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A measure of how much a vehicle resists changes in its rotational speed. Cars with low polar moment of inertia (mass concentrated near the center) change direction more readily, while high polar moment provides stability but slower transitions.
Carroll Smith writes in Tune to Win: "A vehicle with a low polar moment of inertia is one which displays fast steering and cornering response — i.e. a maneuverable vehicle. We achieve this desirable feature by concentrating the mass of the vehicle within the wheelbase and as close to the longitudinal location of the CG as possible. Ergo the mid-engined racing car with minimum overhung mass." He makes the important distinction: "It is important to differentiate between polar moment of inertia and static weight distribution which is the amount of vehicle gross weight supported by the vehicle's rear wheels compared to that supported by the front wheels — with the vehicle at rest. By moving components around it is possible to effect the polar moment without changing either gross weight or static weight distribution." This is why mid-engine cars feel so responsive — not just because of weight distribution, but because mass is concentrated between the axles.