hmmm I just spilled my glass of ice water. Damned!! What??

In dynamics, for evaluating the rotational inertia of an object, there is a radius where you can consider the mass to be located -- that is called the radius of gyration. You then use the radius of gyration in the equation for inertia to come up with the moment of inertia of the object. Well, the moment of inertia of a rotating mass varies with the

**square** of the radius of gyration.

In this case, nobody knows the radius of gyration exactly since it would take some serious measurements and calculations, but we can guess it reasonably closely. It is obviously not the outer radius of the assemblies; most like the radius of gyration is roughly the inside radius of the swept area of contact on the flywheel -- somewhere in that region -- it won't be much if any closer to the inside.

So perhaps the radius of gyration is roughly 4.5" on the large UUC assembly and 3.875" on the stock assembly. That means the UUC would need a 35% lighter weight than stock to equal the same moment of inertia of the stock assembly (4.5^2 / 3.875^2). If we had a better estimate of the radius of gyration for the two, we could get a closer estimate.

People who just focus on weight are missing more than 1/2 of the problem since that radius of gyration has an exponential relationship to inertia.

Chuck