Aero Rules… but OMG the Fecking Rules!

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 (Note that many rules change yearly and these will certainly go out of date.)


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.

24 Hours of Lemons9Go nuts***
AER23As you wish***
Champcar36Take all 12″**
EMRA28Very fair***
Global Time Attack7+ each class“Significant”**
Grid LifePer classNo underbody**
Grid Life Touring Cup34Small splitter, big wing**
Lucky Dog Racing23As you wish***
NASA ST and TT28, 26Limited, but fair**
Ontario Time Attack67Very fair***
SCCA Autocross400ROFL *
SCCA Road Racing722RTFM**
SCCA Time Trials97No, no, yes, yes***
Speed SFCalcSpecific; fair***
SuperSpec Cup17Communism***
TREC24As you wish***
WRL18Simple, loose***
How the aero rules compare, briefly

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’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.

Repurpose parts for zero points!

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.

You’d need to get a wing that high with an open top.


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.


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.

Ready for take off

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 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 use 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.


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.

  • 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.

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 (download) clarified that splitters must be flat, without tunnels, strakes, or splitter diffusers. Any 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.

  • 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.


  • 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.

One thought on “Aero Rules… but OMG the Fecking Rules!”

  1. NASA ST6 is also a little weird in that you could arguably get away with some things for front aero if you bought it as a single molded bumper rather than riveting something over the factory one. AFAIK nobody’s really explored the possibilities inherent in that – I could see 9LR selling one-piece bumpers and #winning.


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