The free-length test

Detach the spring and measure it at rest. Compare against the manufacturer’s specification. A spring that has permanently stretched sits longer than spec and no longer delivers its rated resistance.

This matters more than it sounds. If your "two springs" are actually delivering the resistance of one and a half, every setting in your program has drifted without anyone noticing.

What else to look for

Uneven coil spacing indicates the spring has been overstretched at some point. Surface corrosion changes the fatigue behavior and eventually leads to failure. Any kink or deformation means immediate replacement.

A spring failing under load on a carriage a client is pushing against is a genuine safety event, not an inconvenience.

  • Free length beyond manufacturer spec
  • Uneven or splayed coil spacing
  • Surface corrosion, particularly in humid or coastal environments
  • Any kink, bend or visible deformation
  • Hooks or end fittings that have opened out

Replace as a set

Springs fatigue at different rates depending on which combinations get used most. Replacing one leaves you with a set whose members no longer relate to each other as the chart says.

Replacing the whole set restores the gradient, which is the thing that makes settings meaningful. It is also the only way to be confident that "two springs" means the same thing this year as last.

Use the manufacturer’s springs

Springs must match rate, free length and end fitting. A generic spring with the correct rate but the wrong free length changes resistance at the start of the movement, which is where most exercises begin.

This is not brand protectionism — it is that the spring, the gear bar position and the carriage home position are designed as a system.