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This has been bugging me for quite a while:
"What happens to the spring rate when you cut it shorter?"
K = Spring Rate
d = Wire Diameter
N = Number Of Active Coils
G = Modulus of Rigidity
C =Spring Index, a ratio of Spring Diameter (D) divided by the Wire Diameter (d)
1. Higher Modulus of Rigidity yields higher Spring Rate
This is a matter of aluminum spring vs steel spring of the same dimension give different spring rate
2. Increasing Wire Diameter yields higher Spring Rate
You should be able to imagine this intuitively. In case you can't, from the formula above, K is directly proportional to (d/C3) where C3 is (D/d)3 and we end up with K ~ d4. This reads to Spring Rate increases to the power of 4 for every increment of Wire Diameter.
3. Increasing Spring Diameter yields lower Spring Rate
By logic, to increase spring diameter, you need to get longer wire. Longer wire is less rigid. Less rigid means lower Spring Rate. Enough? If you prefer the difficult way, K is inversely proportional to C3 , again, where C3 is (D/d)3 and gives us K ~ 1/D3. This reads to Spring Rate reduces by root of 3 for every increment of Spring Diameter.
and finally, back to our main discussion..
4. Reducing Number of Coils yields higher Spring Rate
This should be equally important to AE86 and Charmant owners:
- Stock OEM Spring Front: 380mm free-weight length and 1.8 kg/mm
- Stock OEM Spring Rear: 360mm free-weight length and 2.2 kg/mm
I didn't measure them myself.. I got this from an internet forum so take it with a grain of salt.