Motorcycle Rear Shock Spring Rate Calculator
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Recommended Setup
Ideal Spring Rate
9.5 kg/mm
(530 lbs/in)
Leverage Ratio
2.00 : 1
Rear Sprung Mass
0 lbs
Target Race Sag (33%)
0 mm
Note: This calculation assumes a standard linkage curve and approximately 10mm of mechanical preload. Always test actual sag on the bike to fine-tune.
Enter your motorcycle and rider details to see the recommended spring rate.
Understanding Motorcycle Rear Shock Spring Rates
Choosing the correct rear shock spring rate is the foundation of proper motorcycle suspension tuning. A spring that is too soft will cause the bike to wallow, bottom out over bumps, and run wide on corner exits. Conversely, a spring that is too stiff will result in a harsh ride, poor traction, and difficulty initiating turns. This calculator helps you determine the baseline spring stiffness required to support your weight and the bike's mass while maintaining the correct geometry.
How the Calculation Works
The spring rate calculation relies on several physical factors involving leverage and weight distribution:
- Rider Weight: This must include all riding gear. A helmet, leathers, boots, and gloves can easily add 15-25 lbs to your body weight.
- Leverage Ratio: This is the relationship between the movement of the rear wheel and the movement of the shock shaft. If the wheel moves 120mm and the shock moves 60mm, the ratio is 2:1. A higher ratio means the spring needs to be stiffer to support the same weight because the "lever" acting on it is longer.
- Unsprung Weight: The calculator subtracts the weight of the rear wheel, swingarm, and brake components (approximately 30-35 lbs) because the shock does not support this mass; it only controls it.
- Weight Bias: Most motorcycles carry slightly more weight on the rear wheel than the front when static. We estimate the rear sprung mass based on this distribution.
Understanding Sag
The goal of selecting a spring rate is to achieve the correct "Sag" numbers. Sag determines where the suspension sits in its stroke when the bike is loaded.
- Free Sag (Static Sag): How much the bike settles under its own weight without a rider. Typically 5-10mm for street bikes.
- Race Sag (Rider Sag): How much the bike settles with the rider on board. The industry standard target is generally 30% to 33% of total wheel travel for sport/track use, and up to 35-40% for adventure/touring.
Pro Tip: If you can achieve the correct Race Sag but your Free Sag is zero (or the bike tops out), your spring is too soft (requiring too much preload). If you have too much Free Sag, your spring is likely too stiff.
Kg/mm vs. Lbs/in
Spring rates are commonly sold in two units. European and Japanese manufacturers often use kg/mm (kilograms per millimeter), while American manufacturers often use lbs/in (pounds per inch). This calculator provides both to ensure compatibility with whatever brand of spring (Eibach, Öhlins, Racetech, etc.) you are purchasing.
function calculateSpringRate() {
// 1. Get Inputs
var riderWeight = parseFloat(document.getElementById('riderWeight').value);
var bikeWeight = parseFloat(document.getElementById('bikeWeight').value);
var rearBias = parseFloat(document.getElementById('rearBias').value);
var shockStrokeMm = parseFloat(document.getElementById('shockStroke').value);
var wheelTravelMm = parseFloat(document.getElementById('wheelTravel').value);
// 2. Validation
if (isNaN(riderWeight) || isNaN(bikeWeight) || isNaN(shockStrokeMm) || isNaN(wheelTravelMm)) {
alert("Please enter valid numbers for all fields.");
return;
}
if (shockStrokeMm <= 0 || wheelTravelMm <= 0) {
alert("Travel and Stroke must be greater than zero.");
return;
}
// 3. Constants & Conversions
// Standard conversions
var mmToInch = 0.0393701;
// 4. Calculate Physics
// Leverage Ratio = Wheel Travel / Shock Stroke
var leverageRatio = wheelTravelMm / shockStrokeMm;
// Estimate Unsprung Weight (Wheel, Swingarm, Brakes, Chain)
// Average street bike rear unsprung weight is approx 30-35 lbs
var unsprungWeightLbs = 35;
// Calculate Rear Sprung Weight
// Total bike weight * bias% – unsprung weight
var rearTotalWeight = bikeWeight * (rearBias / 100);
var rearSprungWeight = rearTotalWeight – unsprungWeightLbs;
if (rearSprungWeight < 0) rearSprungWeight = 0;
// Effective Rider Load on Rear
// Riders don't sit directly over the axle. Usually 60-70% of rider weight is on the rear.
var riderRearBias = 0.65;
var riderLoadOnShock = riderWeight * riderRearBias;
// Total Sprung Load to support
var totalLoadLbs = rearSprungWeight + riderLoadOnShock;
// Force acting on the shock shaft due to leverage ratio
var forceOnShockLbs = totalLoadLbs * leverageRatio;
// Target Sag Calculation
// We want the spring to hold this weight at roughly 33% of the stroke (Race Sag)
// Spring Formula: F = k * x
// x (displacement) = (Shock Stroke * 0.33) + Preload
// Standard mechanical preload is often around 10mm (0.39 inches)
var shockStrokeInches = shockStrokeMm * mmToInch;
var targetSagInches = shockStrokeInches * 0.33;
var assumedPreloadInches = 0.39; // approx 10mm
// The displacement of the spring at equilibrium
var totalSpringCompressionInches = targetSagInches + assumedPreloadInches;
// Calculate Rate (k) = Force / Displacement
var springRateLbsIn = forceOnShockLbs / totalSpringCompressionInches;
// Convert to kg/mm
// 1 lb/in = 0.0178579673 kg/mm
var springRateKgMm = springRateLbsIn * 0.017858;
// 5. Update UI
document.getElementById('resRateKg').innerHTML = springRateKgMm.toFixed(1) + " kg/mm";
document.getElementById('resRateLbs').innerHTML = "(" + Math.round(springRateLbsIn) + " lbs/in)";
document.getElementById('resRatio').innerHTML = leverageRatio.toFixed(2) + " : 1";
document.getElementById('resSprungMass').innerHTML = Math.round(rearSprungWeight) + " lbs";
// Calculate target sag in mm for display
var targetSagMm = shockStrokeMm * 0.33;
document.getElementById('resSag').innerHTML = Math.round(targetSagMm) + " mm";
// Show Results
document.getElementById('resultsArea').style.display = 'block';
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}