The unsprung mass in a car includes the mass of every component that is no controlled by the spings and dampers in the suspension. This includes the tires, the wheels, the brakes (if they are outboard, rather than inboard), the uprights and wheel bearings, the pushrods, as well as a portion of the mass of the the suspension links and tie rods, the half shafts and the springs and dampers. A key design goal is to reduce the unsprung mass of the car to lowest level possible. A lower unsprung mass gives two main advantages:
- Better grip – with less mass to move up and down, the springs and dampers don’t have to work as hard and can keep the tires in contact with the road more of the time, increasing grip.
- Better braking and acceleration – by lowering the mass of unsprung rotating components like wheels, tires and break disks, there is less mass to spin up and spin down (lower rotational moment of inertia to overcome). This improves acceleration and braking. Lowering unsprung mass is 1.7 more effective at improving acceleration and braking than reducing sprung mass.
The simplest way to reduce the unsprung mass of a car is to use smaller diameter wheels and tires. Reductio ad absurdum this would mean using tiny little wheels the size of a coffee mug. Obviously there is a trade off with reducing the wheel and tire sizes. First, smaller wheels don’t handle road imperfections (like potholes!) as well – this can actually decrease road holding capability. Second, when using outboard brakes, smaller diameter wheels require smaller diameter tires, which reduce braking ability. So if you want to keep your wheel tire diameters big, the only way to reduce your unsprung mass is to simplify the design and “add lightness” through the use of high-tech materials like carbon fiber.
Comfort is an ancillary benefit of lowering the unsprung mass of a car. In the automotive industry, comfort is known as “NHV”: Noise, Harshness and Vibration. By lowering the unsprung mass of the car, it is often possible to lower the spring rate, which lowers the suspension frequency and makes the ride more comfortable.