Shit I don’t understand… #4: square wheels

This is the second in a series of rants about things I don’t understand about car culture, motorsports, and racing.

Staggering

Most high performance sports cars have staggered tire sizes. BMW M4, Chevrolet Corvette, Porsche 911, etc. But here’s something strange: all of the FWD cars have the same size tire in front and back. High performance FWD? Yes, they exist. The Honda Civic Type R is faster than most RWD sports cars. Its time at VIR Grand is 3:03.9. Why do RWD cars have staggered tires but FWD do not? I don’t know.

Load vs. grip

Here are some important laws of friction.

• Amonton’s First Law – The force of friction is directly proportional to the applied load.
• Amonton’s Second Law – The force of friction is independent of the area of contact.
• Coulomb’s Law of Friction – Kinetic friction is independent of the sliding velocity.

If we believe these laws, then it doesn’t matter how wide your tires are. They provide the same grip regardless of the area of contact. Also, it doesn’t matter how heavy your vehicle is. All vehicles with the same tires have the same grip.

It turns out that tires violate all of these rules. If you apply twice the load to a tire, it returns slightly less than twice the grip. Tires also have less grip the faster they are moving, and grip is greatly affected by temperature. That’s because tires aren’t solids, they’re viscoelastic compounds.

The reason why you put wider tires on the rear of RWD sports cars is because it looks cooler. Kidding. The reason is because they have more weight in the rear. A tire with greater width mitigates the loss of friction due to extra weight. Even RWD cars with 50/50 weight distribution put wider tires on the rear because they shift weight onto the rear tires during acceleration. If you want balanced grip while accelerating, you may need a little stagger.

FWD cars typically have a weight distribution of 68:32 or thereabouts. That’s a lot more extreme than RWD cars. With all that load up front, you want a wider tire. What happens with square wheels? The front has less grip under load and the vehicle has a tendency to understeer.

Yaris

On my no-longer-B-Spec Toyota Yaris, I run staggered setups all the time. Up front I use 15×9 and 15×8 and in the rear 15×8 and 15×7. I also run different compounds in the front and rear. This setup is actually illegal in a variety of series where they demand that front and rear tires are not only the same size, but the same brand and model. What does one do in such situations? Pump the rears up to 40 psi, align with no camber, and have a really stiff rear ARB. These things reduce rear tire grip and restore grip balance. But instead of removing grip from the rear, how about adding grip to the front in the form of a wider tire?

What’s up?

So why don’t FWD cars come with staggered tires? So you can rotate them to balance tire wear? Makes some sense, but sounds like the wrong reason for a sports car. Is it because a gorilla stance looks weird? Maybe, but look at the Polaris Slingshot. That thing is wicked looking and shaped like an inverted V.