The purpose of the complete suspension system is to isolate the vehicle body from road shocks and vibrations which would otherwise be transferred to the passengers and load.
It must also keep the tires in contact with the road, regardless of road surface.
A basic suspension system consists of springs, axles, shock absorbers, arms, rods, and ball joints.
The spring is the flexible component of the suspension. Basic types are leaf springs, coil springs, and torsion bars.
Modern passenger vehicles usually use light coil springs.
Light commercial vehicles have heavier springs than passenger vehicles, and can have coil springs at the front and leaf springs at the rear.
Heavy commercial vehicles usually use leaf springs, or air suspension.
Solid, or beam, axles connect the wheels on each side of the vehicle. This means the movement of a wheel on one side of the vehicle is transferred to the wheel on the other side.
With independent suspension, the wheels can move independently of each other, which reduces body movement. This prevents the other wheel being affected by movement of the wheel on the opposite side, and this reduces body movement.
When a wheel strikes a bump, there is a reaction force, and energy is transferred to the spring which makes it oscillate. Oscillations left uncontrolled can cause loss of traction between the wheel and the road surface.
Shock absorbers dampen spring oscillations by forcing oil through small holes. The oil heats up, as it absorbs the energy of the motion. This heat is then transferred through the body of the shock absorber to the air.
When a vehicle hits an obstruction, the size of the reaction force depends on how much unsprung mass is at each wheel assembly.
Sprung mass refers to those parts of the vehicle supported on the springs. This includes the body, the frame, the engine, and associated parts.
Unsprung mass includes the wheels, tires, brake assemblies, and suspension parts not supported by the springs.
Vehicle ride and handling is improved by keeping unsprung mass as low as possible. Wheel and brake units that are small and light follow the road contours without a large effect on the rest of the vehicle.