Glossary

Dual Spring Rate Explained: Combined Rate Formula for Stacked Springs

A dual spring rate setup stacks two springs in series on one damper, usually a soft tender spring on top of a stiffer main spring. Until the tender coil-binds the pair acts at a combined rate lower than either spring; once it binds, the rate jumps to the main spring alone. The combined rate is k = (k1 × k2) ÷ (k1 + k2).

Combined rate examples (lb/in)
Tender	Main	Combined until bind	After bind
200	600	150	600
300	600	200	600
400	800	267	800
500	500	250	500

Why run two springs

Dual springs give a soft initial rate for mechanical grip over small bumps and kerbs, then a much stiffer rate to control ride height under aero load or heavy braking. The transition point is set by the tender spring's free length and rate, so it can be tuned to engage at a chosen ride height.

Setting the crossover

The crossover happens when the tender has compressed to its block height. Work out how much load that takes (tender rate × tender travel to bind) and compare it with the load on that corner at ride height. Setup Genius's dual spring calculator does this arithmetic and shows the rate before and after crossover.

Frequently asked questions

What is the difference between a tender and a helper spring?

A tender spring has a meaningful rate and changes the combined rate until it binds. A helper spring is extremely soft and only exists to keep the main spring captive at full droop.

Can I stack two springs with the same rate?

Yes. Two identical springs in series halve the rate and double the travel, which is sometimes used when a soft, long spring is not available.

Does spring order on the damper matter?

Not for the rate calculation. In practice the softer spring usually goes on top so the heavier main spring sits on the lower perch.

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