Spring Rate Calculator
compiled and edited by Brad #1002
much of the content was provided
by cugino pegaso
Please read the Disclaimer before attempting any work in this
FAQ.
If you have ever wanted to be able to determine what strength spring is on you rear shock or in the front forks you can use this information to help you figure it out. You'll need to be able to measure the spring. That means it is best if it can be removed from the motorcycle. If you don't want to remove the spring you at least need to be able to reach it to measure it. Of course, the springs in the front forks can't be measured without being removed. See Suspension Modification FAQ for more one how to remove the fork springs.
The three measurements you need from your spring are:
Wire diameter - the diameter of the wire used to make the spring. Typically about .5 inches.
Coil diameter - the diameter from center to center of the coil. It can be calculated if you know the outside diameter of the coil and wire diameter (O.D. - Wire Dia.)
Number of coils - the number of free coils. It is often a fractional amount like 4.2.
The formula used to do these calculations is provided in case you are away from your computer and just want to use a calculator. The '^' notation indicates 'to the power of'. This formula applies to steel springs only.
rate = ( wireDia^4 * 1470000 ) / ( coilDia^3 * numCoils )
Related Spring Tips and Notes
Given that we have linkage rear suspensions (1 rear wheel movement translates into less than 1 of shock movement). An increase of 100#s/ at the spring, is less at the wheel. cugino pegaso
To count free coils this is what I do. Do not count the coil at each end that is flattened and really used for seating. starting where the spring takes off from the base coil, I count full coils from this point on, and then do my best guess as to fraction of coil before returning to the base coil at the other end. Basically once the free coils touch the base coil, the spring is done. The base coil can't provide any springing, and thus isn't free. cugino pegaso
Why doesn't the number of coils per inch come into play? If you have ever looked at a progressive spring, you'll find that one end has the coils closer together than the other. The rate is constant across the spring regardless of the coil spacing. What happens is once you begin to use the spring, the coils that are close together finally touch (coil bind) reducing the number of free coils, (making the denominator smaller in the equation) and increasing the spring rate for the remaining unbound coils. cugino pegaso
Other sites to visit with suspension information are; http://www.race-tech.com/ServicesDetail.asp?pid=15 or http://www.eibach.com/index1.htm