Sport / Superbike
Standard / Naked
Sport Touring
Cruiser / Heavy
Adventure / Dual Sport
Motocross / Enduro
Comfort / Novice (Softer)
Standard / Intermediate
Aggressive / Track Day (Stiffer)
Pro Racing (Very Stiff)
Recommended Fork Spring Rate
0.00 kg/mm
0.00 N/mm
*This is an estimated starting point. Verify with static sag measurements.
function calculateSpringRate() {
var riderWeight = parseFloat(document.getElementById('riderWeight').value);
var bikeWeight = parseFloat(document.getElementById('bikeWeight').value);
var bikeType = document.getElementById('bikeType').value;
var styleMod = parseFloat(document.getElementById('ridingStyle').value);
if (isNaN(riderWeight) || isNaN(bikeWeight)) {
alert("Please enter valid numbers for both Rider Weight and Motorcycle Weight.");
return;
}
// Base Calculations
// We establish a baseline spring rate for a standard 170lb rider on a standard weight bike for that class.
var baseRate = 0.0;
var standardBikeWeight = 0;
var weightSensitivity = 0.0; // How much spring rate changes per lb of rider weight
switch (bikeType) {
case 'sport':
// Sportbikes: ~400lb wet, stiffer setup
baseRate = 0.85;
standardBikeWeight = 410;
weightSensitivity = 0.0015;
break;
case 'street':
// Naked/Standard: ~430lb wet, moderate setup
baseRate = 0.80;
standardBikeWeight = 430;
weightSensitivity = 0.0014;
break;
case 'touring':
// Touring: ~600lb wet, progressive but heavy
baseRate = 0.90;
standardBikeWeight = 600;
weightSensitivity = 0.0013;
break;
case 'cruiser':
// Cruiser: ~650lb wet, limited travel often requires stiffer springs to prevent bottoming
baseRate = 0.95;
standardBikeWeight = 650;
weightSensitivity = 0.0016;
break;
case 'adv':
// Adventure: ~500lb wet, long travel allows softer initial rate
baseRate = 0.55;
standardBikeWeight = 500;
weightSensitivity = 0.0012;
break;
case 'dirt':
// MX: ~240lb wet, very long travel, soft rates
baseRate = 0.44;
standardBikeWeight = 240;
weightSensitivity = 0.0010;
break;
default:
baseRate = 0.85;
standardBikeWeight = 400;
weightSensitivity = 0.0015;
}
// 1. Rider Weight Adjustment
// Calculate difference from a "standard" 170lb rider
var riderDelta = riderWeight – 170;
var riderAdjustment = riderDelta * weightSensitivity;
// 2. Bike Weight Adjustment
// Calculate difference from the class average
// Forks support roughly 50% of the bike weight change (simplified distribution)
var bikeDelta = bikeWeight – standardBikeWeight;
// Bike weight has slightly less impact on required rate than rider weight due to unsprung mass ratios
var bikeAdjustment = (bikeDelta * 0.5) * (weightSensitivity * 0.8);
// 3. Final Calculation
var finalRate = baseRate + riderAdjustment + bikeAdjustment + styleMod;
// Safety Limits (prevent unrealistic numbers)
if (bikeType === 'dirt') {
if (finalRate 0.60) finalRate = 0.60;
} else {
if (finalRate 1.40) finalRate = 1.40;
}
// Conversion to Newtons/mm (1 kg/mm = 9.80665 N/mm)
var newtons = finalRate * 9.80665;
// Display Results
var resultBox = document.getElementById('resultBox');
var kgDisplay = document.getElementById('kgResult');
var nDisplay = document.getElementById('nResult');
resultBox.style.display = 'block';
kgDisplay.innerHTML = finalRate.toFixed(2) + " kg/mm";
nDisplay.innerHTML = newtons.toFixed(1) + " N/mm";
}
Why Fork Spring Rate Matters
The spring rate of your motorcycle fork determines how much force is required to compress the spring a specific distance. It is arguably the most critical aspect of your suspension setup. If your springs are too soft, the front end will dive excessively under braking, destabilizing the chassis and reducing cornering clearance. If they are too stiff, the suspension won't absorb bumps, leading to a harsh ride, loss of traction, and rider fatigue.
Correct spring rates ensure the suspension operates in the "sweet spot" of its travel, maximizing grip and control whether you are commuting, touring, or racing.
How the Calculation Works
This calculator estimates the ideal linear spring rate based on several physical factors:
Rider Weight: This is the most significant variable. The calculator assumes a "wet" weight, so be sure to include your helmet, boots, leathers, and any luggage you carry consistently.
Motorcycle Weight: Heavier bikes require stiffer springs to support the chassis mass. We compare your bike's weight against the class average.
Motorcycle Type: A motocross bike with 300mm of travel requires a much softer spring rate (e.g., 0.44 kg/mm) compared to a sportbike with 120mm of travel (e.g., 0.90 kg/mm) to achieve the correct sag.
Riding Style: Aggressive track riders create higher G-forces under braking, requiring stiffer springs to maintain geometry. Touring riders often prefer a softer setup for comfort.
Pro Tip: Always verify your spring rate choice by measuring Sag.
Understanding Sag
Once you install springs, you must measure "Sag" to confirm the rate is correct for your weight. Sag is how much the bike settles under its own weight and the rider's weight.
Static Sag (Free Sag): How much the bike settles under its own weight (without rider). Usually 20-30mm for sportbikes.
Rider Sag (Race Sag): How much the bike settles with the rider on board. Usually 30-40mm for sportbikes and 95-105mm for dirt bikes.
If you cannot achieve the correct Rider Sag without compromising Static Sag (i.e., you have to preload the spring too much, leaving zero static sag), your spring rate is likely too soft. If you have too much static sag but correct rider sag, the spring might be too stiff.
Metric vs. Imperial Units
Suspension springs are globally measured in either kg/mm (Kilograms per millimeter) or N/mm (Newtons per millimeter).
Conversion factor: 1 kg/mm ≈ 9.8 N/mm.
While some older American manuals use lbs/in, the industry standard for modern motorcycles is metric. This calculator provides both for your convenience.