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The unsprung mass (colloquially unsprung weight) of a
vehicle
A vehicle (from la, vehiculum) is a machine that transports people or cargo. Vehicles include wagons, bicycles, motor vehicles (motorcycles, cars, trucks, buses, mobility scooters for disabled people), railed vehicles (trains, trams), wa ...
is the
mass
Mass is an intrinsic property of a body. It was traditionally believed to be related to the quantity of matter in a physical body, until the discovery of the atom and particle physics. It was found that different atoms and different elementar ...
of the
suspension
Suspension or suspended may refer to:
Science and engineering
* Suspension (topology), in mathematics
* Suspension (dynamical systems), in mathematics
* Suspension of a ring, in mathematics
* Suspension (chemistry), small solid particles suspend ...
,
wheel
A wheel is a circular component that is intended to rotate on an axle Bearing (mechanical), bearing. The wheel is one of the key components of the wheel and axle which is one of the Simple machine, six simple machines. Wheels, in conjunction wi ...
s or
tracks (as applicable), and other components directly connected to them. This contrasts with the
sprung mass
Sprung mass (or sprung weight), in a vehicle with a suspension, such as an automobile, motorcycle, or a tank, is the portion of the vehicle's total mass that is supported by the suspension, including in most applications approximately half of the ...
(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
axle
An axle or axletree 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 the vehicle, with the wheels rotating around the axle. In the former case, bearing ...
s, wheel
bearings, wheel hubs,
tire
A tire (American English) or tyre (British English) is a ring-shaped component that surrounds a Rim (wheel), wheel's rim to transfer a vehicle's load from the axle through the wheel to the ground and to provide Traction (engineering), t ...
s, and a portion of the weight of
driveshaft
A drive shaft, driveshaft, driving shaft, tailshaft (Australian English), propeller shaft (prop shaft), or Cardan shaft (after Girolamo Cardano) is a component for transmitting mechanical power and torque and rotation, usually used to connec ...
s,
springs,
shock absorber
A shock absorber or damper is a mechanical or hydraulic device designed to absorb and damp shock impulses. It does this by converting the kinetic energy of the shock into another form of energy (typically heat) which is then dissipated. Most sh ...
s, and suspension links.
Brake
A brake is a mechanical device that inhibits motion by absorbing energy from a moving system. It is used for slowing or stopping a moving vehicle, wheel, axle, or to prevent its motion, most often accomplished by means of friction.
Background ...
s that are mounted
inboard
An inboard motor is a marine propulsion system for boats. As opposed to an outboard motor where an engine is mounted outside the hull of the craft, an ''inboard motor'' is an engine enclosed within the hull of the boat, usually connected to a pr ...
(i.e. as on the drive shaft, and not part of the wheel or its hub) are part of a vehicle's sprung mass.
Effects
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
Ride quality refers to a vehicle's effectiveness in insulating the occupants from undulations in the road surface (e.g., bumps or corrugations).
A vehicle with good ride quality provides a comfort for the driver and passengers.
Importance
Good r ...
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. On dirt roads and on some softly paved roads, the induced motion generates small bumps, known as corrugations,
washboarding
Washboarding or corrugation
is the formation of periodic, transverse ripples in the surface of gravel and dirt roads. Washboarding occurs in dry, granular road material
with repeated traffic, traveling at speeds above . Washboarding creates an ...
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
Rear-wheel drive (RWD) is a form of engine and transmission layout used in motor vehicles, in which the engine drives the rear wheels only. Until the late 20th century, rear-wheel drive was the most common configuration for cars.
Most rear-wheel ...
car with
Hotchkiss drive
The Hotchkiss drive is a shaft drive form of power transmission. It was the dominant means for front-engine, rear-wheel drive layout cars in the 20th century. The name comes from the French automobile manufacturer Hotchkiss, although other makers, ...
, a
live axle
A beam axle, rigid axle or solid axle is a dependent suspension design in which a set of wheels is connected laterally by a single beam or shaft. Beam axles were once commonly used at the rear wheels of a vehicle, but historically they have als ...
supported by simple
leaf spring
A leaf spring is a simple form of spring commonly used for the suspension in wheeled vehicles. Originally called a ''laminated'' or ''carriage spring'', and sometimes referred to as a semi-elliptical spring, elliptical spring, or cart spring, it ...
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.
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
Filter, filtering or filters may refer to:
Science and technology
Computing
* Filter (higher-order function), in functional programming
* Filter (software), a computer program to process a data stream
* Filter (video), a software component tha ...
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.
Unsprung mass and vehicle design
Unsprung mass is a consideration in the design of a vehicle's suspension and the materials chosen for its components.
Beam axle
A beam axle, rigid axle or solid axle is a dependent suspension design in which a set of wheels is connected laterally by a single beam or shaft. Beam axles were once commonly used at the rear wheels of a vehicle, but historically they have als ...
suspensions, in which wheels on opposite sides are connected as a rigid unit, generally have greater unsprung mass than
independent suspension
Independent suspension is any automobile suspension system that allows each wheel on the same axle to move vertically (i.e. reacting to a bump on the road) independently of the others. This is contrasted with a beam axle or deDion axle system in w ...
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 mass by connecting them directly to the body (as in a
de Dion tube
De Dion rear axle
A de Dion tube is a form of non-independent automobile suspension. It is a considerable improvement over the swing axle, Hotchkiss drive, or live axle. Because it plays no part in transmitting power to the drive wheels, it is ...
rear suspension). Lightweight materials, such as
aluminum
Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. It has ...
,
plastic
Plastics are a wide range of synthetic or semi-synthetic materials that use polymers as a main ingredient. Their plasticity makes it possible for plastics to be moulded, extruded or pressed into solid objects of various shapes. This adaptab ...
,
carbon fiber
Carbon fiber-reinforced polymers (American English), carbon-fibre-reinforced polymers (Commonwealth English), carbon-fiber-reinforced plastics, carbon-fiber reinforced-thermoplastic (CFRP, CRP, CFRTP), also known as carbon fiber, carbon compo ...
, and/or hollow components can provide further weight reductions at the expense of greater cost and/or fragility.
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
HORIBA MIRA Ltd. (formerly the Motor Industry Research Association) is an automotive engineering and development consultancy company headquartered near Nuneaton in Warwickshire, United Kingdom. It provides product engineering, research, testing, ...
in November 1924. This lecture was published as a 100-page paper.
Inboard brake
An inboard brake is an automobile technology wherein the disc brakes are mounted on the chassis of the vehicle, rather than directly on the wheel hubs. Its main advantages are twofold: a reduction in the unsprung weight of the wheel hubs, as this ...
s can significantly reduce unsprung mass, but put more load on half axles and (constant velocity)
universal joint
A universal joint (also called a universal coupling or U-joint) is a joint or coupling connecting rigid shafts whose axes are inclined to each other. It is commonly used in shafts that transmit rotary motion. It consists of a pair of hinges lo ...
s, and require space that may not be easily accommodated. If located next to a differential or transaxle,
waste heat
Waste heat is heat that is produced by a machine, or other process that uses energy, as a byproduct of doing work. All such processes give off some waste heat as a fundamental result of the laws of thermodynamics. Waste heat has lower utility ...
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
The Chapman strut is a design of independent rear suspension used for light cars, particularly sports and racing cars. It takes its name from, and is best known for its use by, Colin Chapman of Lotus.
The characteristic feature of the Chapma ...
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 mass 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 affecting their poor reputation for road-holding.
See also
*
Sprung mass
Sprung mass (or sprung weight), in a vehicle with a suspension, such as an automobile, motorcycle, or a tank, is the portion of the vehicle's total mass that is supported by the suspension, including in most applications approximately half of the ...
Notes
External links
{{DEFAULTSORT:Unsprung Mass
Mass