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. While two springs may look about the same side by side, it doesn’t take a huge difference in the wire diameter to significantly affect the spring rate. Conversely, two springs with markedly different lengths and diameters may actually have the same spring rate.While two springs may look about the same side by side, it doesn’t take a huge difference 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. Manufacturers may make a single coil spring somewhat progressive so it has a rising spring rate as it is compressed. This can be done by varying the wire diameter or the mean diameter over the length of the coil spring, or by changing the pitch spacing. Pitch is the center-to-center distance between adjacent coils on a spring. As the lower rate coils on a progressive spring collapse during compression, the overall spring rate rises as fewer active coils remain in action.Manufacturers may make a single coil spring somewhat progressive so it has a rising spring When a vehicle sits on coil springs on a straight axle, the spring rate is applied vertically in a 1:1 ratio. That is to say a 400-pound-rated spring compresses 1 inch for each 400 pounds applied to that corner of the vehicle. However, coil springs used with A-arms will typically be rated much higher as they work through a leverage ratio. Force is applied through the tire at the end of the A-arm, but the shock is attached at some shorter distance from the A-arm point of rotation.When a vehicle sits on coil springs on a straight axle, the spring rate is applied vertica A traditional leaf spring pack has some internal friction to its movement. However, a coil spring has no such friction. As such, shocks used in coil applications need to have greater damping action than those for leaf applications. It’s not uncommon to see dual shocks used on vintage Broncos with front coil springs.A traditional leaf spring pack has some internal friction to its movement. However, a coil By Jay Kopycinski Enjoyed this Post? Subscribe to our RSS Feed, or use your favorite social media to recommend us to friends and colleagues!