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      09-22-2008, 12:41 PM   #2
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Drives: 2016 BMW M2
Join Date: Sep 2006
Location: Woodbridge, VA

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2016 BMW X5 35d  [0.00]
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.
It does make sense - less rotational inertia. In a disk, I = MR^2. Since the radial distance is squared, the distribution of the mass is a greater factor in inertia than the mass itself.