Cutting Springs: Rate per coil calculation

[Mucci]

Maybe someone's already done this but I couldn't find it. I was curious how much the spring rate increased with the amount of coil removed.

 

According to the Blue Book, the spring wire diameter and spring OD is the same, front and rear, for 1502, 1602, 1802, 2002, 2002ti, & 2002tii. 

With that information all you need to do is count the amount of active coils you have to figure out rate:

 

 

(* ignore "front" in the above pic. This is the same for the front & rear.)

 

I.E. - My 1975 2002 had 6 active coils in front, stock. I cut 1.5 coils so the front rate went from 120.7 in/lbs to 161 in/lbs. 

 

...of course this is all in a perfect world where deterioration doesn't exist and all materials are identical, but hey, it's a ballpark. 

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Formula:  in/lbs = Gd^4 / (8ND^3)

 

d = wire size (inches)
D = Mean Diameter (inches)
N = Number of active coils
G = Shear Modulus of Material

 

d = .465"

Spring outer diameter = 4.992"
D = 4.527"
N = 6 (front on my '75 2002)
G = 11,500,000 (Oil Tempered MB - ASTM A229 gen. purpose spring steel) 
 

 

(Calculator here: https://www.engineersedge.com/calculators/comp_spring_k_pop.htm)

 

Dimensions from the Blue Book:

 

 

 

 

 

 

Edited February 15, 2021 by Mucci
duplicate images

[robspeed]

Easy way is just count the coils before and after, divide smaller number by larger number, and there's your percentage.