Polar moment of inertia

Imagine spinning a barbell around its middle. With the weights near the center it spins easily; with the weights at the ends it resists. Cars are the same. When heavy components like the engine, fuel and driver sit close to the middle, the car has a low polar moment and changes direction eagerly. When mass sits at the ends, it has a high polar moment.

A low polar moment, common in mid-engine layouts, makes the car responsive and quick to rotate, but also quick to spin once the rear lets go, and harder to catch. A high polar moment feels more stable and lazy: it resists rotating, and when it does slide, it does so more slowly.

For drivers, the practical lesson is to know which kind of car you are in. In a low-polar-moment car, be smooth with lifts and corrections and react early to a slide. Adding weight at the extremes, such as a heavy spare in the trunk, raises the moment slightly.