Standard Telescopic / USD (Most Sport/Street Bikes)
Cruiser / Raked Out (More weight on rear)
Adventure / Dual Sport (Long Travel)
Commuting / Touring (Comfort Focus)
Street Sport / Canyon Carving
Track Day / Aggressive Race Pace
Recommended Rate (Metric):0.00 kg/mm
Recommended Rate (Newton):0.00 N/mm
Est. Static Sag Target:0 mm
*This calculation assumes standard fork geometry. Always verify with a suspension tuner or specific manufacturer data sheets before purchasing.
function calculateSpringRate() {
// 1. Get input values
var riderWeight = parseFloat(document.getElementById('riderWeight').value);
var bikeWeight = parseFloat(document.getElementById('bikeWeight').value);
var forkType = document.getElementById('forkType').value;
var ridingStyle = document.getElementById('ridingStyle').value;
// 2. Validation
if (isNaN(riderWeight) || isNaN(bikeWeight) || riderWeight <= 0 || bikeWeight <= 0) {
alert("Please enter valid positive numbers for Rider and Bike weight.");
return;
}
// 3. Logic Configuration
// Baseline: A 400lb bike + 160lb rider (560 total) typically uses ~0.85 kg/mm for street.
// Formula Logic: Linear approximation based on total load deviation from baseline.
var totalLoadLbs = riderWeight + bikeWeight;
var baselineLoad = 560;
var baselineRate = 0.85; // kg/mm
// Determine weight distribution/geometry factor
var typeFactor = 0;
var sagTarget = "";
if (forkType === 'standard') {
// Standard bias
typeFactor = 1.0;
sagTarget = "30 – 35 mm";
} else if (forkType === 'cruiser') {
// Less weight on front usually, but heavier bikes.
// Often use softer springs relative to weight due to rake angle, but weight is high.
// We adjust the effectiveness.
typeFactor = 0.95;
sagTarget = "35 – 45 mm";
} else if (forkType === 'adventure') {
// Long travel needs softer rates to use the travel, but heavy loads.
typeFactor = 0.90;
sagTarget = "50 – 65 mm";
}
// Calculate Rate adjustment based on weight
// Approx rule: Add 0.01 kg/mm for every 7 lbs over baseline?
// Let's use: (Difference in lbs) * 0.0013
var weightDifference = totalLoadLbs – baselineLoad;
var weightAdjustment = weightDifference * 0.0013;
// Determine Style Modifier
var styleModifier = 0;
if (ridingStyle === 'soft') {
styleModifier = -0.05; // Softer for comfort
if(forkType === 'standard') sagTarget = "35 – 40 mm";
} else if (ridingStyle === 'standard') {
styleModifier = 0;
} else if (ridingStyle === 'aggressive') {
styleModifier = 0.10; // Stiffer for track/race braking
if(forkType === 'standard') sagTarget = "25 – 30 mm";
}
// 4. Final Calculation
// Formula: (Base + WeightAdj) * TypeFactor + Style
var calculatedKg = (baselineRate + weightAdjustment) * typeFactor + styleModifier;
// Safety limiters (Sanity check for motorcycles)
if (calculatedKg 1.6) calculatedKg = 1.6;
// Convert to N/mm (1 kg/mm = 9.807 N/mm)
var calculatedN = calculatedKg * 9.80665;
// 5. Display Results
document.getElementById('resKg').innerHTML = calculatedKg.toFixed(2) + " kg/mm";
document.getElementById('resN').innerHTML = calculatedN.toFixed(2) + " N/mm";
document.getElementById('resSag').innerHTML = sagTarget;
document.getElementById('results').style.display = 'block';
}
Understanding Front Fork Spring Rates
Choosing the correct front fork spring rate is the single most critical step in setting up a motorcycle's suspension. The springs support the weight of the bike and the rider, determining the chassis geometry and how the bike handles braking forces and bumps. If your springs are wrong, no amount of clicker adjustment (compression or rebound damping) will fix the handling issues.
What is Spring Rate?
Spring rate refers to the stiffness of the spring. It is defined by Hooke's Law and measures how much force is required to compress the spring by a specific distance.
kg/mm (Kilograms per millimeter): The most common unit in the aftermarket industry (e.g., Öhlins, RaceTech). A 1.0 kg/mm spring requires 1 kg of force to compress it by 1 mm.
N/mm (Newtons per millimeter): The standard metric scientific unit, often used by OEM manufacturers (e.g., Showa, KYB). To convert kg/mm to N/mm, multiply by roughly 9.8.
Why Rider Weight Includes Gear
When inputting your weight into the calculator, it is vital to include your full riding gear. A helmet, leathers, boots, and gloves can easily add 15 to 25 lbs (7-11 kg) to your static weight. Since the suspension must support this total mass, omitting gear weight will result in a recommendation that is too soft, leading to excessive brake dive and poor handling.
Interpreting Your Results
The calculator provides a theoretical baseline based on standard weight distributions. Here is how to interpret the numbers:
Too Soft (Rate too low)
If your spring rate is too low, the front end will dive excessively during braking. This compresses the fork completely (bottoming out), causing instability and loss of traction at the front tire when you need it most. It also steepens the rake angle dangerously, making the bike feel twitchy.
Too Stiff (Rate too high)
If the rate is too high, the suspension will not compress enough over bumps, transferring harsh energy to the rider's hands. The tire will skip over road imperfections rather than tracking the ground, reducing mechanical grip. You will also struggle to get heat into the front tire.
Static Sag vs. Rider Sag
Once you install the correct springs, you must set the preload to achieve the correct "Sag." Sag is how much the bike settles under its own weight and the rider's weight.
Street Sag Target: Typically 30mm to 35mm (approx 25-30% of total travel).
Track Sag Target: Typically 25mm to 30mm (stiffer for higher G-forces).
If you cannot achieve these sag numbers with your preload adjusters, your spring rate is incorrect.