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Bill Verburg Bill Verburg is offline
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Join Date: Dec 2000
Posts: 23,329
Originally Posted by 1QuickS View Post
Steve is right. Rotational moment of inertia relates to how much torque is required for rotational acceleration. A pair of wheels can weigh the same but one will have higher rotational inertia making it swallow more torque to accelerate in rotation. The maximum impact of rotational inertia comes at the maximum radius of the part. Rubber is heavy and is at the maximum radius so tire weight affects rotational moment of inertia the most.

For those interested: J=(.5*Pi*R^4)
  • J is rotational moment of inertia
  • Pi is 3.14159
  • R is radius from rotational axis to point of mass

It is obvious that the distance (Diameter and width of tire in this case) affects torque requirement to accelerate to the 4th power.
It's actually way more complicated than that due to the geometry and where in the geometry the various mass is located.

There is no 4th power exponential, only exp 2

The rotating torque curves are 2nd degree and are also dependant on the linear acceleration of the vehicle as that determines the circular acceleration of the wheel/tire assembly. Inertial cost is is proportional to the mass, summed mass distribution radius and acceleration.

The gearing cost is separate and is linear in nature, it is ~12lb-ft per extra inch of OD
Bill Verburg
'76 Carrera 3.6RS(nee C3/hotrod), '95 993RS/CS(clone)
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