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).