How do different contact patches affect a cars handling and responsiveness? Example - If I go from a 195/60-15 tire to a 205/60-15 tire, the contact patch will be made longer and wider.

Which is better? Longer, or wider? could one possibly counteract the other?

Say you run a taller skinny tire in front, and a wider shorter tire in rear.. would the additional length in the front contact patch help reduce the understeer one would experience by having wider tires in the rear?
example - 225/45 front - 245/40 rear


I'm really just trying to understand how the differences in contact patch, be it lengthening it or widening it, will affect the cars handling.


(btw - I drive a 240sx.. so if this stuff seems a bit absurd to do on a fwd car thats why)

 

A longer contact patch will provide more traction for acceleration and braking. A wider contatct patch will provide more traction for lateral acceleration. Which one is best is dependent upon your use. Since this is the RoadRacing/AX forum, I'll suppose that handling is paramount - ok I just reread your post, duh. So for you, I'd go as wide as effectively possible, while still keeping the general diameter of the stock tire/wheel setup.

 

ok.. how would that affect turn in, and steering response?

 

Wider I get but I'm curious about the assumption that the contact patch gets longer with a tire that is effectively the same diameter. Or one that isn't for that matter. I like extreme models as tools to clarify understanding...

Imagine a 6" wide, 24" diameter cylinder made of an enormously hard material on a surface of the same stuff (no compliance in either). Its contact patch is 6" wide and almost unmeasurably "long". If you increase the the width to 8", the contact patch length doesn't change - right? How about if you increase the diameter of the cylinder to 30"? 30 feet? Surely though, if the cylinder diameter is increased to some huge diameter approaching infinity, that intersection between the cylinder and the plane gains some dimension 90* to its width - right? Umm - maybe not.

If the cylinder is more compliant, how does the situation change? I'm not sure that it does, if the vertical load is the same in all cases (i.e., the cylinder footprint area doesn't increase just because it gets heavier and squashes outward). In fact, the pressure over the area of the footprint will actually decrease with a wider tire, at the same vertical load. How is that factored in?

In the real world of tires and pavement, I think that might be safe to say that there are too many variables involved to be able to make generalizations beyond (a) more grip at any given tire is better, and (the oft ignored b) decreasing grip at the end that IS sticking is bad juju.

Kirk

 

Remember contact patch size is determined by tire presure, not tire size.

 

Then why do drag racers use extremely wide slicks?

aj

 

Wide, but with a deep sidewall. Wide, I'd guess, is always good, because it's going to add to the crosswise dimension of the contact patch. Dragsters use a wide tire with a deep, soft sidewall, and low pressure. Under acceleration, both the deformation due to the twisting of the tire against the pavement, and the increased downward pressure due to weight transfer, tend to increase the fore-and-aft contact patch.

 

Should contact patch be determined by tire pressure AND size?

 

...and construction so therefore, by implication, brand and model.

K

 

In theory, the size of the tire influences the shape and the air pressure influences the size. Thats why lower pressure on the same tire gives you more grip.

 

makes sense

 

But changing tire width increases contact patch. Because tires are rubber, and are pliable, wouldn't the contact patch be longer on a taller tire as well?