Unsprung Mass
Unsprung mass is the mass of wheels and other components that move substantially with the road side of the suspension rather than with the supported body.
What belongs in the term
Wheels, tires, hubs, brakes, and wheel carriers are largely unsprung. Parts of control arms, springs, dampers, driveshafts, and steering links are shared between sprung and unsprung motion, so engineering models allocate an effective portion rather than treating every component as entirely one or the other.
Lower unsprung mass can make it easier for the wheel to follow short road inputs and can reduce forces transmitted through the suspension. The outcome still depends on tire stiffness, wheel rate, damping, geometry, and component strength. A lighter wheel or brake is therefore not an isolated guarantee of comfort or grip.
For wheel and body motion, see Suspension Kinematics and Vehicle Dynamics. Construction choices that determine which parts move with the wheel are covered in Suspension Construction.