Springs
SPRINGS
Springs are the most important part of the suspension system, other than the tires. They are what holds the bike up, support it under braking and accelerating, and pushes the wheels into dips in the road. Without proper spring selection, the system will not work correctly, no matter what adjustments are made. Oil height is part of the spring component.
Spring Math: To rate an unknown spring:
(11,500,000 x (wire diameter) ^(4)) / (8 x (ID + wire diameter) ^(3) x active coils) Example: wire diameter = .489; ID = 2.275; active coils = 5.666 Your rate is = 687.0 (I checked this against a Hypercoil spring that I rated on my digital spring scale at 686 lbs/in, and on another spring using new variables that I rated at 805 lb/in, so it works)
Note: The paint on rear shock springs is very thick, you must get a very accurate measure of wire diameter for this formula to work. You may have to remove some paint with a razor blade to get the calipers onto bare metal. I found that the blue Hypercoils paint added 0.011" to 0.012" to the actual wire diameter.
Also: This formula assumes that the spring matirial is a high quality silicon spring. Lower grade springs may give erroneous results. The formula only works for non-tapered, non-progressive springs.
To figure out active coils:
Hold the spring upright and start from the bottom. When the flat end coil comes in contact with the first coil, that’s zero. Up from there, count the number of turns until it touches the other flat end coil. In most cases, it won't end up on an even number. Divide the full turn into 10 units. (Active coils = 8.5; or 9.2; or 7.8, etc.). I found that using a degree wheel or protractor gave me the most accurate divisions.
To figure out the combined rate using multiple springs:
The formula for two springs is: 1/K + 1/K2 = 1/K3 For three springs: 1/K + 1/K2 + 1/K3 = 1/K4 (K=spring rate) Example: You have an #80 tender and a #370 spring combination. 1 divided by 80 + 1 divided by 370 = 1 divided by K3
- 65.8 = K3
or
Combined Spring Rate = (Spring Rate 'A' x Spring rate 'B') ÷ (Spring Rate 'A'+Spring Rate 'B') Example: if the rate for spring 'A' is 200 and the rate for spring 'B' is 500, then: Combined Spring Rate = (200*500) ÷ (200+500) = 143
If you need to cut a spring to obtain a desired rate use this formula:
(K1 x Ac = K2 x AC) (K = spring rate) Example: a 60 lb. spring with 8.5 active coils =78.5 lbs. x 6.5 active coils Or 60 x 8.5 = 78.5 x 6
