Shock absorbers are the part of a car's suspension system that do exactly as their name suggests. They dampen the vertical motions resulting from driving on bumpy roads so passengers are not jostled about. Shock absorbers do not handle this job entirely by themselves, rather they work together with springs to absorb and dissipate vertical movement.
Without the dampers, the energy collected by the springs would just bounce until all of the energy wore off. Driving a car with springs but no shock absorbers would require a stomach of steel, motion sickness medicine and probably safety pads for protection, as it would be a bumpy, bouncy ride. Shock absorbers dampen unwanted spring movements by converting the kinetic energy stored in the springs into heat, which is then removed by use of hydraulic fluid.
The part is essentially a pump. The upper mount of the damper is connected to the car's frame and the lower mount is hooked up the the car's axle, near the wheel. The most common design is a twin-tube setup where the upper mount contains a piston rod, which is connected to a piston. The piston sits in a tube filled with hydraulic liquid, called the pressure tube. This is housed inside the lower mount, which is also referred to as the reserve tube.
When the car hits a bump in the road, the spring will coil and uncoil and the energy from this action transfers to the shock absorber. The piston rod travels into the pressure tube, and the up-and-down motion causes hydraulic fluid to slowly seep through a number of small holes in the piston. As this process continues, the piston's motion will slow and bring the spring to a stop.
Shock absorbers go through two different cycles - extension and compression. The compression cycle occurs when the piston moves down into the pressure tube, compressing the hydraulic fluid. The extension cycle is when the piston moves back up and compresses the collected fluid in the space above the piston.
When shock absorbers begin to wear out, they become less effective at handling the car's movements. This will lead to abnormal wear on your vehicle's tires and other components of the suspension system, and it can have an adverse effect on the car's handling. The absorbers may also be leaking hydraulic fluid, which will result in weaker responses and make for a bouncier ride.
One simple way to assess the health of your car's shock absorbers is to perform a bounce test. Put pressure on the bumper so the car actually dips a bit and get the car rocking up and down before you release. The car should come to a stop after only one or two bounces, but if it continues to move, the dampers could be worn.
When the shock absorbers wear out, replacing them will not only keep your ride smooth and comfortable, but it can improve the safety of a car. Weak dampers can make it difficult for the tires to stay on the ground when you slam on the brakes. The action will set the springs in motion, but the shock absorbers may not be able to dissipate the spring energy, which can prolong the bouncing. In some situations, this may even cause the tires to hop instead of staying in contact with the road surface.