Americans call them dampers, a name that describes their function more accurately than shock absorbers.

There's normally one for each wheel of a vehicle and they live in the dark recesses behind the wheels as part of the suspension system.

Shocks are needed to dampen the oscillations, which a spring will try to make if it has been flexed and suddenly released. Think of a weight hanging on a light coil spring. If you pull it down to stretch the spring and then suddenly release it, it will oscillate or bounce up and down for a long time as the spring keeps contracting and extending.

Replace the weight with a car wheel and the light spring with a much heavier coil spring, leaf spring or torsion bar and it becomes clear why we need dampers to quickly dampen the up-and-down bouncing. We don't want to drive a vehicle whose wheels keep jumping up and down every time we go over a bump.

All modern dampers are of the hydraulic, tubular, telescopic design. Inside them, at the bottom end of the rod protruding from the housing, is a piston that can slide up and down a tube filled with hydraulic fluid.

The top end of the rod is attached to the car body and the bottom of the housing is attached to the suspension components carrying the wheel.

When the wheel goes over a bump, the piston is forced to move down inside the tube, which means that some of the hydraulic fluid previously below the piston must find its way elsewhere. This is made difficult for the fluid by forcing it through constricting openings that create resistance to the free flow of the fluid. The same thing happens when the spring tries to rebound.

Now the piston has to move upward in the tube, but again its movement is hampered by fluid being forced through constricting openings. In this way the oscillations are damped very quickly by fluid friction.

The original telescopic damper suffered from the disadvantage that air bubbles will form in the fluid if the piston is forced to move rapidly up and down. This reduces the effectiveness of the damper because air, unlike fluid, will have little difficulty squashing through narrow openings.

The problem of aeration was counteracted by filling the space above the fluid with gas (usually nitrogen) under pressure. A further step forward was adjustable dampers, which allow one to change the degree of stiffness.

On more expensive cars this control is being taken over by the on-board computer in response to sensors, allowing both a smooth ride for comfort and a firm suspension when needed for roadholding.

Dampers wear out gradually. The symptoms of worn shocks include the following:

l Excessive roll in turns;

l Excessive nose-dive when braking;

l Uneven tyre wear, especially "cupping", caused by tyres jumping up and down;

l Vehicle veers in cross-winds.

How long shock absorbers will last depends on factors such as the original quality, how hard the shock has to work and the loading of the vehicle.

Remember to take the vehicle to an experienced technician once every 20000km and let him drive the car, examine the tyres, and visually inspect the dampers for dents, fluid leakage or worn mounting bushes.

Fluid seepage or "weeping" is not uncommon on shock absorbers and does not affect their performance, but a substantial leak down the side of the damper indicates a worn or damaged seal, which means the shock should be replaced or the strut refurbished.

Replacing a shock absorber on a separate spring and damper suspension is simple. On a strut suspension it's more complicated and best left to a professional.