Unsprung weight
From Free net encyclopedia
In a ground vehicle with a suspension, the unsprung weight is mass of the suspension, wheels or tracks (as applicable), and other components directly connected to them, rather than supported by the suspension. (The mass of the body and other components supported by the suspension is the sprung mass.) Unsprung weight includes the mass of components such as the wheel spindles, wheel bearings, tires, and a portion of the weight of driveshafts, springs, shock absorbers, and suspension links. If the vehicle's brakes are mounted outboard (i.e., within the wheel), their weight is also part of the unsprung weight.
Effects of Unsprung Weight
The reaction forces of the wheels or tracks to bumps and surface imperfections are transmitted by the suspension to the body. The greater the unsprung mass, the larger the forces exerted on the body and its occupants. Therefore, the smoothness and stability of the ride and the mechanical loads are closely related to the ratio of unsprung mass to sprung mass.
The inertia of the unsprung mass affects the suspension's ability to follow the shape of the road, ground or track. As a result, high unsprung weight has a negative effect on overall handling and braking ability, especially on imperfect surfaces.
Pneumatic or elastic tires (tyres) help by providing some springing for most of the (otherwise) unsprung mass, but the damping that can be included in the tires is limited by considerations of fuel economy and overheating. The shock absorbers, if any, damp the spring motion also and must be less stiff than would optimally damp the wheel bounce. So the wheels execute some vibrations after each bump before coming to rest. On dirt roads and perhaps on some softly paved roads, these motions form small bumps, known as "corduroy" because they resemble smaller versions of the bumps in roads made of logs. These cause sustained wheel bounce in subsequent vehicles, enlarging the bumps. These bumps are more common on US dirt roads than on French dirt roads, because French car design has paid more attention to reducing unsprung weight than American car design has, and possibly also because there is a wider range of resonant frequencies in French wheels or a wider range in the speeds of French drivers.
High unsprung weight also exacerbates wheel control 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 leaf springs), vertical forces exerted by acceleration or hard braking combined with high unsprung mass can lead to severe wheel hop, compromising traction and steering control.
Though this is usually not considered important, at least in the popular literature, there is a positive effect. 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 weight tending to uncouple them.
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 rather than the suspension (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 make a big difference, but put more load on half axles and (constant velocity) universal joints and require space that may not be easily accommodated.