Engine Tuning and Chassis
Suspension Motion Ratio Calculator
Calculate suspension motion ratio from measured spring and wheel movement. Measure through the relevant travel range because the ratio may be nonlinear.
Measurements used for motion ratio
Replace defaults only after identifying where each value came from.
Purpose and reference point
Calculate suspension motion ratio from measured spring and wheel movement — use the equation to compare setups while keeping the test conditions explicit.
Measure through the relevant travel range because the ratio may be nonlinear — that condition defines when motion ratio is comparable with another result.
Spring or damper travel: Measured spring-axis displacement — a compatible entry should identify whether the reading is taken at the source, charger, battery, or accessory.
The Wheel vertical travel entry represents measured wheel-center vertical displacement — before calculating, use a measurement or specification from the exact component and operating condition being evaluated.
Spring rate is defined here as spring rate for wheel-rate context — keeping that definition intact requires you to use a measurement or specification from the exact component and operating condition being evaluated.
A related vehicle record may need to calculate diagonal cross-weight percentage from four scale readings, a relationship covered by the Cross-Weight Percentage.
How the result is derived
In “ motion ratio = spring travel ÷ wheel travel,” the equation links spring or damper travel, wheel vertical travel, and spring rate to motion ratio.
No term beyond spring or damper travel, wheel vertical travel, and spring rate is introduced in “ motion ratio = spring travel ÷ wheel travel.”
What a changed result indicates
Motion ratio answers “Calculate suspension motion ratio from measured spring and wheel movement.”
Use consistent axes and remove free play from measurements — when that condition changes, compare separate calculator runs instead of blending the inputs.
If the next task is to estimate four-stroke engine airflow for carburetor sizing, continue with the Carburetor CFM.
Reasons the real vehicle may differ
Traction, grade, wind, temperature, driver input, and control-system intervention remain outside this simplified model — for spring rate, the page specifically expects spring rate for wheel-rate context.
Notes for a later comparison
For Suspension Motion Ratio, the raw notes should preserve both spring or damper travel—defined as measured spring-axis displacement—and spring rate, meaning spring rate for wheel-rate context.
When reviewing the result later, use “Measure through the relevant travel range because the ratio may be nonlinear” as the test for whether the original operating case still applies.
How to compare another reading
For Suspension Motion Ratio, a useful second case changes spring or damper travel only when the revised value still means measured spring-axis displacement. Keep wheel vertical travel on its original definition, measured wheel-center vertical displacement, so the comparison answers the same vehicle question.
If spring rate also changes, the new value represents spring rate for wheel-rate context and should be documented as a separate operating case rather than attributed to spring or damper travel alone.
Input and comparison questions
What measurement source fits Spring or damper travel when it represents measured spring-axis displacement?
Because spring or damper travel represents measured spring-axis displacement, use a source tied to the exact vehicle, component, and operating period described by the other fields.
How does the warning “Measure through the relevant travel range because the ratio may be nonlinear” affect Motion ratio?
The condition “Measure through the relevant travel range because the ratio may be nonlinear” is not corrected automatically by the numeric inputs, so create a separate suspension motion ratio case when it changes.