Worn shock absorbers can adversely affect the way a car handles and brakes. Worn shock absorbers affect tire wear and ride quality for the passengers. A vehicle with worn shock absorbers may continue bouncing after driving over bumps or may feel as though it is constantly bobbing when driving on the freeway. Worn shock absorbers can also contribute to front-end "dive" when the brakes are applied.
A shock absorber's primary function is to dampen or resist the coil spring's "springing" action. It smoothes the initial bump (compression) and controls the bouncing "rebound" of the spring. Worn shock absorbers are no longer able to properly react to changes when the tire contacts the road surface during cornering, or when varying surface conditions are encountered (pot holes and speed bumps).
Premature shock absorber wear may be a result of driving habits—either by aggressive driving on rough road surfaces, or in the case of commercial vehicles, if recommended payloads are exceeded.
Shock absorbers can be tested by the old fashion "bounce test." (Many modern cars use lightweight body panels, so be careful not to apply excess force to the fenders as you can easily damage them.) First, bounce the car up and down by applying pressure to each corner of the vehicle, one at a time. Then, press down and release (to get full suspension travel) three times and let go—if the shocks are good, the vehicle should rise and fall and then settle. A good shock absorber will resist the constant bouncing of the spring.
The bounce test is not the sole indicator of worn shock absorbers. Visually inspect the shock absorbers for signs of leaking hydraulic oil. The shock absorbers or suspension struts should appear dry and free of oily, dusty residue. Look at the shocks on each side of the vehicle (both sides front and both sides rear) for comparison; they should appear visually similar.
On 1960s and 1970s American cars, vehicle weight was often a contributing factor to the necessity to replace shock absorbers every 30,000 miles or less. Worn shock absorbers on a very heavy car seriously compromise road handling to the point that the car may become dangerous to drive.