Coil Spring

A coil spring is a length of spring steel wound into a helix that supports part of the vehicle's weight at each corner and absorbs the shocks transmitted by an uneven road. It is by far the most common type of spring in modern car suspension, found on the great majority of passenger vehicles, and its job is to allow the wheel to move up and down relative to the body so that the body remains comparatively level and the tyre stays in contact with the surface. Without a spring, every bump would be transmitted directly into the structure and the occupants.

The spring works by storing energy as it is compressed. When a wheel strikes a bump, the spring shortens and stores the energy of the impact; as the wheel drops into a dip, the spring extends and releases that energy. The stiffness, or rate, of the spring is determined by the diameter of the steel wire, the diameter of the coil, the number of active turns and the properties of the steel itself, which gives engineers a wide range of parameters to tune the ride to a particular car. Crucially, a coil spring on its own would oscillate up and down repeatedly after a disturbance, so it always works in concert with a damper, which controls and dissipates that bouncing motion.

For the vehicle and its occupants, the coil spring delivers the balance of comfort and control that defines a car's ride. A softer spring soaks up bumps for a plush ride but allows more body roll and pitch; a firmer spring keeps the body flatter for sportier handling at the expense of comfort. The spring also maintains a consistent contact load at the tyre, which is essential for grip, braking and steering. Its compactness compared with older spring types allows suspension to be packaged efficiently, freeing space within the wheel arches and the body.

The coil spring rose to prominence as suspension design evolved away from the leaf springs of earlier and heavier vehicles. It is light, durable and, because its behaviour is easily predicted and adjusted, well suited to the independent suspension layouts that dominate modern cars. Variations include progressive-rate springs, whose coils are unevenly spaced so that stiffness increases as the spring compresses, giving a soft initial response with greater control under heavy load, and uprated springs fitted to lower or stiffen a car.

In practical terms coil springs are robust and long-lived, but they can sag with age, lowering the ride height, or fracture, particularly where corrosion has weakened the steel, which produces a knocking noise and uneven stance. They are a cornerstone of the suspension system and are most often paired with a damper inside a MacPherson strut, where spring and damper are combined into a single structural unit. They stand alongside leaf springs and torsion bars as the principal means by which a car is sprung.