Rotational Inertia
The rotational inertia topic was so big, it took me two months (June and August '99) to get it sorted out. Here's the deal: There is this rule of thumb among racers that adding weight to something that rotates is far more detrimental to performance than if you add it to the body of the car. This is absolutely true, and by bumbling through some physics, and after slipping and falling on a radian, I managed to get a few formulas figured out that could tell you just how much worse.

Any moving object has kinetic energy, as does an object sitting in place and rotating. An object that is both rotating and moving (like a rolling wheel, for example), has kinetic energy from both, meaning that accelerating or decelerating that rolling object will take more power than one that is just sliding along. How much more power is the question.

The answer, it turns out, depends on how the weight is distributed on the wheel. An extra pound on the tread of a rolling tire has as much kinetic energy as 2 lbs on the floor of the car. As you move toward the center of the wheel, the rotational effect drops until, at the center, a pound is just a pound. The formula I derived to determine the exact relationship between weight on a wheel and weight in the car isn't worth repeating here for one simple reason. It requires that you know the moment of inertia of the wheel, and measuring that is virtually impossible. What you need to know is that changing to tires that are 1 lb heavier will effectively add 8 lbs to the car (four tires, remember) and that adding a pound to the wheels will effectively add somewhere around 6 lbs to the car.

That only considers acceleration and braking; handling is dramatically affected by unsprung weight as well, but no simple formula is going to tell you how big the effect is.

Sport Compact Car :Technobabble: September 2001
The "Technobabble" Study Guide

By Dave Coleman