Unsprung Mass - Unsprung Weight and Vehicle Design

Unsprung Weight and Vehicle Design

Unsprung weight is largely a function of the design of a vehicle's suspension and the materials used in the construction of suspension components. Beam axle suspensions, in which wheels on opposite sides are connected as a rigid unit, generally have greater unsprung weight than independent suspension systems, in which the wheels are suspended and allowed to move separately. Heavy components such as the differential can be made part of the sprung weight by connecting them directly to the body (as in a de Dion tube rear suspension). Lightweight materials, such as aluminum, plastic, carbon fiber, and/or hollow components can provide further weight reductions at the expense of greater cost and/or fragility.

Inboard brakes can significantly reduce unsprung weight, but put more load on half axles and (constant velocity) universal joints, 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 motor racing. They also make anti-dive suspension characteristics harder to achieve as the moment created by the act of braking is not reacted 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's patented independent rear suspension (IRS) similarly reduced unsprung weight by replacing the upper wishbone arms of the suspension with the drive shafts, as well as mounting the brakes inboard in some versions.

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 impacting the reputation for road-holding.