karlooz

on the rear and smaller diameter on the front? better heat capacity/dispersion? less rotations per mile giving you better longevity?

Mike Buck

IIRC, the larger rear is for durability/strength. Think that is what my buddy that works all the ALMS races for Michelin told me

I will see him tomorrow and double check

smlporsche

Another possible explanationis that with a larger diameter tire the contact patch will also be larger, all other things being equal.

Bull

Actually, "all other things being equal", the shape of the contact patch will change, but not the size. At least that is my understanding.

Geoffrey

Larger diameter tires will have a large contact patch than a smaller diameter tire. This is because the OD of the tire has less of an arc and more of it is contacting the ground. Additionally, a larger diameter tire on the same size rim has more air in it and a larger sidewall which aids in the loading of a tire. For instance, for the guys running Daytona, you would want to run a 680 rear tire over a 650 on the banking, particularly on the right rear tire where there will be substantial loading. I would not be surprised if the 650 diameter tires on a stock class car come apart during the club races.

Bull

You will find that IF all other things remain equal (as stated above), including weight of the car, tire compound, pressure, etc., the contact patch will not only become longer as you state, it will also become more narrow. The shape of the patch will change, not the total area.

Agreed, other factors will change which are very important under dynamic loading situations. My only point was that under static loading, a larger diameter tire won't change the patch size, only the shape, if all else remains equal.

analogmike

On most cars including the GT3 cup.RS.RSR it's about loading, which Geoffrey mentioned. A larger/taller tire will support heavier loads and more side loads. Remember all the people blowing out super low profile 18" rear tires on the heavier stock cars (Hoosiers and Michelins)? they were using a tire that could not support the heavy loads on the rear of a 911. I think Hoosier even claimed the tires were designed for the front axle. Using taller tires solved the problems.

Bull

Absolutely, as it did for me. Just the change from a 285 to a 295 worked wonders on several fronts. There are many goods reasons why a taller tire could be an improvement.

Joe Weinstein

The notion that the tire contact patch area won't change is incorrect. If the tire was a
perfectly flexible balloon, then that would be true, like the elementary physics lesson
would teach. However, modern tires with their stiff sidewalls are significantly rigid,
and consider the run-flat tires too. The other elementary physics lesson concerns a
rigid cube. The surface area contacting the ground does not change with increasing
weight, only the pressure per square inch.
A modern tire is somewhere between the perfectly flexible balloon and the solid
block, so all other things being equal, a wider/taller tire will increase tire contact patch,
and the contact patch shape will change, and the average contact pressure will go down.
The one sure calculation is that the contact patch area times the contact pressure will
be the same for either tire. NB: the contact pressure is not constant over the whole patch
either, for the same tire-as-significantly-rigid reason.
Tire tests also show that maximum lateral friction increases with weight (as you'd expect),
but the increase is less than one-to-one. Therefore, within reason a given load placed on
a wider tire is like a lesser load on an identical tire, and should stick better. The head of
Michelin's F1 tire team said that in the absence of tire regulations, tire widths would be
significantly wider.
Joe Weinstein

karlooz

great answers guys. since i have a street car and the ABS can't handle too much diameter stagger, would using a 17" wheel/tire be better for loading? the rear tire diameter is still stock-ish so the contact patch is the same as with an 18" wheel/tire but the 17" tire has more volume.

Geoffrey

You can run different diameter tires if you change the number of teeth on your front ABS rings.

Bull

Particualerly since generally a wider, shorter contact patch is better for our purposes than a longer, more narrow patch.

karlooz

can it be done on a 996? joel reiser on the pca tech q&a said it couldn't be done.

hmmm.... i'll have to look into it some more.

Geoffrey

Yes, you can.

GT3 Nut

Please explain how this can be done on a 996 GT3 and where I would get the parts. Is it a simple thing to do? Any ECU programming needed?