Mechanics & specs
Reading spring tension charts
The short answer
Spring rate is measured in pounds per inch — how much force it takes to stretch the spring one inch. It is the only figure that compares meaningfully across brands. Color codes and terms like "light" or "heavy" are internal to each manufacturer and do not transfer.
Why lb/in is the only portable number
A spring rated at 2 lb/in requires 2 lb of force for the first inch of stretch, 4 lb by two inches, and so on. That is a physical property, comparable between any two springs from any two manufacturers.
"Medium" is not. Nor is red, blue or green. Where a manufacturer publishes rates, you can compare directly; where they do not, you are relying on their internal scale.
| Descriptor | Portable? | Notes |
|---|---|---|
| Rate in lb/in | Yes | The only directly comparable figure |
| Color code | No | Brand-specific; no industry standard |
| Light / medium / heavy | No | Relative to that manufacturer’s own set |
| Fraction (½, 1, 1½) | Partly | Consistent within a brand only |
Rate is not the whole story
Two springs with the same rate can feel different if their free lengths differ, because the pre-stretch at the carriage home position differs. This is why replacement springs must match the original specification, not just the rate.
Fitting a spring of correct rate but wrong free length changes the resistance at the start of the movement, which is exactly where most exercises begin.
Checking your own springs
Measure free length unloaded and compare against the manufacturer specification. A spring that has stretched permanently sits longer than spec and delivers less resistance than the chart says.
Do this across the whole set at once. Springs fatigue at different rates depending on which combinations you use most, and a set that has drifted apart makes settings unreliable.
Frequently asked questions
- What does lb/in mean for springs?
- Pounds of force required to stretch the spring one inch. It is the only spring figure that compares directly across brands.
- Are spring colors standard across brands?
- No. Each manufacturer uses its own color key, and the same color can mean very different resistances on different machines.
- How do I know if a spring has fatigued?
- Measure its unloaded free length against the manufacturer specification. A permanently stretched spring sits longer and delivers less resistance than the chart indicates.