Coil springs are simple devices that allow us to support the weight of a vehicle with the suspension. Associated with the spring is what we refer to as the spring rate expressed as pounds per inch. If we have a coil rated at 300, then it would compress 1 inch if a 300-pound weight was placed on it. It would compress 2 inches if a 600-pound weight was placed on it, and so on. Too light a spring rate and the suspension has too much sway and bottoms out easily on large bumps. Too stiff a spring rate and the vehicle rides horribly and bounces around instead of soaking up rough terrain.
There are three-dimensional properties that determine the spring rate of a coil. These are: coil wire diameter, coil spring mean diameter, and number of active coils as shown in this diagram. Larger wire diameter equals higher spring rate. However, as the mean diameter or the number of active coils increases, the spring rate decreases. This is counter-intuitive and often confuses some enthusiasts.