Originally Posted by Mech33

Not quite... Let's agree that the force supported by the contact patch is the same regardless of the tire. For a 60% rear weight biased car, that's roughly 30% of the car's weight W on each of the rear tires. So 30% of the car's weight (0.3*W) is the total force applied from the tire to the ground through the "contact patch". That total 0.3*W force on the patch creates a pressure distribution over the area of that patch. For simplicity, if we assume the pressure over the patch is roughly constant (a good assumption if your tires wear uniformly), then the force, area, and pressure are simply related by: Force = Pressure x Area If you look at a piece of tire in the middle of the contact patch, it has the contact patch ground pressure on one side, internal tire air pressure on the other side, and some shear forces in the rubber around the edges of that section. But if the tire section is sitting flat and properly inflated, these internal tire rubber forces are relatively small the the tire contact patch pressure is essentially the internal tire air pressure. So if you have 35 psi (pounds per square inch) in your tire, then a 3100 lb car with 30% of the weight over each rear wheel as roughly this size contact patch: Area = Force / Pressure = (0.3 * 3100 lb) / (35 lb / in^2) = 26.6 square inches For a 12" wide rear tire and an approximation of a simple rectangular contact patch, that's a 2.2" long x 12" wide patch. Sounds reasonable, no? So far, tire diameter hasn't factored into any of these calculations... There is a second order effect on contact patch area for narrow profile tires. For a fixed tire outer diameter, but larger wheel diameter, the profile gets shorter, and the sidewall stiffness goes up. The stiffer the sidewall, the higher the actual pressure at the edges of the contact patch where the sidewall touches the road. In the above calculation, we assumed pressure was uniform, but in actuality the edges are a bit higher. For the same tire air pressure, this causes a little more of the car's weight to be supported at the edges of the patch and less in the center, reducing the "contact patch" area slightly. Also, the wider the tire, the wider the aspect area of the contact patch area, so the less the impact of the sidewall pressure on the overall contact patch area, so the area goes up slightly for the same tire pressure... Another second order effect is the overall tire diameter. At the long edges of the contact patch where the curvature of the tire lifts up and off the road, the stiffness there is higher just like at the sidewall, so the local contact patch pressure there is higher. The larger the diameter, the less this effect. Anyway, in conclusion, contact patch area is virtually completely a function of the tire air pressure. So does a 991 GT3 RS rear 21" tire have a larger contact patch than a 991 GT3 20" tire? Depends on the tire pressure. But does it have a more uniform contact patch pressure? Yes, slightly. Why is more uniform pressure important? Tire material coefficients of friction are non-ideal and decrease with load. That is, if you take a piece of rubber loaded with force F, vs. two identical pieces of rubber each loaded with half the force F/2, the two half-loaded pieces with have more total friction force available than the single higher loaded piece. This is the fundamental reason why everything in motorsports is about being "smooth" (to reduce peak load transients on the tire that result in far less friction holding ability than the steady-state turning load), why sway bar tuning affects balance (distributing more or less load on the inside or outside tire changes total available grip on that axle), etc... End rant...

Originally Posted by Mike in CA

Great analysis, Mech33. As you point out, there are several factors affecting contact patch size and some have more influence than others. Still, if all other parameters are equal, the larger the diameter of the tire the longer and, overall, the larger the contact patch will be. It is, in fact, one of the reasons manufacturers keep increasing tire diameter where it's possible.

Originally Posted by Mech33

To be more technically accurate, they would be increasing diameter to keep the tire contact patch *pressure* more uniform, not increase the patch itself...

Originally Posted by Mike in CA

I accept that you know more about this than I do. Thanks for the clarification....

Originally Posted by Mech33

Not quite...

Let's agree that the force supported by the contact patch is the same regardless of the tire. For a 60% rear weight biased car, that's roughly 30% of the car's weight W on each of the rear tires. So 30% of the car's weight (0.3*W) is the total force applied from the tire to the ground through the "contact patch".

That total 0.3*W force on the patch creates a pressure distribution over the area of that patch. For simplicity, if we assume the pressure over the patch is roughly constant (a good assumption if your tires wear uniformly), then the force, area, and pressure are simply related by:

Force = Pressure x Area

If you look at a piece of tire in the middle of the contact patch, it has the contact patch ground pressure on one side, internal tire air pressure on the other side, and some shear forces in the rubber around the edges of that section. But if the tire section is sitting flat and properly inflated, these internal tire rubber forces are relatively small the the tire contact patch pressure is essentially the internal tire air pressure.

So if you have 35 psi (pounds per square inch) in your tire, then a 3100 lb car with 30% of the weight over each rear wheel as roughly this size contact patch:

Area = Force / Pressure = (0.3 * 3100 lb) / (35 lb / in^2) = 26.6 square inches

For a 12" wide rear tire and an approximation of a simple rectangular contact patch, that's a 2.2" long x 12" wide patch. Sounds reasonable, no?

So far, tire diameter hasn't factored into any of these calculations...

There is a second order effect on contact patch area for narrow profile tires. For a fixed tire outer diameter, but larger wheel diameter, the profile gets shorter, and the sidewall stiffness goes up. The stiffer the sidewall, the higher the actual pressure at the edges of the contact patch where the sidewall touches the road. In the above calculation, we assumed pressure was uniform, but in actuality the edges are a bit higher. For the same tire air pressure, this causes a little more of the car's weight to be supported at the edges of the patch and less in the center, reducing the "contact patch" area slightly.

Also, the wider the tire, the wider the aspect area of the contact patch area, so the less the impact of the sidewall pressure on the overall contact patch area, so the area goes up slightly for the same tire pressure...

Another second order effect is the overall tire diameter. At the long edges of the contact patch where the curvature of the tire lifts up and off the road, the stiffness there is higher just like at the sidewall, so the local contact patch pressure there is higher. The larger the diameter, the less this effect.

Anyway, in conclusion, contact patch area is virtually completely a function of the tire air pressure. So does a 991 GT3 RS rear 21" tire have a larger contact patch than a 991 GT3 20" tire? Depends on the tire pressure. But does it have a more uniform contact patch pressure? Yes, slightly.

Why is more uniform pressure important? Tire material coefficients of friction are non-ideal and decrease with load. That is, if you take a piece of rubber loaded with force F, vs. two identical pieces of rubber each loaded with half the force F/2, the two half-loaded pieces with have more total friction force available than the single higher loaded piece. This is the fundamental reason why everything in motorsports is about being "smooth" (to reduce peak load transients on the tire that result in far less friction holding ability than the steady-state turning load), why sway bar tuning affects balance (distributing more or less load on the inside or outside tire changes total available grip on that axle), etc...

End rant...

Originally Posted by orthojoe

Please don't run Hoosiers if you are novice. Not to beat a dead horse, but don't be that guy.

Originally Posted by mooty

$500-700 set of tire is a BIG difference.
some of us destroy a set per weekend!

Originally Posted by Mech33

Why is more uniform pressure important? Tire material coefficients of friction are non-ideal and decrease with load. That is, if you take a piece of rubber loaded with force F, vs. two identical pieces of rubber each loaded with half the force F/2, the two half-loaded pieces with have more total friction force available than the single higher loaded piece. This is the fundamental reason why everything in motorsports is about being "smooth" (to reduce peak load transients on the tire that result in far less friction holding ability than the steady-state turning load), why sway bar tuning affects balance (distributing more or less load on the inside or outside tire changes total available grip on that axle), etc... ...

Originally Posted by PhilT3

So if the price wasn't a factor, and Hoosiers were made in the 20'' size suitable for the GT3, would you go 19" wheels or 20...?

I guess I'm really trying to figure out what is the better handling/performing wheel diameter for track duty?