Originally Posted by swamp2
18s are optimal performance tires for handling for the M3. All timed Nurburgring runs use 18s.
First: The effect of size is more important than mass when calculating the moment of intertia. Wheels (aluminum) are much more dense than tires (rubber and steel and...). In any wheel for the M3 the outer "cylindrical" barrel portion of the wheel is the part that is the dominant contributor to the moment of intertia (I = integral r^2 dm) since it is furthest from the axis of rotation. The tires horizontal cylindrical portions (tread portions) will be rougly equal comparing 18, 19 or 20. Is is the sidewall the becomes smaller or larger, again this part of the tire is closer to the rotation axis.
Second: Tire deformation. Tires need to deform just the right amount to grip, absorb bumps without loosing traction, brake, etc. 18s are known to do this just right. 20s have way too short of a sidewall for proper lateral or vertical defection. 19s are a decent compromise between looks and performance.
These effects are fairly small but will be measurable on a timed track with a good driver.
I think I'm going to have to respectfully disagree with your first point. A 1 inch larger diameter rim that is overall 1lb heavier with 3lbs less wrapped around the outside, rear wheel example, should rotate much easier. The rim will play a much lesser roll than what's wrapped around the outer edge of it because that extra 3lbs is at the farthest point. The 1lb is only an extra .5inch farther from center. The 3lbs of tire is much farther from center and is 3 times heavier than that 1lb of rim. Plus the overall weight of the rim & tire is 2lbs less than the 19.
If you look at the wheel & tire from the center out this is what you get. Take the 20" setup for example. The overall diamter is 26.73 or a radius of 13.365 inches. Zero to 9.5 inches from center they are the same(19 inch diameter). Then the next .5 inch is 1lb heavier. Then the next 3.365 inches is 3lbs lighter than the 19" setup.
I'm not so sure about your second point except for the fact that each car is engineered from the factory to each specific sidewall. Otherwise your argument wouldn't hold any water for the Porsche 911 GT3 or the new Corvette ZR-1. Both of which use tire diameter or sidewall depths which violate what you say. Thinner sidewalls allow the suspension engineers to more accurately dial in a suspension because there is less tire variable to compensate for.