The basic principle of any form of suspension is the isolation of one component or medium from an adjacent component or medium. 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.
Variations Of Isolation
In order to achieve the isolation required it is necessary to use a device or mechanism which will locate securely to the vehicle and locate just as well onto to the components which will eventually support the wheel/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 deforming further as a result of extra vehicle loading and/or road impact whilst travelling.
Common Factors Of The Three Types Of Suspension
There are three main types of suspension found on today’s vehicles; steel, rubber and fluid/pneumatic. Each of these have 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, many years and has the advantage of being able to locate, an axle for example, without much in the way of extra linkages. The coil spring, which is probably the most common form of car/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 the 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 (moveable) and the other to the structure/bodywork (fixed). 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 will simply compress and the torsion bar will absorb the movement by twisting about its length.
Absorbing That Energy
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 (fixed) and the suspension linkage (moveable); upon impact the force from the tyre/wheel is transferred through the strut to the rubber spring.
Fluid/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. The deflection within the unit is provided by the compression of a volume of gas, usually nitrogen. Interlinked systems can be as simple as the Hydrolastic/Hydragas suspension found on British Leyland/Rover vehicles of some years ago, or as complex as the type fitted to some Citroen models. These systems provide bump energy absorption via pressure spheres and/or the controlled flow of a special fluid from one part of the vehicle to another.
Varying Degrees Of Shielding
As previously mentioned, all spring mediums are able to absorb varying degrees of impact thus shielding, to a reasonable extent, the vehicle occupants 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 concerned 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/pneumatic suspensions are to a large extent self damping thus removing the requirement for external damping.