Most modern cars have shock absorbers or hydraulic dampers as part of their suspension system. Usually combined with coil springs the shock absorbers primary function is to smooth out the cars ride and dampen any bounce generated by the springs as the car travels over the ground.
The shock absorber is constructed of hollow metal tubing which is sealed with end caps creating a chamber or cylinder which is filled with hydraulic oil or fluid. A piston located inside the chamber is connected to a rod which passes through seals at one end. The piston is able to move up and down the chamber but due to narrow passages in the piston the oil slows or dampens its movement.
The chamber end of the shock absorber (bottom) is bolted to the axle, trailing arm or strut while the piston rod end (top) is bolted to a reinforced section of the bodywork. This braces the suspension of the car slowing any bounce due to the resistance provided by the shock absorber.
There are several different types of shock absorber but the most common are either telescopic or strut. Telescopic shock absorbers are used with most suspension systems including trailing arm, wishbone, leading arm and swing axles. Strut shock absorbers or inserts are similar in design although are an integral part of the coil spring. The most common system of this type is the MacPherson strut which can be used on both front and rear wheels.
Shock absorbers generally have a fixed life expectancy although this can vary significantly depending on driving style, distance covered and terrain. They are generally non serviceable although should be checked regularly for early signs of wear or failure. The common point of failure is the seal where the piston rod exits the chamber. As the seals wear they are likely to allow oil to leak which will eventually reduce their efficiency resulting in poor ride and road holding. Other checks should include signs of damage from road debris including significant dents to the chamber; corrosion, pitting or scoring of the piston rods which will cause premature seal wear or damage to the mounting bolts, brackets or bushes.
When replacing shock absorbers it is generally recommended to so in axle pairs to ensure balanced suspension performance across the width of the vehicle.