Glossary
What is Motion Ratio? How to Measure It and Use It in Setup
Motion ratio is the ratio of spring (or damper) movement to wheel movement, a number between roughly 0.5 and 1.0 for most race cars. It captures the suspension's leverage and is the link between the spring you bolt in and the stiffness the tyre feels. Spring and damper motion ratios can differ if they mount at different points.
| Suspension | Typical MR | Notes |
|---|---|---|
| MacPherson strut | 0.90–1.00 | Spring nearly on the wheel axis |
| Double wishbone, outboard coilover | 0.65–0.85 | Depends how far inboard the lower mount sits |
| Pushrod / rocker | 0.40–0.80 | Set by bellcrank geometry; often adjustable |
| Live axle, coil spring | 0.80–1.00 | Spring close to the axle tube |
Measuring motion ratio
With the spring removed and the damper disconnected or at mid-stroke, raise the wheel through a known distance and measure the change in distance between the spring seats. Repeat at several points through travel to see whether the ratio rises or falls. Do the same for the damper if it mounts separately.
Why it matters
Two cars with identical springs but different motion ratios will have very different wheel rates and ride frequencies. Any comparison of spring rates between cars, or even between front and rear of the same car, is meaningless without the motion ratio.
Frequently asked questions
Is a higher motion ratio better?
Not inherently. A ratio near 1.0 means less mechanical leverage, which keeps spring rates sensible and reduces bending loads, but packaging often forces lower ratios.
Why is my damper motion ratio different from my spring ratio?
If the damper and spring mount at different points on the arm, each has its own leverage. Record both; the damper ratio affects how damping forces reach the wheel.
Does motion ratio change with ride height?
Yes, slightly, because the arm angles change. On progressive rocker systems it can change a lot, which is sometimes designed in deliberately.
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