kgThe share of rider, passenger and luggage weight carried by the rear wheel.
Wheel travel divided by shock stroke, e.g. 200 mm / 80 mm = 2.5.
mm
Result
Spring rate at the shock153.2 N/mm
How it is calculated, with your numbers
- Spring rate at the shock
k = load × g × lr² / sag100 × g × 2.5² / 40 = 153.2 N/mm
Working uses the metric units the formula is written in.
- load Load added at the rear wheel (kg)
- lr Average leverage ratio (wheel travel / shock travel)
- sag Extra wheel movement you accept for that load (mm)
- k Spring rate at the shock
What it means
A rear shock is driven by a linkage or swingarm that multiplies wheel movement. With a leverage ratio of 2.5, the shock moves 1 mm for every 2.5 mm at the wheel, but feels 2.5 times the force. The two effects multiply, so the spring rate at the shock goes with the square of the ratio.
This gives a first estimate for the spring a suspension specialist would fit; progressive linkages change the ratio through the travel.
Assumptions and limits
- Average leverage ratio; linkage bikes are progressive.
- Load as a weight at the rear contact patch.
Sources
- Foale, T., Motorcycle Handling and Chassis Design: The Art and Science, 2002.
Related calculators
Suspension sag calculatorStatic and rider sag from three measurements, as millimetres and percent of travel.Fork spring rate calculatorFork spring rate from the load on the front wheel and the sag you want.Payload and GVWR calculatorHow much rider, passenger and luggage the bike can legally carry, and what is left.