Include full riding gear (helmet, boots, suit). Typically adds 15-20 lbs.
Total weight of the bike with fluids and fuel.
Standard linkage: 2.5 – 3.0. Direct/PDS: 1.0. Twin Shock: 1.0.
The actual travel of the shock shaft, not wheel travel.
Recommended Spring Rates
Imperial Rate:–
Metric Rate:–
Newton Rate:–
Note: This recommendation targets a ~30-33% race sag figure. Round to the nearest commercially available spring rate (e.g., if result is 9.3 kg/mm, choose a 9.5 kg/mm spring).
Understanding Motorcycle Spring Rates
Choosing the correct rear spring rate is the single most important step in setting up your motorcycle's suspension. The spring supports the weight of the bike and the rider, maintaining the correct geometry and allowing the suspension hydraulics to work effectively. If your spring is too soft, the bike will wallow, bottom out, and steer slowly due to excessive rear sag. If it is too stiff, the ride will be harsh, traction will suffer, and the suspension won't utilize its full travel.
How the Calculation Works
This calculator uses a physics-based approach to estimate the required stiffness (rate) of your rear coil spring based on load and mechanical advantage. Here are the key factors:
Rider Weight: This must include all riding gear. A rider wearing full leathers, boots, and a helmet can easily weigh 20lbs more than their street clothes weight.
Vehicle Weight: The "Wet Weight" represents the bike ready to ride. The calculator applies a weight distribution algorithm (approx 52% rear bias for bike, 65-70% rear bias for rider) to determine the load on the rear axle.
Leverage Ratio: This is the mechanical advantage of the swingarm and linkage. On a bike with a 2.5:1 ratio, 100lbs of force at the wheel creates 250lbs of force at the shock. This is why lighter bikes with high leverage ratios often need stiffer springs than heavy bikes with direct-acting shocks.
Sag: The Critical Metric
The goal of selecting a spring rate is to achieve the correct "Sag." Sag is the amount the suspension compresses under the weight of the bike and rider from a fully extended position.
Free Sag (Static Sag): The compression under the bike's own weight. Typically 5-10mm.
Race Sag (Rider Sag): The compression with the rider on board. Typically 30-35% of total travel for street/sport bikes and 33% for dirt bikes.
If you have to add excessive preload (compressing the spring manually) to achieve the correct Race Sag, your spring is too soft. If you have zero preload and still have too little sag, your spring is too hard.
Common Leverage Ratios
If you are unsure of your leverage ratio, consider these general rules of thumb:
Modern Sportbikes with Linkage: 2.0 to 3.0 (Average 2.5).
Dirt Bikes with Linkage: 3.0 to 4.0.
KTM PDS (No Linkage): Close to 1.0 (Progressive spring required).
Twin Shock Cruisers: Usually close to 1.0 (Direct acting).
function calculateSpringRate() {
// 1. Get Inputs
var riderWeight = document.getElementById('riderWeight').value;
var bikeWeight = document.getElementById('bikeWeight').value;
var leverageRatio = document.getElementById('leverageRatio').value;
var shockStroke = document.getElementById('shockStroke').value;
// 2. Validate Inputs
if (riderWeight === "" || bikeWeight === "" || leverageRatio === "" || shockStroke === "") {
alert("Please fill in all fields to calculate the spring rate.");
return;
}
riderWeight = parseFloat(riderWeight);
bikeWeight = parseFloat(bikeWeight);
leverageRatio = parseFloat(leverageRatio);
shockStroke = parseFloat(shockStroke);
if (riderWeight < 0 || bikeWeight < 0 || leverageRatio <= 0 || shockStroke <= 0) {
alert("Please enter valid positive numbers.");
return;
}
// 3. Calculation Logic
// Assumptions for weight bias (Generic Sport/Street Standard)
var bikeRearBias = 0.52; // 52% of bike weight on rear
var riderRearBias = 0.65; // Rider sits mostly over rear shock/swingarm pivot
// Calculate static load on rear wheel
var loadRearWheel = (bikeWeight * bikeRearBias) + (riderWeight * riderRearBias);
// Calculate force transmitted to the shock shaft
var forceAtShock = loadRearWheel * leverageRatio;
// Target: We want the spring to support this weight at roughly 33% of the shock stroke (Race Sag).
// However, we also account for installed preload (typically ~0.4 to 0.5 inches).
// Formula: K = Force / (Displacement).
// Displacement = (Stroke * TargetSag%) + InstalledPreload.
var targetSagPercent = 0.33;
var estimatedPreload = 0.45; // inches
var totalSpringCompression = (shockStroke * targetSagPercent) + estimatedPreload;
// Calculate Rate in lbs/in
var rateLbsPerInch = forceAtShock / totalSpringCompression;
// Convert to Metric
// 1 lb/in = 0.017857 kg/mm approx, or divide by 55.997
var rateKgPerMm = rateLbsPerInch / 56.0;
// Convert to Newtons/mm
// 1 kg/mm = 9.80665 N/mm
var rateNewton = rateKgPerMm * 9.80665;
// 4. Display Results
document.getElementById('resLbs').innerHTML = Math.round(rateLbsPerInch) + " lbs/in";
document.getElementById('resKg').innerHTML = rateKgPerMm.toFixed(1) + " kg/mm";
document.getElementById('resNewton').innerHTML = Math.round(rateNewton) + " N/mm";
document.getElementById('resultBox').style.display = "block";
}