CalculatorsRear Shock Spring Rate Calculator
Rear Shock Spring Rate Calculator
A coil has one rate. This estimates the lb/in that sags the requested percent of stroke with about 70% of rider weight on the rear wheel.
Inputs
Results
- Spring rate
- 475 lb/in
- Spring rate
- 83 N/mm
The Result is indicative and educational. It does not constitute a design, a technical opinion or medical advice and does not replace calculations or verification by a person with the appropriate qualifications and authorisations. Before purchase, installation or use in practice – in particular in electrical installations – the Result should be verified by a qualified specialist. Details: Terms of Service, point 4.
How the number is made
Rate (lb/in) = (rider lb × 0.70 × leverage) ÷ (stroke in inches × sag fraction). N/mm = lb/in × 0.175. Leverage is rear-wheel travel ÷ shock stroke, or the frame maker’s value at sag. Example: 80 kg, leverage 2.5, 55 mm stroke, 30% sag → 308.6 ÷ (2.165 × 0.30) = 475 lb/in = 83 N/mm.
Progressive linkages sag differently from a constant-leverage bike. Springs come in 25 or 50 lb/in steps, so round to the nearest one and confirm with a riding-kit sag check on the shock.
Related calculators
- Fork Sag and PSI Calculator
Set the front before you judge the rear spring on one ride.
- Flip-Chip Geometry Calculator
A flip chip can change leverage and bottom-bracket height together.
Questions
Where do I find leverage?
The frame maker publishes a leverage curve. Use the average, rear travel ÷ shock stroke (150 mm on a 55 mm stroke is 2.73), or the value at sag if they print one.
Why both lb/in and N/mm?
US springs are sold in lb/in. European springs are often N/mm. Same part, two labels.
Does the spring rate include the bike weight?
No. It takes about 70 percent of rider weight with kit as the rear load. That is a little above the usual 60–65% rear bias, which roughly covers the frame of a 15 kg bike. A 25 kg e-bike needs about 10% more rate for an 80 kg rider, and more for a lighter one.
Example products
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- Coil spring
Buy the rate you calculated and one step either side if the shop will swap them.
$30–$80