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      09-23-2008, 11:42 PM   #26
rzm3's Avatar

Drives: E92 M3
Join Date: Aug 2007
Location: USA

iTrader: (16)

Originally Posted by AZE92M3 View Post
Can someone please explain to me why this 20" rim setup wouldn't perform as well as the 19" rim setup detailed below? Let me explain that both the 19's & 20's are the same rim and same tire brand just different sizes.

19" rim scenario:
Front wheel weight = 22lbs
Rear wheel weight = 25lbs
Front tire weight = 25lbs
Rear tire weight = 29lbs
TOTAL = 101lbs

20" rim scenario:
Front wheel weight = 23lbs
Rear wheel weight = 26lbs
Front tire weight = 24lbs
Rear tire weight = 26lbs
TOTAL = 99lbs

Here is the link where I found the tire weights. 19's are 255/35/19 & 295/30/19. 20's are 255/30/20 & 285/30/20. Wheel weights come from the manufacturer.

Here is my conclusion. The 20" rim setup will perform better for 2 reasons. 1) The overall wheel & tire weight is 2lbs less. Not a big difference but a difference none the less.
2) Since the 20" tires weigh less than the 19's, even if the overall weight were the same, the 20's would perform better because there is less weight farther from the center of rotation. Since the overall tire diameter is roughly the same even though the rims are heavier the tire is the farthest point from the center. Having a lighter tire will perform better because of less rotational mass. Plus the overall package being lighter.

I'm no engineer but it logically makes sense to me that when you distribute the weight closer to the center of rotation it improves performance. Does this make sense? Thanks for any and all suggestions.
What is more dense? Rubber or metal? With 20s you are putting more metal at a farther distance from the center, hence higher rotational inertia (I) even though the tires weigh a bit less.

With the same design, 20s will for sure have higher "I" than 19s. Period.

Now, how much impact does that extra "I" have on the performance of the car? I don't know. Probably need to do some math to figure out.
2011 E92 M3 DCT
1991 Spec Miata
2008 E92 M3 DCT - KW Competition 2-way / Brembo 365 BBK / OMP ARS racing seats / rear seats delete / GT4 aero / AiM EVO4 data logger / SmartyCam racing camera
2008 E92 M3 6MT
2008 E92 335xi