumb line) and to the wheel centerline at the back. A recent technical bulletin (https://timetrials.scca.com/announcements/296-max-category-splitter-technical-bulletin) clarified that splitters must be flat, without tunnels, strakes, or splitter diffusers. Although I’ve also read that splitter diffusers are allowed, up to 84 square inches on each side. (https://timetrials.scca.com/pages/class4fullrules). Confusing. Any other ducting must result in a tube to cool brakes or radiators.
• Canards are allowed, max 1.2 sq feet (207 sq in), 4″ proud, and within the perimeter of bodywork. Canards may have endplates, but it’s unclear if endplates are included in the allotted area. In any case, end plates can’t connect to a splitter, or be be within 3″ of one.
• You can only have one wing or one spoiler, but no active aero.
  • A spoiler can be 10″, any angle, and must remain within the perimeter of the bodywork.
  • A wing can have two elements, must be less than body width (mirrors), max roof height +10″, and no part of the wing can be 6″ in front of the rear axle or behind the rear bodywork.
• Side skirts can be 3″ outboard, 12″ inboard, with 3″ ground clearance.
• No flat bottoms or underbody aero.
• Diffusers are allowed, but can’t extend forward in front of the rear axle, or more than 6″ behind the bumper.

The Unlimited category is looser on bodywork (must bear a noticeable resemblance to production counterparts) and allows pretty much anything except active aero. This is also where underbody aero (flat bottom) would place you.

SCCA Road Racing

In a 700+ page rule book that includes Formula cars, you’re bound to find a lot of different rules for aero. There are so many classes that there’s really no standardization. For example, for splitters, the rules vary between 2″, 2.5″, 3″, 4″, 5″, and 6.5″.

For wings there are generally three sizes, all of which can use a Gurney flap up to 1/2″ in size.

• Street classes don’t allow aero unless the car came with it, except cars that came with factory active aero are not allowed. Never mind that there are no street cars with active aero that will do anything for a car at 40 mph. Reynolds numbers at this speed (~150k Re) are so low that wings aren’t efficient and create more drag than downforce. Whoever wrote this rule doesn’t understand how aero works.
• Prepared classes allow for some minor aero, like a huge spoiler, but there are some weird exceptions. For example, your splitter can’t have fences in front of the tire, or that puts you into the Modified class. You can modify the bodywork somewhat, but a fastback is a grey area. In a 380 page rulebook there isn’t one mention of the word “fastback”, and so a S2000, Miata, or whatever with an aftermarket top has to run with the big dogs, no matter if it was otherwise 100% stock. Seriously: a fastback at 40 mph is not worth a performance advantage.
• Modified classes allow you all the bodywork changes you want. You can also add a wing, and the rules are generous. The wing can have two elements and a total of 8 square feet (1152 square inches), with end plates measuring 200 square inches each. This is the proper dimensions of a low speed wing! The wing can be placed 6″ above the roof, but can’t extend past the bumper.

• Small – Super Touring wings are limited to 8.5″ chord and 48.25″ wide, and an end plate measuring a total of 64 sq-in. The APR GTC200 wing is also acceptable. The wing must be mounted 6″ below the roof, and in this case, the APR wing seems like a good option.
• Medium – Some of the GT classes are limited to 10.75″ chord and 100 sq-in end plate. Some of the classes can use a 64″ wing, some can use a 72″ wing.
• Large – The GT2, STO, STU, T1 etc. classes can use a 12″ chord wing up to 72″ wide, with 144 sq-in end plates. The APR GTC500 wing is also acceptable.

There’s probably a lot more details in the SCCA aero rules, but 700 pages? Fuck. That.

Speed Ventures

This group has several TT categories for different cars, like Corvettes, Nissans, Subarus, etc, and when I tried to research the rules they just opened up Facebook pages. I don’t have time for that nonsense, so I went with the two divisions that did have printed rules, 86 Cup and Honda Time Attack. Both series have a point system, and when you exceed a certain number, you go into the next class. Both divisions have an online calculator that makes classing easy.

86 Cup

This series for the Scion FRS, Subaru BRZ, and Toyota 86. For aero, they go into a lot of detail, too much for me to list here. I applaud this level of point-based leveling, down to .125 points for some items. All in all, the points look very well balanced versus other options, well done.

Honda Time Attack

The aero rules for this all-Honda series are similar to the 86 Cup rules, but they group some items into “levels” of aero. For example:

• Level 0 Front Aero: stock front bumper OEM/OEM style lip (s2000 CR’s or CR front end must take Level 1 points)
• Level 1 Front Aero: aftermarket front bumper or any aftermarket lip; (CR lip must not have splitter built-in)
• Level 2 Front Aero: Non-tunneled ‘flat’ splitter element. (These points include a set of spats and canards.)
• Level 3 Front Aero: Splitter incorporating 3D bottom design- tunnels etc. (These points also include spats and canards.)

SuperSpec Cup

SuperSpec started as Supermiata, and those rules were the bomb. The basis is the same, 2300 lbs competition weight and 140 hp max, but the new rules allow other cars with the same spec. Let’s take a moment to celebrate a sprint series that uses Hankook RS4 tires. Too bad the races are nowhere near me.

The aero rules are bit like communism: “an equal distribution of poverty.”

• You can have an airdam (vertical plus or minus 5 degrees), but no splitter.
• You can have a flat spoiler up to 13″, but no wing. You can’t really balance a wing very well without a splitter, so this makes sense.
• Aftermarket bodywork is allowed, but must follow the OEM shape. Gives people a chance for some personalization, nice.

And that’s it. If the rules allowed wings, splitters, fastbacks, or anything else, it would add cost and ruin the parity. This series is all about fairness and economy, and even though I’m an aero-first guy who likes to have options, I like these rules.

World Racing League (WRL)

WRL is an endurance racing series based on lbs/hp, and it allows aero in a very simple fashion. Basically, small items (spoiler, side skirts, canards, VGs) cost you .1 lbs/hp each, to a maximum of -.6 modifier. Medium items (airdam/splitter, wing, flat underbody, diffuser) cost you -.2 each, also to a maximum of -.6. And if you have a multi-element or active aero wing it’s another -.4.

I saw no mention of splitter length, wing size, roofline shape, etc., and so there’s a lot of freedom in these rules. Noice.

Conclusions

• As I look over these rules, one of the things that strikes me is the almost universal ban on active aero. Active aero really isn’t that beneficial (at the club racing level), and it’s an area that would be fun to experiment with, so it’s a pity everyone is scared of it.
• A diffuser doesn’t work very well unless you have a flat bottom or tunnel, and I always find it humorous to see rules that allow diffusers and ban flat bottoms. There are rare cases where a diffuser on a car without a flat bottom works, but most of the time it just adds weight to the rear of the car.
• It would be difficult to create an aero package that could run in many different series, without changing anything. The common denominator would be the least effective aero. It would be better to have parts that are adjustable (for example splitter length and wing height).
• I was going to end this post with a spreadsheet comparing the various rules in an easy to scan format. But rules change yearly and I’m not going to do that kind of maintenance.