Suspension Motion Ratio Calculator readings come from arm lengths, measured travel, a droop-to-bump sweep or a rocker, then show stroke needed, wheel rate share and offset effects.
Motion ratio is shock or spring travel divided by wheel travel. Pick how you want to measure it.
Wheel Travel and Shock Stroke
Wheel Offset or Spacer
Moving the wheel outward lengthens the lever from the inner pivot, which lowers the ratio.
Measuring Precision
Used to show how much a small reading error moves a measured ratio.
Suspension Motion Ratio Calculator Methods
The Suspension Motion Ratio Calculator works out how far a shock or spring moves for each inch the wheel moves. It can find that ratio from arm lengths, from a single travel measurement, from a sweep between droop and bump, or from a pushrod and rocker. Coilover buyers, custom suspension builders, and racers sizing springs and shocks can use the result to check stroke, wheel rate, and the effect of a wheel offset change.
Arm Lengths and Shock Angle
On a control arm, the shock moves in proportion to how far it sits from the inner pivot. Hyperco measures from the pivot to the point under the spring and from the pivot to the ball joint, then divides the first by the second. A Grassroots Motorsports forum member puts it simply, saying a spring at 6 in on a 12 in arm gives a 50% ratio.
$$MR = \frac{d_1}{d_2} \times \cos\theta$$
Here d1 is pivot to shock mount, d2 is pivot to wheel center, and θ is the shock’s lean away from the arm’s direction of motion. With 11 in and 16 in and a 10° lean, the Suspension Motion Ratio Calculator gives 0.677. Measure that angle from the line the arm swings through, not from true vertical, because a tilted arm changes what counts as upright.
Measured Travel and the Droop-to-Bump Sweep
Measuring travel directly skips the geometry and captures everything at once. Suspension Secrets jacks the wheel in 5 mm steps from 50 mm of droop to 50 mm of bump, with the spring removed and the anti-roll bar disconnected, and records the damper length at each step. A single reading of 2.4 in of shock travel over 4.0 in of wheel travel gives 0.600.
That single reading carries some measuring error, so the calculator shows how much a 1/16 in tape reading moves the result. At the defaults it is about ±0.016, which is why longer travel measurements give steadier numbers. The Measuring Precision panel lets you set a finer tool, such as a 0.5 mm caliper.
The sweep mode splits the travel into a droop half and a bump half. The defaults read 0.600 from droop to ride height and 0.680 from ride height to bump, a 13% rise, with 0.640 overall. Suspension Secrets notes the ratio can change through the travel because of the arc the suspension follows, and the alert flags any linkage that changes by more than 10%.
Pushrod and Rocker Setups
On a pushrod car, the rocker multiplies whatever the pushrod does. Suspension Secrets recommends the travel measurement method for these bell crank layouts, since arm lengths alone miss the rocker. The Suspension Motion Ratio Calculator takes the pushrod’s own travel ratio and multiplies it by the rocker’s shock-side arm divided by its pushrod-side arm.
$$MR = \frac{x_{pushrod}}{x_{wheel}} \times \frac{r_{shock}}{r_{pushrod}}$$
With 3.0 in of pushrod travel for 4.0 in of wheel travel and a 1.2:1 rocker, the result is 0.750 × 1.2, or 0.900. That rocker alone adds 44% to the wheel rate, because rate follows the ratio squared. A 1.5:1 rocker pushes the ratio to 1.125, which the tool flags because ratios above 1.0 are rare outside rocker setups.
Motion Ratio or Installation Ratio
The names are not used consistently. Penske Shocks defines installation ratio as the inverse of motion ratio, while Suspension Secrets calls the shock-to-wheel travel ratio the installation ratio and derives a separate motion ratio from it. Wikipedia’s entry also notes that motion ratio is sometimes used for the inverse.
The calculator always reports shock or spring travel divided by wheel travel, which is below 1.0 on most cars. The first result card also shows the inverse, 1.477 at the defaults, so either convention can be read straight off the result. Check which one a spring or shock maker means before plugging their number into another formula.
What the Ratio Means for Springs and Shocks
The hero figure of the Suspension Motion Ratio Calculator gives the ratio and turns it into plain terms, such as the shock moving 0.68 in for every 1 in of wheel travel. The four cards then apply it to spring rate, shock stroke, shock angle, and wheel offset. The alert under them reports the method used and any problem with stroke or ratio.
Share of Spring Rate at the Wheel
The first card squares the ratio to show how much of the spring rate reaches the tire, which is 45.8% at the defaults. It also shows the shock force for every 100 lb at the wheel, 148 lb here, since the shorter lever makes the spring work harder. Carry the ratio into the Wheel Rate Calculator to turn a spring rate into the rate the tire feels.
Shock Stroke for Your Wheel Travel
The second card of the Suspension Motion Ratio Calculator multiplies the planned wheel travel by the ratio to find the stroke the shock needs. At the defaults, 5.00 in of wheel travel needs 3.39 in of stroke, so a 4 in shock allows 5.91 in of wheel travel with 0.61 in to spare. QA1 gives typical ratios of 0.50 to 0.66 for independent front suspension and about 1.0 for a shock on a solid axle, which is why the same wheel travel needs very different strokes.
Travel Lost to Shock Angle
In arm-length mode, the third card shows how much the lean costs. A 10° lean takes 1.5% off the ratio, so the upright value of 0.688 drops to 0.677. At 20° it falls to 0.646, since the loss grows quickly with the cosine of the angle.
In the other modes the same card shows something different. Measured mode gives the reading error range, sweep mode shows whether the ratio rises or falls through the travel, and rocker mode shows how much the rocker multiplies the rate. Each version answers the question that matters most for that way of measuring.
Wheel Offset and Spacers
Moving the wheel outward lengthens the lever from the inner pivot, which lowers the ratio. The default 1 in change takes the ratio from 0.677 to 0.637, a 5.9% drop that softens the wheel rate by 11.4%. Hyperco measures to the wheel center rather than the ball joint on reverse offset wheels for the same reason.
The same offset change also moves the tire relative to the steering axis. The Scrub Radius Calculator shows that side of a wheel or spacer swap. Check both before ordering wheels for a car with a set spring rate.
Suspension Motion Ratio Calculator Limits
All lengths and travels must be above zero, and the shock angle must be below 80°. If the entries produce a ratio outside 0 to 5, the tool stops and asks you to check which reading is the shock and which is the wheel. An inward offset change close to the full pivot-to-wheel distance is rejected, because it would leave almost no lever arm.
Each mode gives a ratio for one position or one average over the travel. A Grassroots Motorsports forum thread points out that the arc the wheel follows around the instant center also shifts the ratio on unequal-length arms. Use the travel sweep when the linkage is complex, and take the ride height value for spring choice.
Measuring Mistakes That Skew the Ratio
Leaving the spring or anti-roll bar connected during a travel sweep makes the car lift off its stands or bind the arm. Suspension Secrets removes the spring and disconnects the bar first so the wheel moves freely. Measure from the same two mounting points at every step.
Measuring the shock angle from true vertical on a tilted arm gives the wrong correction. Entering the installation ratio in place of the motion ratio, or reversing shock and wheel travel, produces a figure above 1.0 on a normal car. The alert flags that case, so check the inputs whenever the ratio comes out higher than expected.
Motion Ratio Questions From Builders
What is a suspension motion ratio?
Motion ratio is the distance the shock or spring moves divided by the distance the wheel moves. On most suspensions the shock sits partway along an arm, so it moves less than the wheel and the ratio is below 1.0. At the defaults the ratio is 0.677, meaning the shock moves about two-thirds as far as the wheel.
How do you measure motion ratio on a car?
The quick way is to measure pivot-to-shock and pivot-to-wheel distances on the arm and divide them, which is the arm-length mode in the Suspension Motion Ratio Calculator. The more accurate way, used by Suspension Secrets, is to remove the spring, jack the wheel through its travel in small steps, and record how far the shock moves each time. The travel method captures shock angle and linkage effects automatically.
What is a typical motion ratio?
QA1 gives about 0.50 to 0.66 for independent front suspension and about 1.0 for a shock mounted on a solid axle. Struts usually sit close to 1.0 because the spring rides almost directly above the wheel, and a Grassroots Motorsports forum member measured about 55% on a 1968 Mustang. Pushrod cars can go above 1.0 when the rocker multiplies the travel.
Does moving the shock or spring mount change the ratio?
Moving the mount farther out along the arm raises the ratio, and moving it in lowers it. The same Mustang owner moved the spring mount out by 1 in and raised the ratio from about 55% to about 65%. Because rate follows the ratio squared, that change stiffens the wheel rate by roughly 40% with the same spring.
Why does the motion ratio change through the travel?
The arms swing in arcs, so the angle between the arm and the shock changes as the wheel moves. Suspension Secrets recommends graphing the ratio when it varies noticeably, and the sweep mode in the calculator shows the droop and bump halves separately. A ratio that rises toward bump makes the wheel rate stiffen as the suspension compresses.