In Reply to: So the question is.... posted by Austin on November 20, 2000 at 12:59:03:
For one thing, assuming the tires weigh the same, which I think is plausible since the lower-profile tires have beefier side-walls, the tire's mass would be farther from the axis, increasing your rotational mass regardless of wheel weight.
Then, there is the larger circumference of the rim. Even with a forged 2 or 3-piece wheel, the center could be lighter, but it would be very difficult to make the rim light enough to compensate for the size because it is bigger than the 15 and, like the tire, it is farther from the axis.
I cannot be any more explicit than this, though, since the actual figure depends not only on overall mass, but on how much mass there is at each distance from the axis. It would be really cool if wheel and tire manufacturers would measure these figures and publish them. Then maybe they could justify charging $2000 for a wheel!
The higher polar moment of inertia of larger wheels is not, in my opinion, a good enough reason not to get 17" wheels for a street car. See my post below about the handling advantages.
The ride quality is better with 15s. The acceleration is a little better with 15s. But 17s can give a bit more grip and they look SO NICE. Those DTM teams REALLY wanted to look nice. That's why they had 18" wheels. They wanted those Mercedes drivers to say, "Damn, look at those nice wheels," as the M3s passed them. ;)
If you're going to 17's, how light do they have to be to approach the same rotational mass characteristics of the stock 15's? Can it be done? I have 17's and definitely noticed my car accelerates slower than it did with the 15's.
Well I took out the car on a test run with the 15's the other night. I noticed a significant increased amount of acceleration through all of the gears. My first test was running with my friends 97 E36 M3. I was a half a car length behind him up to 100 mph. With the 17's on he dogged me completely. I also noticed the handling did not suffer much around the street. So my conclusion is keep the 17's for show and 15's speed on the street.