The job of a car suspension is to maximize the friction between the tyres and the road surface, to provide steering stability with good handling and to ensure the comfort of the passengers. If a road were perfectly flat, with no irregularities, suspensions wouldn't be necessary. But roads are far from flat. Even freshly laid roads have subtle imperfections that can interact with the wheels of a car. It's these imperfections that apply forces to the wheels. A bump in the road causes the wheel to move up and down perpendicular to the road surface. The magnitude, of course, depends on whether the wheel is striking a giant bump or a tiny speck. Either way, the car wheel experiences a vertical acceleration as it passes over an imperfection.
Without a structure in between, all of the wheel's vertical energy is transferred to the frame, which moves in the same direction. In such a situation, the wheels can lose contact with the road completely. Then, under the downward force of gravity, the wheels can slam back into the road surface. What is needed is a system that will absorb the energy of the vertically accelerated wheel, allowing the frame and body to ride undisturbed while the wheels follow bumps in the road.
The suspension system has two basic functions, to keep the car's wheels in firm contact with the road and to provide a comfortable ride for the passengers. A lot of the system's work is done by the springs. Under normal conditions, the springs support the body of the car evenly by compressing and rebounding with every up-and-down movement. This up-and-down movement, however, causes bouncing and swaying after each bump and is very uncomfortable to the passenger. These undesirable effects are reduced by the shock absorbers. The shock absorber is important in the safety of the car on the road as it irons out the continued bouncing of the car body and more importantly, the tendency for the body to sink down rapidly when the steering is changing the direction of travel. The effect can be unstable steering and another fact which is often overlooked accelerated tyre wear.
In order to provide flexibility to the suspension, various ball joints are used in the system, particularly on the commonly found lower wishbone. Some later vehicles have combinations of suspension arms and ball joints. Ball joints have rubber covers to prevent the ingress of water and road dirt. A split cover will eventually lead to joint failure and a potentially dangerous situation if the joint comes apart and allows the suspension to collapse.
A further item in the suspension system is the anti roll bar. Some car manufacturers have roll bars at both the front and the rear. The roll bar is kept under torsion and is fitted across the vehicle and is designed to remove the twisting motion of the body and suspension due to uneven surfaces. It is clamped to the body and the ends are connected to the suspension with some form of link. The most common failure at the roll bar can be the rubber bushes in these links. A result of a worn or broken link is often heard as a knocking noise as the car travels over bumpy ground.
Shock absorbers and struts can wear out and affect handling. If your car bounces excessively over bumps and leans hard in corners, your shock absorbers could be worn. Look behind the road wheel for the shock absorber or strut and check for signs of leaking oil. This is a sure sign of a failing shock absorber or strut. Check your shock absorbers or struts for leakage frequently. Also pay attention to how your car handles. If you notice the ride deteriorating take your car in to have them checked.
Ball joints wear and can cause your car to wander while driving down the road. This is dangerous as they can separate and cause you to lose control. Check that ball joint covers are secure and not split as the entry of water and road dirt will certainly accelerate wear
Ball joints, roll bar links and shock absorbers will be checked when your car is inspected annually for Mot, but regular inspections are essential as the Mot test only checks that at the time of test, the vehicle complies with minimum standards required for lawful use on the road.