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Old 01-05-2013, 05:23 AM
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Chris90 Chris90 is offline
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Location: Massachusetts
 
Join Date: Apr 2003
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Mein Auto: '04 330i
Quote:
Originally Posted by bmw_or_audi View Post
When it comes to larger wheels, the penalty is not simply the extra weight, but also the increased moment of inertia (AKA rotational inertia). Even with identical weight, the 19" will have higher moment of inertia than 18", which will affect acceleration. Moment of inertia for a small mass m scales with m * r^2, where r is the radius about the axis of rotation. Note how it is a squared term. The big heavy rim of the wheel will be one inch further out from the axle, increasing moment of inertia by about 11% (for the rim alone). I would have to make a back of the envelope calculation to know to what extent this affects acceleration, but in the hair splitting regime it may make a very real difference.
A 3500 lb car would have 1/2 (3500 lb) V^2 kinetic energy, while the KE of four 40 lb wheel/tires would be 2 (40lb) V^2. Add 11% and it would be 2 (44 lb) V^2.

8 /1750 lbs = 0.5%. Like losing 1 hp. Not anything you would notice.
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