Glossary
What is Wheel Rate? Formula and How It Differs from Spring Rate
Wheel rate is the effective spring rate measured at the wheel rather than at the spring. Because most suspensions mount the spring inboard of the wheel, the spring moves less than the wheel does, and the leverage reduces its effective stiffness. Wheel rate = spring rate × (motion ratio)², where motion ratio is spring travel divided by wheel travel.
| Motion ratio | MR² | Wheel rate (lb/in) | Typical suspension |
|---|---|---|---|
| 1.00 | 1.00 | 500 | Coilover on strut, nearly vertical |
| 0.85 | 0.72 | 361 | Coilover on lower arm near the ball joint |
| 0.70 | 0.49 | 245 | Spring well inboard on a wishbone |
| 0.55 | 0.30 | 151 | Pushrod/rocker with soft bellcrank ratio |
Why the ratio is squared
Leverage reduces the force at the wheel once and also reduces the displacement at the spring once, so the effective stiffness is scaled twice. Small motion ratio changes therefore have a large effect: moving a spring perch from 0.75 to 0.65 motion ratio cuts the wheel rate by a quarter.
Using wheel rate in setup
Wheel rate feeds directly into ride frequency, roll stiffness distribution and total lateral load transfer distribution. Record it alongside the spring rate for every setup so a spring change on a car with a different motion ratio can be compared like for like.
Frequently asked questions
Is the motion ratio constant?
Rarely. It changes through suspension travel as the geometry moves, which is called a rising or falling rate. Measure it at ride height and, ideally, at full bump and droop.
Does tyre stiffness affect wheel rate?
The tyre is a spring in series with the suspension. The combined 'ride rate' is lower than the wheel rate alone. For ride frequency calculations the tyre spring rate matters on very stiff cars.
How do I measure motion ratio?
Remove the spring, jack the wheel through a known travel (say 25 mm) and measure how far the spring seats move. Divide spring travel by wheel travel.
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