Virtually every car that is on the road today has a set of springs either in coil over form (the twisty kind) or leaf springs. You may have heard the term “spring rate” and wondered exactly what it meant and what it entailed.
Put simply, spring rate is the amount of weight that is needed in order to compress a coil over spring a certain distance. Springs are rated by the amount of pounds it takes to compress them one inch or the amount of kilograms that it takes to compress the spring by 1 millimeter. The number will all depend on whether or not it is stated as standard (pounds and inches) or metric (kilograms and millimeters) form.
Springs that have a lower spring rate are said to be “softer” while springs that have a higher spring rate are said to be “stiffer.” Depending on the application that you want the spring for will depend on whether or not you want a softer spring rate or a stiffer spring rate. For example, a passenger car may have a need for a softer rated spring where as a race car may have the need for a stiffer rated spring.
Typically on a spring there will be one or two values that are listed. If there are two, the first number will represent how much force is needed to compress the spring by the first inch and the second will represent how much additional force will be needed in order to compress the spring the second inch. So a 10lb 25lb progressive spring for example will need 10 pounds of force in order to compress the spring the first inch and an additional 13 pounds to compress it the second inch for a total of 25 pounds.
In coil over springs there are three factors that will affect the spring rate and these are:
- Diameter of the Spring Wire: The diameter or thickness of the spring wire will directly affect the spring rate as the greater the diameter the greater the spring rate. So the thicker the spring wire becomes the harder the spring will be to compress.
- Diameter of the Spring Itself: When the diameter of the spring itself gets bigger, or wider, then the spring rate will be affected. The affect will be a decrease in spring rate, meaning it will be easier to compress the spring.
- Height or Number or Active Coils on the Spring: As the spring gets taller, or the active coils increase then it becomes easier to compress the spring. Therefore a spring that is taller has a decreased spring rate whereas a spring that has less active coils or is shorter will be harder to compress and therefore will have a higher spring rate.
As there are different types of springs there will also be different types of spring rates. As previously mentioned, a progressive spring will be stated in two different spring rates as will a linear spring where a normal spring has only one fixed spring rate. The spring that is right for you and your vehicle again all comes down to the application you desire and the softness of stiffness you wish to achieve from the springs.