The Dumbest Aero Rules? Splitter Rules

As someone who is building a car to fit into multiple racing classes, I need to keep up with the aero rules in different series. It’s important to know not only what is allowed in each class, but how various aero components are weighed vs other performance modifications. I gave a broad overview of several aero rules in Aero Rules… but OMG the Fecking Rules!, but wanted to dive deeper in one area.

Through this journey, I’ve leaned that most rules are written by people who have a Childs understanding of car aerodynamics. That’s not a typo; let me explain.

I used to read the Jack Reacher series of novels by Lee Childs, and in every book the author would make a dumb technical error relating to firearms. I know firearms because I was a nerdy reloader for several years, and in this field, where things can literally blow up in your face, as Jack Reacher would say, “details matter.”

As a character, Jack Reacher understood this, but his creator did not. After the author incorrectly referred to a trio of Thompson submachine guns as “grease guns” I emailed Lee Childs and said that while I enjoyed his writing, he needed a technical editor. I offered to copyedit his next book for free.

Note #1: The M1928A1 “Tommy gun” and M3 “grease gun” fire the same ammunition and perform the same role, but they look nothing alike and even someone who knows nothing about firearms wouldn’t confuse the two. I mean, one of them looks like a tool for squirting out grease, the other one is in gangster movies. Note #2: Writing the author directly and offering my services may seem like a bold move, but this is exactly how I became a globe trotting motorcycle journalist for Moto-Euro magazine.

I wasn’t surprised when I didn’t receive a reply from Mr Childs. And so I also wasn’t surprised when he fumbled again in a later book referring to to a rifle as a “M14 Garand”. <sigh>

There’s no such thing as a M14 Garand. I own a M1 Garand, and I’ve shot a civilian M14, and while they are similar rifles, they don’t even use the same cartridge. Calling a rifle a “M14 Garand” is as idiotic as saying that Reacher’s new car is a “Mustang Corvette.” Yes, it’s that stupid.

To make matters worse, in the same story Reacher gets a ride from a woman in an enormous pickup truck. Astounded by the size of the truck, Reacher notes that it’s a Honda. Come on now!

Honda doesn’t make an enormous pickup truck, and now I was certain about two things: Childs knows as much about firearms as he does trucks. These are man’s-man topics, as baked into Jack Reacher’s DNA as the fisticuffs he engages in. After these two gaffes I let out a guffaw and could no longer read anything from that charlatan.

Let’s bring this back to aero rules. It’s difficult to write a racing rulebook, it takes the input of specialists with specific knowledge on suspension, tires, safety, engines, and aero. For whatever reason, rulebooks, like the Reacher books, get published with a Childs understanding of aero. And this results in some silly rules.

Of all the nonsensical rules I’ve seen, from banning fastbacks on convertibles, to equating wings with spoilers, to allowing diffusers but not flat bottoms… the most Childs-like rules are the splitter rules.

Flat splitter nonsense

Splitters separate the air above and below the splitter blade. They create downforce via a high pressure zone on top of the splitter blade, and a low pressure zone below it. You know what else creates downforce in the same manner? A wing.

Just like a wing, a splitter creates much more downforce through suction than from pressure. You can vastly improve the performance of a splitter by adding camber. Most people do this by installing splitter diffusers (splitter ramps), which add curvature over a small area in front of the wheel wells. Time attack cars often curve the entire rear of the splitter’s trailing edge upwards, thereby creating a splitter diffuser across the full width of the car. And some cars also curve the front upwards as well.

However you do it, adding curvature creates a Venturi, accelerating air under the lowest part of the splitter. This in turn drops the pressure, resulting in suction and downforce.

Given that this is how a splitter works, it’s surprising how many rulebooks specify that a splitter must be flat (or horizontal, or without curvature). A rule that states that a splitter must be flat is akin to a rule that states that a wing must be flat! I think we can all agree just how well a wing like that would work.

Splitters without side plates

Because splitters behave similarly to a wing, they also benefit from some attention paid to the outer edge. Look at any wing and you’ll see end plates. Look at any pro-level race car with a splitter, and you’ll see various things on the end of the splitter, which I’ll collectively refer to as side plates. These devices trap high pressure air, change the stagnation point, promote extraction, or in other ways improve the splitter’s functionality.

If there’s room on the end of your splitter blade, side plates are a no brainer. And yet, how many racing rules state that splitters can’t have anything on the ends of them? Many of them! Just for fun, I’ll pick on the SCCA autocross rules:

Front splitters are allowed but must be installed parallel to the ground… The splitter must be a single plane with the top and bottom surfaces parallel… A front splitter and its associated features shall not function as a diffuser… Splitter fences or longitudinal vertical members that serve to trap air on top of the splitter by preventing it from flowing around the sides of the car are not allowed.

You see the same verbiage from NASA, SCCA road racing, and numerous other club racing rules. Given that the splitter rules don’t allow side plates, I find it surprising they allow wings with end plates. I mean, it’s the same damn thing.

Splitter width and length

Time attack cars don’t race wheel to wheel, and so rear wings and front splitters are often much wider than the car. For example, in the Global Time Attack rules, in the Limited class, you’re allowed to use a splitter that extends 10″ in front and is 14” wider than the body.

On the other hand, most wheel to wheel racing rules limit the span of wings and splitters to body width. This is understandable, as you don’t want aero parts to hit each other on track. However, there’s no standardization on the length of the splitter lip. When you consider how few racing rules mention the chord of a wing, it’s amazing how many rules there are on splitter length. Again, this is the same thing! Here’s a smattering of splitter lengths:

• 12” Champcar
• 6″ – NASA ST1-ST4
• 5″ – SCCA Time Trials Nationals, SCCA GT1, GT2
• 4″ – NASA ST5
• 3″ – Grid Life Touring Cup, SCCA STU
• 2″ – SCCA STO, GT3, Super Touring, T1
• 0″ – SCCA Street Prepared

I’ll make fun of the SCCA autocross rules one more time, because it’s such low-hanging fruit. Did you see that last item on the list? The SCCA autocross Street Prepared rules allow you to have a splitter, but it can’t stick out past the bumper. Here’s the exact wording:

“A spoiler/splitter may be added to the front of the car below the bumper. It may not extend rearward beyond the front most part of the front wheel well openings, and may not block normal grille or other openings, or obstruct lights. Splitters may not protrude beyond the bumper. “

WTF? You can have a splitter but it can’t protrude beyond the bumper? How is that possible? Perhaps when this rule was written (1973?), all cars had underbite bumpers, but show me a modern car that you can fit a splitter to that doesn’t extend beyond the bumper!

I don’t want to just pick on SCCA autocross rules, because splitter rules across most car racing rulebooks show a misunderstanding of how splitters work. Whoever is writing these splitter rules could have easily written the following rule for wings: “Cars may use a rear wing, but the wing must be completely flat, installed horizontally to the ground, and with no curvature. Vertical members that serve to separate air above and below the wing are not allowed.”

I’m curious, who was the first rules lawyer that decided to castrate splitter performance? I feel like they have a lot to answer for. Some may argue that cost cutting is the reason, but you can make splitter diffusers and side plates for $10. Heck, you can make a fully curved splitter for free by selecting a warped piece of plywood!

But in the end, I guess it doesn’t matter who started this, because virtually every other rules writer copied and pasted the same absurdity into their rulebooks. That’s on all of them for having a Childs understanding of aerodynamics.