## How to Rate Coil Springs Without a Spring Rater Tool

Mathematical Spring Rating Formula

Not many people have access to a spring rating tool. You can come a lot closer than you would think just using some dial calipers and a measuring tape to measure the spring rate of your springs. Here is how to do it:

SPRING RATE = GD4/8NM3

G = Torsional Modulus for Steel 11.25 x 106

D = Wire Diameter in Inches

N = Number of Active Coils

M = Mean Coil Diameter in Inches. Mean Diameter Is:

If using I.D. = 1 Wire plus Inside Diameter

If using O.D. = 1 Wire minus Outside Diameter

8 = A Constant for all Coil Springs

The “G” Factor is always the same for all coil springs made from

steel (11.25 x 106 can also be written as 11,250,000).

EXAMPLE: 10 active Coils and a mean coil diameter of 5.00 inches and a wire size of .625

11,250,000 x .625 x .625 x .625 x .625 = 171,661,370

8 x 10 x 5.0 x 5.0 x 5.0 10,000

(Constant) x (Active Coils) x (Mean Dia.) x (Mean Dia.) x (Mean Dia.)

Spring Rate = 171,661,370 / 10,000

Spring Rate = 171.66 lbs./per inch

HOW TO DETERMINE ACTIVE COILS OF A COIL SPRING:

Count total number of coils, subtract a coil for each coil that touches, these are dead coils. Ground flat ends are a dead coil. Start count with cut-off end facing you directly above would be one and so on. Not all coil rings are even coiled. You can have .125, .25, .5 or .75 of a coil (Example 10.25 Coils).