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Concept
Unsprung mass
Summary
The unsprung mass (colloquially unsprung weight) of a vehicle is the mass of the suspension, wheels or tracks (as applicable), and other components directly connected to them. This contrasts with the sprung mass (or weight) supported by the suspension, which includes the body and other components within or attached to it. Components of the unsprung mass include the wheel axles, wheel bearings, wheel hubs, tires, and a portion of the weight of driveshafts, springs, shock absorbers, and suspension links. Brakes that are mounted inboard (i.e. as on the drive shaft, and not part of the wheel or its hub) are part of a vehicle's sprung mass.
The unsprung mass of a typical wheel/tire combination represents a trade-off between the pair's bump-absorbing/road-tracking ability and vibration isolation. Bumps and surface imperfections in the road cause tire compression, inducing a force on the unsprung mass. The unsprung mass then reacts to this force with movement of its own. The motion amplitude for small duration and amplitude bumps is inversely proportional to the weight. A lighter wheel which readily rebounds from road bumps will have more grip and more constant grip when tracking over an imperfect road. For this reason, lighter wheels are sought especially for high-performance applications. However, the lighter wheel will soak up less vibration. The irregularities of the road surface will transfer to the cabin through the suspension and hence ride quality and road noise are worse. For longer duration bumps that the wheels follow, greater unsprung mass causes more energy to be absorbed by the wheels and makes the ride worse.
Pneumatic or elastic tires help by restoring some spring to the (otherwise) unsprung mass, but the damping possible from tire flexibility is limited by considerations of fuel economy and overheating. The shock absorbers, if any, also damp the spring motion and must be less stiff than would optimally damp the wheel bounce. So the wheels still vibrate after each bump before coming to rest.
Official source
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