Worth the Weight

What’s the difference between sprung and un-sprung weight? Which is more important?

The terms refer to the weights of the parts that are supported by the car’s springs, versus those that are not. For example, the chassis and all the parts on it are “sprung weight.” The springs hold the chassis up. The wheels and suspension arms are un-sprung weight, and are not supported by the springs. Racers look for ways to shave weight off of any parts, but trims made to un-sprung parts are particularly attractive because they allow the suspension to be more responsive. It’s simple: lighter parts have less inertia, so less force is required to make them change direction. Imagine waving your arm while holding a tennis ball versus a bowling ball—it’s much easier to wave the lightweight tennis ball. Likewise, the lighter a car’s wheels, tires, and moving suspension parts are, the more quickly the suspension can cycle over bumps to maintain traction.

Wheels, tires, suspension arms, and hub carriers move with the shocks and represent un-sprung weight. And the shocks themselves? The shafts are un-sprung weight, the shock bodies are sprung weight.