Coil springs make up part of the suspension system, which is the link between the vehicle body and the wheels. The purpose of suspension is to:
locate the wheels, whilst allowing them to move up and down, and steer
maintain the wheels in contact with the road and minimise road noise
distribute the weight of the vehicle to the wheels
reduce vehicle weight as much as possible – in particular the unsprung mass
resist the effects of steering, braking and acceleration
work in conjunction with the tyres and seat springs to give acceptable ride comfort
This full list is difficult to achieve completely and so different vehicles have different suspension systems, these depend on what the vehicle is designed to do. A compromise is reached, one that fits the fiscal demands of the overall car. A sports car will dedicate more of the design budget to the suspension than a small economical car.
The effect of suspension
As a wheel hits a bump in the road, it is moved upwards with quite some force. An unsprung wheel is affected only by gravity, which will try to return the wheel to the road surface. However, most of the energy will be transferred to the body. When a spring is used between the wheel and the vehicle body, most of the energy in the bouncing wheel is stored in the spring and not passed to the vehicle body. The vehicle body will only move upwards through a very small distance compared to the movement of the wheel.
These parts of the suspension system take up the movement or shock from the road. The energy of the movement is stored in the spring. The actual spring itself can be in many different forms, ranging from a steel coil to a pressurised chamber of nitrogen. Soft springs provide the best comfort, but stiff springs can be better for high performance. Vehicle springs and suspension therefore are made to provide a compromise between good handling and comfort. Coil springs can have applications for comfort, economy or performance. They are a popular choice for many vehicles, they require little attention and can be easily replaced following failure. Popular alternatives to coil springs can include leaf springs or air bags.
Independent suspension systems
Coil springs are generally used with independent suspension systems – the springs are usually fitted on each side of the vehicle, between the stub axle assembly and the body. The spring remains in the correct position because recesses are made in both the stub axle assembly and body. The spring is always under compression due to the weight of the vehicle and hence holds itself in place.
Coil spring features
The coil spring is a torsion bar wound into a spiral. It can be progressive if the diameter of the spring is tapered conically. A coil spring cannot transmit lateral or longitudinal forces, hence the need for links or arms. It produces little internal damping. No maintenance is required and high travel is possible.