What is a coil spring?
A coil springs is a piece of bar that has been twisted to form the shape of a helix (spiral). It works in an up and down motion as a spring by using the vertical displacement to twist the bar making the spring. Therefore, it is effectively a torsion bar (similar to an antiroll bar) that works in a vertical motion.
The coil spring is the most common type of spring used within the automotive industry and the motorsport industry. This is because; it is light, variable in diameter, length and rate and is compact. All of these features mean that it can be designed very easily to control a system predictably and reliably.
Coil springs can have variable diameters throughout their length as well as different coil spacing and wire diameter. This means they are very tuneable and can have varying stiffness throughout their compression or extension range.
The Purpose of the Coil Spring
The main purpose of the spring in terms of motorsport is to keep the tyre in contact with the ground as much as possible regardless of cornering forces and road surface. If a wheel is moved vertically upwards by a bump in the road, the spring must be able to return the wheel back to the ground as fast as possible so grip can be re-gained. Alternatively, it needs to be able to compress enough to account for the bump so the car is not bounced in the first place but simply moves over the bump.
Another purpose of the coil spring is to limit the amount of body roll, squat or dive that occurs during cornering, acceleration and braking. Body is roll is undesirable but often having some squat can be useful to give rear end traction and having some dive can deliver some more front end grip under braking conditions.
The coil spring determines the amount that the suspension and therefore the wheel can compress or extend. The stiffer the spring, the lesser the movement of the wheel at a constant force.
The spring can also be used to determine the ground clearance of the vehicle by installing longer or shorter springs. It also determines the frequency that will control the wheel during its designed movement.
The stiffness of a spring is referred to as its spring rate and is measured in a number of units. One is in newtons per meter which is the scientific way to state the spring rate. This specifies how many units of force it takes to compress the spring by 1 meter. However, as most springs in the automotive and motorsport industry on compress by millimetres, the more common measurement is in newtons per millimetre. The higher the number, the stiffer the spring.
A stiff spring is often associated with motorsport as it prevents the body from rolling during cornering which provides more feedback to the driver and allows better control of the car when cornering. However, there still needs to be an element of softness to the spring as not all racing surfaces are perfectly smooth and flat. If a spring was too stiff, the car would bounce into the air and lose contact with the surface and therefore loose grip. Therefore, a fine line is to be found in spring stiffness so that the best in terms of compression and rebound can be found as well as resisting body roll.
One way of being able to run a soft enough spring to compress well but also be stiff enough to withstand cornering forces is to use an anti-roll bar in parallel. Therefore, the cornering forces are shared across the coil spring and the roll bar, increasing the effective spring rate at the wheel.
Coil springs can be used in series, parallel or both. Within motorsport and cars they are always used in parallel with one spring per side across the front and rear. The spring rates usually match side to side but differ from front to rear due to different masses at each end and wanting different handling characteristics.
What is a Helper Spring?
An often misunderstood part of suspension systems in the form of coil overs is the purpose of a helper spring. A helper spring is a smaller less stiff coil spring that is sat above or below the main coil spring. A common misconception is that a helper spring contributes to the stiffness of the car making it soft over bumps etc. this is incorrect. When the car is sat with its weight fully on the wheels, the helper spring is fully compressed. The helper spring only becomes useful when the wheel has left the ground or is in full droop.
When the wheel is in full droop, the coil over is fully extended and the spring can become totally unloaded and can leave the seat on the damper. This is not ideal as it can cause the damper to accelerate faster than it is designed to and cause damage or feel extremely hard and it can also cause the spring to rock over and damage the seat or the damper. Therefore, the helper sprig is there so that when full droop occurs, it extends and keeps the main coil spring pressed firmly into its seat, preventing it from lifting and rattling around. This improves the response of the coilover and maintains its predictability and improves the tune ability of the handling.