This is the second in a series of rants about things I don’t understand about car culture, motorsports, and racing.
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.
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?
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.
11 thoughts on “Shit I don’t understand… #4: square wheels”
Ian, you are going to love this … Some cars DO have reverse stagger. Audi RS3 for example … 255’s in front, 235’s in the back …
Are those legit sizes or compound markers? I remember being very surprised to learn that the “staggered” setup on the GT350R, for example, was using manufacturer-specific compounds and used identically sized tires but the sizing labels made it easy to differentiate the compounds between front and rear.
Personally in Miataland I like square for easy rotation, but I’m not chasing every tenth either.
One of my friends has one of these cars … 255/30-19 in the front, 235/35-19 on the rear. The compound seems to be the same, like treadwear 200 front and rear. Delivered this way from the factory.
Miatas have slightly more weight in the front, especially if you remove the spare tire, so I don’t think they should be staggered. Square is probably ideal.
There it is for all to see: I suck at car knowledge. Yay Audi RS3!
Hm. My impression is that the main reason RWD cars come with staggered tire setups is that the manufacturers want them to understeer at the limit for liability reasons (well, that and the fact that it looks cool). Putting wider tires up front to make it square is pretty common for Porsche and BMW owners.
This “shit I don’t understand” series is educating… me.
Ian, lots of FWD race cars (remember Grand Am ST?) have ‘reverse stagger’ by running a lot of rear camber to decrease rear tire footprint. Just one way to tune grip balance and stay within the rules. Great blog BTW :)
So not actually thinner, but operationally thinner. Huh.
Square setup seems ideal for my 86 (which is similar to the Miata in the comment above).
One suggestion that I have not tried, but would likely work for a FWD: use worn-out tires in the rear and new ones in the front.
I recall seeing an episode of either Best Motoring or Hot Version, testing Hondas on whatever touge course they were on. The fastest of them was running a 265 front width, and 225 or 215 rear, if I recall correctly. Looked a bit strange, but made a lot of sense given the weight distribution.
I’ve considered running a skinnier or lower grip compound on the rear of my Miata, when I was autocrossing that. Had all the rear grip in the world, but I was never able to get the alignment it needed to have the front end grip I wanted. A bit unorthodox for those cars, sure, but… it’s an idea.