Active Coils Formula
The active coil count drops out of the rate equation: Na = G*d^4/(8*D^3*k). Use the working spring design calculator to back-solve coils from a target load and travel, then check stress and solid height.
Rearranging the rate equation
Spring rate k = G*d^4/(8*D^3*Na) solved for the coil count:
Na = G*d^4 / (8 * D^3 * k)
Total vs active
End coils that touch the seat do not deflect: closed ends add about two inactive coils (Nt = Na + 2), which also sets solid height.
Design flow
Pick the load point first (k = F/x), back-solve Na, then verify stress, index and travel on the working calculator.
Frequently asked questions
Why does adding coils soften a spring?
Each active coil is a torsion element in series; more coils means more compliant wire length, so rate falls as 1/Na.
Can active coils be a half count?
Yes - half-coil counts are normal in layouts; manufacturing tolerance on Na is one reason measured rate differs from calculated.
Related pages
- Spring design calculator - back-solves wire and coils from load, travel and index.
- Spring rate calculator - forward direction: rate from geometry.
- Spring pitch calculator - solid height and coil spacing from the count.