| Active Coils | Spring Rate |
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Designing a Coil Spring
Unlike a measured spring rate, this works the other way: it predicts the rate from the spring’s physical design before you ever compress it. Four things set it, the wire diameter, the mean coil diameter, the number of active coils, and the material’s shear modulus, and they combine in a way that makes wire thickness by far the strongest lever.
Which Dimension Does What
Wire diameter enters to the fourth power, so a small increase in wire thickness stiffens the spring dramatically. A larger coil diameter softens it quickly because it enters as a cube. Adding active coils softens the spring in direct proportion, while removing or grinding coils stiffens it. That is why cutting a coil spring raises its rate rather than just lowering ride height.
Keeping the Index Sensible
The spring index, coil diameter divided by wire diameter, should usually fall between about four and twelve. Below that the spring is hard to wind and highly stressed; above it the spring becomes prone to buckling and tangling. Checking the index alongside the rate keeps a design both effective and manufacturable.
Frequently Asked Questions
Why does cutting a spring make it stiffer?
Because rate is inversely proportional to active coils. Removing coils reduces the count, which raises the rate, while also lowering ride height.
What shear modulus should I use?
About 11.5 million psi for most spring steel and music wire. Stainless and other alloys differ slightly, so use the material’s published value when precision matters.
Is this the rate at the wheel?
No, it is the spring’s own rate. The rate felt at the wheel also depends on the suspension motion ratio.