Motorcycle Fork Spring and Shock Spring Tech Specs

Compression coil springs hold up a motorcycle whether it is a street, cruiser, enduro, off-road, or motocross bike. (Air has also been used and is still used in some applications, KYB PSF and Showa SFF Air).  Springs are the foundation of a suspension's performance.  With out the right spring rates you can not begin to achieve better performing suspension.

Spring Rate
Spring rate is a numerical value often expressed in kg/mm (kilograms per millimeter) or N/mm (Newtons per millimeter)

For example a 0.40kg/mm spring would compress 1mm for every .40kilogram weight placed on it.  4 kilograms placed on top of the spring would compress the spring 10mm.

Selecting the correct spring rate for the bike, rider weight/height, riding level, and riding application is crucial & starting point foundation for suspension setup.  The right spring rates setup the proper chassis height of the bike when the rider is aboard.  This is very important as it directly affects handling characteristics and suspension performance.

Spring Types

• Linear (Straight Rate):  Constant rate throughout the entire range of the spring.  Coils are evenly spaced apart.  Most commonly used spring type for motorcycles.
• Dual Rate: Whenever two or more different spring rates are stacked on top of each other a dual/triple rate spring is created.  The spring with closer/tighter spring coils will run out of travel sooner.  Once the coils bottom out (bind) together this effect makes the overall spring rate stiffer as the other spring continues to compress.
• Progressive:  Coils are not evenly spaced apart, instead the coils will be spaced tightly on one end and far apart on the other end.  Allows the spring rate to change minutely as it compresses but progressively build as the closer coils begin to bottom out.  Progressive springs are often found on KTM bikes with the linkageless swingarm design.

Free Length
The length of spring in its natural state (uncompressed or extended)

Spring Preload
The length of the spring that is compressed from its free length.  As spring preload is increased the spring requires more initial force to move.  As spring preload load is decreased the spring required less initial force to move.  Adding preload does not make the spring rate stiffer, instead the initial force required to compress the spring farther is changed, but the rate change stays the same.  The main function of spring preload is setting the ride height (suspension sag)

Example:

A linear 5.2kg/mm shock spring with 10mm of preload will require 53 kilograms of force to compress the spring another millimeter.  The same 5.2kg/mm spring with 5mm of preload will only require 27 kilograms of force to compress the spring an additional millimeter.

Useful Spring Equations

Springs in a series: 

1/Kt = 1/k1 + 1/k2 + 1/k3 + …… 

Kt = Total Spring Rate

K1 = First Spring Rate

K2 = Second Spring Rate

K3 = Third Spring Rate

Springs in parallel (Think front forks!)

Kt = K1 + k2 

Kt = Total Spring Rate

K1 = First Spring Rate

K2 = Second Spring Rate