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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 handling and braking for good active safety and driving pleasure, and keeping vehicle occupants comfortable and reasonably well isolated from road noise, bumps, and vibrations. These goals are generally at odds, so the tuning of suspensions involves finding the right compromise. The suspension also protects the vehicle itself and any cargo or luggage from damage and wear. Design of front and rear suspension of a car is different.
Springs and dampers
All suspensions use springs to absorb impacts and dampers (or shock absorbers) to control spring motions. A number of different types of each have been used:
Springs
Dampers or shock absorbers
The shock absorbers damp out the, otherwise resonant, motions of a vehicle up and down on its springs. They also must damp out much of the wheel bounce when the unsprung weight of a wheel, hub, axle and sometimes brakes and differential bounces up and down on the springiness of a tire. The "corduroy" bumps found on dirt roads are caused by this wheel bounce. These bumps are more common in US cars, where solid rear axles are common, than they are in e.g. French cars where unsprung weight tends to be low and suspensions well damped.
Suspension types
Suspension systems can be broadly classified into two subgroups - dependent and independent. These terms refer to the ability of opposite wheels to move independently of each other.
A dependent suspension normally has a live axle (a simple beam or 'cart' axle) that holds wheels parallel to each other and perpendicular to the axle. When the camber of one wheel changes, the camber of the opposite wheel changes in the same way.
An independent suspension allows wheels to rise and fall on their own without affecting the opposite wheel. In this case, the wheels are either not connected at all or are connected through universal joints with a swing axle. Suspensions with other devices, such as anti-roll bars that link the wheels in some way are still classed as independent.
A third type is a semi-dependent suspension. In this case, a swing axle is used, but the wheels are also connected with a solid tube, most often a deDion axle.
Interconnected suspensions (mechanically interconnected, such as anti-roll bars; and hydraulically or pneumatically interconnected, e.g. SAE 2005-01-3593, SAE 2003-01-0312) have also been attempted to achieve a better compromise among vertical, roll and pitch properties.
Dependent suspensions
Dependent systems may be differentiated by the system of linkages used to locate them, both longitudinally and transversely. Often both functions are combined in a set of linkages.
Examples of location linkages include:
In a front engine rear drive vehicle, dependent rear suspension is either "live axle" or deDion axle, depending on whether or not the differential is carried on the axle. Live axle is simpler but the unsprung weight contributes to wheel bounce.
Independent suspensions
The variety of independent systems is greater and includes:
New suspension technologies in development include a system from the Bose Corporation which uses computer-controlled motors to automatically adjust the suspension to changing road surfaces, keeping a vehicle level and in contact with the road even at high speed over bumpy roads, or in hard cornering.
Armoured fighting vehicle suspension
This M3 Grant tank's suspension has road wheels mounted on wheel trucks, or bogies.
Military AFVs, including tanks, have specialized suspension requirements. They weigh up to seventy tons and are required to move at high speed over very rough ground. Their suspension components must be protected from land mines and antitank weapons. Tracked AFVs can have as many as six or eight road wheels on each side. Many wheeled AFVs have six or eight wheels, to help them ride over rough ground.
The earliest tanks of the Great War had fixed suspensions—with no movement whatsoever. This unsatisfactory situation was improved with leaf spring suspensions adopted from agricultural machinery, but even these had very limited travel.
Speeds increased due to more powerful engines, and the quality of ride had to be improved. In the 1930s, the Christie suspension was developed, which allowed the use of coil springs inside a vehicle's armoured hull, by redirecting the direction of travel using a bell crank. Horstmann suspension was a variation which used a combination of bell crank and exterior coil springs, in use from the 1930s to the 1990s.
By the Second World War the other common type was torsion-bar suspension, getting spring force from twisting bars inside the hull—this had less travel than the Christie type, but was significantly more compact.
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