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The unsprung mass (colloquially unsprung weight) of a vehicle is the mass of the suspension, or Continuous track (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 , wheel bearings, wheel hubs, , and a portion of the weight of , springs, , and suspension links. that are mounted inboard brake (i.e. as on the drive shaft, and not part of the wheel or its hub) are part of a vehicle's sprung mass.
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. On dirt roads and on some softly paved roads, the induced motion generates small bumps, known as corrugations, washboarding or "corduroy" because they resemble smaller versions of the bumps in roads made of logs. These cause sustained wheel bounce in subsequent axles, enlarging the bumps.
High unsprung mass also exacerbates wheel control issues under hard acceleration or braking. If the vehicle does not have adequate wheel location in the vertical plane (such as a rear-wheel drive car with Hotchkiss drive, a live axle supported by simple ), vertical forces exerted by acceleration or hard braking combined with high unsprung mass can lead to severe wheel hop, compromising traction and steering control.
A beneficial effect of unsprung mass is that high frequency road irregularities, such as the gravel in an asphalt or concrete road surface, are isolated from the body more completely because the tires and springs act as separate filter stages, with the unsprung mass tending to uncouple them. Likewise, sound and vibration isolation is improved (at the expense of handling), in production automobiles, by the use of rubber bushings between the frame and suspension, by any flexibility in the frame or body work, and by the flexibility of the seats.
The term "unsprung mass" was coined by the mathematician Albert Healey of the Dunlop tyre company. He presented one of the first lectures taking a rigid analytical approach to suspension design, "The Tyre as a part of the Suspension System", to the Institution of Automobile Engineers in November 1924. This lecture was published as a 100-page paper.
can significantly reduce unsprung mass, but put more load on half axles and (constant velocity) , and require space that may not be easily accommodated. If located next to a differential or transaxle, waste heat from the brakes may overheat the differential or vice versa, particularly in hard use, such as racing. They also make anti-dive suspension characteristics harder to achieve because the moment created by braking does not act on the suspension arms.
The Chapman strut used the driveshafts as suspension arms, thus requiring only the weight of one component rather than two. Jaguar independent rear suspension (IRS) similarly reduced unsprung mass by replacing the upper wishbone arms of the suspension with the drive shafts, as well as mounting the brakes inboard in some versions.
motor scooter-type motorcycles use an integrated engine-gearbox-final drive system that pivots as part of the rear suspension and hence is partly unsprung. This arrangement is linked to the use of quite small wheels, further affecting their poor reputation for road-holding.
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