Can anyone tell me the way to figure out rotational inertia. Let me clarify what I am asking. I learned that because of rotational inertia, reducing the weight of a wheel for instance by 1 pound, equates to a reduction in the "perceived" weight the car "feels" by much more than 1 pound because of the concept of rotational inertia. I also know that the diameter of a wheel plays into this as well. I am contemplating changing the wheels on my S80 T6 and want to be able to calculate how much of a difference a reduction or addition in weight, and an enlargement in diameter would make. Any help would be appreciated.

Hey Crazy,

Have a look here for a good discussion of rotational inertia:

http://www.mta.ca/faculty/Courses/Physics/1551/Slide%20shows/Rolling%20can%20derby/Rotational%20Inertia%20-%20soup%20can%20derby/
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Dave in Virginia; 98 V70 T5(M): IPD MOTRONIC+ ECU Upgrade, Sport Muffler, VDO Boost Gauge, IPD Anti-Sway Bars; 88 245 DL; 67 MGB

While I do not know how to do it , it would be far easier to measure rotational inertia than it would be to calculate it. For instance, you could spin each wheel at the same speed and find out how much energy it takes to stop the wheel, twice the energy = twice the inertia. You could spin them on a wheel balancer and see how long they spin after you shut it off.

In order to calculate the difference, you would have to know the weight of the wheel/tire assembly at many different distances from the center. You might be able to calculate the difference if the wheels/tires were identical but different size, or different weights.

For instance, any weight at the outer edge of the wheel has far more inertia than the same weight near the center of the wheel.

I do not understand what you could use this information for.

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96 855R, 95 855,854, 90 744 Ti - 325,000 miles driven