How Car Suspension Works

A spring absorbs the bump. Without something to control it, the spring then throws the car back at you.

The basic principle of any form of suspension is the isolation of one component or medium from an adjacent one. In the case of a motor vehicle it is desirable, if not essential, that the car body and its occupants are isolated from the harsh road surface. There are of course hundreds of components which would not take too kindly to the relatively high levels of vibration induced by road travel, but in this article comfort for the driver and passengers is all that will be considered.

Achieving the isolation

In order to achieve the isolation required, it is necessary to use a device which will locate securely to the vehicle and locate just as well onto the components which will eventually support the wheel and tyre assembly. The best thing for this task is some form of spring, usually positioned one at each corner of the vehicle, but there are variations on this theme. The spring used will, to a certain extent, deform under the weight of the static vehicle, then deform further as a result of extra vehicle loading or road impact whilst travelling.

Three types, three things in common

There are three main types of suspension found on today's vehicles: steel, rubber and fluid or pneumatic. Each of these has certain factors in common. They will deform as a result of impact, they require a robust mounting system, and they all have a tendency to rebound, sometimes at an alarming rate.

Common forms of springing

Steel springing will normally take one of three forms: leaf, coil or torsion bar. The leaf spring has been fitted to many vehicle types over many years and has the advantage of being able to locate, for example, an axle, without much in the way of extra linkages. The coil spring, which is probably the most common form of car and light commercial springing, has no such advantage, requiring turrets, linkages and platforms for accurate location, but it does have the very useful attribute of being compact and therefore easier to incorporate into modern vehicle design. The less common torsion bar is quite simply a square or round section bar fixed at one end to the suspension arm and at the other to the structure or bodywork.

The operation of each of the above as a result of impact is for the leaf spring to deflect whilst increasing in length, the coil to simply compress, and the torsion bar to absorb the movement by twisting about its length.

Rubber and fluid systems

Rubber suspension is much the same as steel in that it is the principle of deflection and temporary deformation which absorbs the impact. Unlike steel, rubber has the ability to absorb a larger amount of energy per unit of its mass, and it also produces much less rebound. Mounting the rubber suspension unit is a relatively simple case of mounting a strut assembly between the structure and the suspension linkage; upon impact the force from the tyre and wheel is transferred through the strut to the rubber spring.

Fluid and pneumatic suspension systems, which rely on the transfer of fluid, sometimes under gas pressure, from one area of the car to another, can be by their very nature quite complex. Some systems require only a pressure sphere mounted at each wheel to provide the absorption required, with deflection provided by the compression of a volume of gas, usually nitrogen. Interlinked systems can be as simple as the Hydrolastic and Hydragas suspension found on British Leyland and Rover vehicles of some years ago, or as complex as the type fitted to some Citroën models. These systems provide bump energy absorption via pressure spheres and the controlled flow of a special fluid from one part of the vehicle to another.

Why the damper matters

All spring mediums are able to absorb varying degrees of impact, thus shielding the vehicle occupants to a reasonable extent from the harshness of the road surface. However, springs have the nasty habit of bouncing, and this aspect of their character must be brought under some form of control if the vehicle is not to leap around like a scared rabbit.

This is the task of the shock absorber, or more correctly, the damper. To drive a vehicle on the road without some form of effective damping is akin to a suicide mission, and a frightening experience to say the least. The suspension damper not only controls the magnitude of spring deflection but also shortens the time taken for the spring to return to its normally laden state. This also has the desirable effect of keeping the tyres in contact with the road. Fluid and pneumatic suspensions are to a large extent self damping, thus removing the requirement for external damping.