Sprung mass and Unsprung mass

The weight of the engine, transmission, various mechanical and electrical components, passenger cabin, passengers and various other components whose weight is supported by the suspension of a car in total is called sprung mass. The weight of the wheels, tyres, brakes are considered to be the unsprung mass of a vehicle which is defined as the mass between the road and the suspension.

The stiffness of the springs determines to what extent sprung mass reacts to the movements of a car. Loosely sprung suspensions are found in luxury cars that are set up for a comfortable ride as they can absorb bumps on the road and cushion the impact. However, a soft suspension will dive and squat under braking and acceleration and roll a lot during cornering. A stiff suspension set up will reduce dive and squat, it will also minimise body roll. But a stiff suspension compromises on ride quality which means that the ride will be bumpy and hard as the stiff springs are not capable of absorbing bumps on the road very well.

Higher the unsprung mass of a car higher the force required to move the car forward. Higher force used to move the car forward means higher the force exerted on the springs of the suspension of a car. This effects the balance of the sprung mass of a car which affects handling, therefore car maker constantly try to reduce the unsprung mass of a car in addition to lowering the sprung mass of a car. But modern luxury and creature comforts demanded in a car by customers adds to the sprung weight of a car which affects handling and fuel economy. Car manufacturers are constantly in the quest to find materials that can be used to build a car which are highly rigid and weigh less.