Unsprung weight

In a ground vehicle
A vehicle is a device that is designed or used to transport people or cargo. Most often vehicles are manufactured, such as bicycles, cars, motorcycles, trains, ships, boats, and aircraft....

 with a suspension
Suspension (vehicle)
Suspension is the term given to the system of springs, shock absorbers and linkages that connects a vehicle to its wheels. Suspension systems serve a dual purpose — contributing to the car's roadholding/handling and braking for good active safety and driving pleasure, and keeping vehicle occupants...

, the unsprung weight (or the unsprung mass) is the mass of the suspension, wheel
A wheel is a device that allows heavy objects to be moved easily through rotating on an axle through its center, facilitating movement or transportation while supporting a load, or performing labor in machines. Common examples found in transport applications. A wheel, together with an axle,...

s or track
Caterpillar track
Continuous tracks or caterpillar tracks are a system of vehicle propulsion in which modular metal plates linked into a continuous band are driven by two or more wheels...

s (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
Sprung mass
In a vehicle with a suspension, such as an automobile, motorcycle or a tank, sprung mass is the portion of the vehicle's total mass that is supported above the suspension, including in most applications approximately half of the weight of the suspension itself...

.) Unsprung weight includes the mass of components such as the wheel axle
An axle is a central shaft for a rotating wheel or gear. On wheeled vehicles, the axle may be fixed to the wheels, rotating with them, or fixed to its surroundings, with the wheels rotating around the axle. In the former case, bearings or bushings are provided at the mounting points where the axle...

s, wheel bearing
Bearing (mechanical)
A bearing is a device to allow constrained relative motion between two or more parts, typically rotation or linear movement. Bearings may be classified broadly according to the motions they allow and according to their principle of operation as well as by the directions of applied loads they can...

s, wheel hubs, tire
A tire or tyre is a ring-shaped covering that fits around a wheel rim to protect it and enable better vehicle performance by providing a flexible cushion that absorbs shock while keeping the wheel in close contact with the ground...

s, and a portion of the weight of driveshaft
A drive shaft, driveshaft, driving shaft, propeller shaft, or Cardan shaft is a mechanical component for transmitting torque and rotation, usually used to connect other components of a drive train that cannot be connected directly because of distance or the need to allow for relative movement...

s, springs
Spring (device)
A spring is an elastic object used to store mechanical energy. Springs are usually made out of spring steel. Small springs can be wound from pre-hardened stock, while larger ones are made from annealed steel and hardened after fabrication...

, shock absorber
Shock absorber
A shock absorber is a mechanical device designed to smooth out or damp shock impulse, and dissipate kinetic energy. It is a type of dashpot.-Nomenclature:...

s, and suspension links. Even if the vehicle's brake
A brake is a mechanical device which inhibits motion. Its opposite component is a clutch. The rest of this article is dedicated to various types of vehicular brakes....

s are mounted outboard
Inboard brake
An inboard braking system is an automobile technology wherein the disk brakes are mounted on the chassis of the vehicle, rather than directly on the wheel hubs...

 (i.e., within the wheel), their weight is still considered part of the unsprung weight.

Effects of unsprung weight

The unsprung weight of a wheel controls a trade-off between a wheel's bump-following ability and its vibration isolation. Bumps and surface imperfections in the road cause tire compression—which induces a force on the unsprung weight. The unsprung weight then responds to this force with movement of its own. The amount of movement, for short bumps, is inversely proportional to the weight - a lighter wheel which readily moves in response to 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. In contrast, a heavier wheel which moves less will not absorb as much vibration; the irregularities of the road surface will transfer to the cabin through the geometry of the suspension and hence ride quality
Ride quality
Ride quality refers to the degree of protection offered vehicle occupants from uneven elements in the road surface, or the terrain if driving off-road. A car with very good ride quality is also a comfortable car to ride in. Cars which disturb vehicle occupants with major or minor road...

 and road noise are deteriorated. For longer 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 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 corrugations, washboarding
Washboarding is the name of the process which results in unmetalled roads developing a series of regular bumps with short spacing in the road surface...

 or "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.

High unsprung weight 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
Hotchkiss drive
The Hotchkiss drive is a system of power transmission. It was the dominant form of power transmission for front-engine, rear-wheel drive layout cars in the 20th century...

, a live axle
Live axle
A live axle, sometimes called a solid axle, is a type of beam axle suspension system that uses the driveshafts that transmit power to the wheels to connect the wheels laterally so that they move together as a unit....

 supported by simple leaf spring
Leaf spring
Originally called laminated or carriage spring, a leaf spring is a simple form of spring, commonly used for the suspension in wheeled vehicles...

s), vertical forces exerted by acceleration or hard braking combined with high unsprung mass can lead to severe wheel hop, compromising traction and steering control.

As mentioned above, there is a positive effect of unsprung mass. 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
Electronic filter
Electronic filters are electronic circuits which perform signal processing functions, specifically to remove unwanted frequency components from the signal, to enhance wanted ones, or both...

 stages, with the unsprung weight 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.

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
Beam axle
A beam axle is a suspension system, also called a solid axle, in which one set of wheels is connected laterally by a single beam or shaft...

 suspensions, in which wheels on opposite sides are connected as a rigid unit, generally have greater unsprung weight than independent suspension
Independent suspension
Independent suspension is a broad term for any automobile suspension system that allows each wheel on the same axle to move vertically independently of each other. This is contrasted with a beam axle, live axle or deDion axle system in which the wheels are linked – movement on one side affects...

 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
De Dion tube
A de Dion tube is an automobile suspension technology. It is a sophisticated form of non-independent suspension and is a considerable improvement over the alternative swing axle and Hotchkiss drive types. A de Dion suspension uses universal joints at both the wheel hubs and differential, and uses a...

 rear suspension). Lightweight materials, such as aluminum, plastic
A plastic material is any of a wide range of synthetic or semi-synthetic organic solids used in the manufacture of industrial products. Plastics are typically polymers of high molecular mass, and may contain other substances to improve performance and/or reduce production costs...

, carbon fiber
Carbon fiber
Carbon fiber, alternatively graphite fiber, carbon graphite or CF, is a material consisting of fibers about 5–10 μm in diameter and composed mostly of carbon atoms. The carbon atoms are bonded together in crystals that are more or less aligned parallel to the long axis of the fiber...

, and/or hollow components can provide further weight reductions at the expense of greater cost and/or fragility.

Inboard brake
Inboard brake
An inboard braking system is an automobile technology wherein the disk brakes are mounted on the chassis of the vehicle, rather than directly on the wheel hubs...

s 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. Jaguar's patented independent rear suspension
Jaguar independent rear suspension
Jaguar's independent rear suspension unit has been a common component of a number of Jaguar production cars since 1961, passing through three major changes of configuration up to 2003 and still used in the full-sized Jaguar XJ...

(IRS) further 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.
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