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Re: Gearing versus inertia (archive)

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Posted by Adrian on November 20, 2000 at 11:15:25:

In Reply to: Gearing versus inertia posted by Stan on November 20, 2000 at 10:28:30:

..Well the two tires are essentially identical in diameter so gearing is the a nonissue. So the main thing would the Moment of Inertia differences between the two sets of wheels. MOI is essentially telling us about the inertia effect when we try to increase or decrease the rotating speed of the wheel and tire. The same amount of mass further from the wheel center increases the MOI so larger wheels and tires actually have to be less massive than stock to have the same MOI. Another way to look at is is that the rolling stock acts like a flywheel. There is a radius squared effect here which is quite powerful. BTW, roughly what are the wheel and tire weights? Thanks.


The polar moment of inertia is proportional to the 4th power not squared ! makes a big difference and could explain the acceleration differences, I wish manufacturers would quote tyre weights.... The further away from the centre the greater the difference.

By the way most aftermarket 17's are real heavy approx 11-12 kg's per wheel the factory 16's are about 8kg,s I think. I am buying some dymags for my 911 C2 RS a rear 10 x 18 weighs less than 8kgs - now that is light !! - Shame about the cost !

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