Yes it's the shock spring rate i'm looking for. I found a link to a formula to figure the rate by measurements of the spring, number of coils,etc, but the link was dead. I won't give up.
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The formula to calculate coil spring's rate is as follows
Symbols
d = wire diameter
Do = coil's outer diameter
D = mean diameter
D = Do -d
E = Young's Modulus of Material
v = Poisson's ratio of Material
G = Shear Modulus of Material
G = E ÷ 2 ( 1 + V)k = spring rate
Na = Number of Active coils
k = Gd^4 ÷ (8D^3 na)Most suspension springs are "closed and ground".
With these springs,
Na (Number of active coils) is
Na = number of total coils - 2Now, here's the catch.
You really can't accurately calculate spring rate UNLESS you know exactly what kind of material is used.
Young's modulus is essentially modulus of elasticity. You can think it as the ratio of tensile stress to tensile strain.
Poisson's ratio is essentially a measurement of deformation (either expansion or contraction) of a material, in a direction perpendicular to the direction of applied force.
Shear modulus of material is essentially the ratio of shear stress to shear strain.
G = stress / strain Young's modulus and Shear modulus (AKA Modulus of rigidity) can vary with both cold work and heat treating.
The material itself, you can guess, in some cases.
For example, Penske says their springs are made out of "l
ightweight chrome silicon spring steel",
so, you can assume they are ASTM A401, then shear modulus can be found in the steel manufacturer's catalogue.
Any calculation that skips Young's modulus, Share modulus, etc is useless.
For example, take a look at this video.
His formula is
k = 9913 x d^4 ÷ (D^3 na)He says in the video "9913, it's just a shear strength of the steel ..."
well, he clearly doesn't understand the physics.
And his calculation is wrong.
This video's explanation is excellent. (It may not be entertaining. But, if you don't like physics and math, it's not going to be entertaining anyway.)
More practical way to figure out spring rateIf you have access to hydraulic press, you can use it, then calculate spring rate from Hook's law.
Hook's law
k = F ÷ x k = spring rate
F = force applied
x = distance traveled (how much spring is compressed)
Apply force to spring, measure it, then measure how much the spring gets compressed.
This is actually far better way than the first one.