Novice / Beginner
Amateur / Intermediate
Expert / Pro
Vet Class (30+)
Recommended Spring Rates
Fork Spring Rate
0.00 kg/mm
0.0 N/mm
Shock Spring Rate
0.00 kg/mm
0.0 N/mm
*These are estimated starting points. Fine-tuning via race sag measurement is recommended.
function calculateSpringRates() {
// 1. Get Inputs
var weightInput = document.getElementById('riderWeight').value;
var displacement = document.getElementById('bikeDisplacement').value;
var discipline = document.getElementById('discipline').value;
var skill = document.getElementById('skillLevel').value;
// 2. Validate Input
if (!weightInput || weightInput <= 0) {
alert("Please enter a valid rider weight.");
return;
}
var weight = parseFloat(weightInput);
// 3. Define Baselines (Based on average modern geometry for 450cc, 175lb rider)
// Baseline: 450cc, 175lbs, Amateur, MX
var baseFork = 0.48; // kg/mm
var baseShock = 5.4; // kg/mm
var baseWeight = 175;
// 4. Calculate Weight Adjustments
// Logic: For every 20lbs diff, fork changes ~0.01, shock changes ~0.2
var weightDiff = weight – baseWeight;
var weightSteps = weightDiff / 20; // How many 20lb increments
var forkWeightAdj = weightSteps * 0.01;
var shockWeightAdj = weightSteps * 0.2;
// 5. Calculate Bike Displacement Adjustments
var forkDispAdj = 0;
var shockDispAdj = 0;
switch(displacement) {
case "450":
forkDispAdj = 0;
shockDispAdj = 0;
break;
case "350":
forkDispAdj = -0.01;
shockDispAdj = -0.1;
break;
case "250":
forkDispAdj = -0.02; // Lighter bike
shockDispAdj = -0.2;
break;
case "125":
forkDispAdj = -0.04; // Significantly lighter
shockDispAdj = -0.6;
break;
case "85":
// Mini bikes use completely different geometry, but we simulate a scale down
forkDispAdj = -0.15;
shockDispAdj = -1.5;
break;
}
// 6. Calculate Discipline Adjustments
var forkDiscAdj = 0;
var shockDiscAdj = 0;
switch(discipline) {
case "mx":
forkDiscAdj = 0;
shockDiscAdj = 0;
break;
case "sx":
// Supercross requires much stiffer springs for jump faces
forkDiscAdj = 0.04;
shockDiscAdj = 0.4;
break;
case "enduro":
// Woods requires compliance
forkDiscAdj = -0.02;
shockDiscAdj = -0.2;
break;
case "desert":
// High speed needs stability but not SX stiffness
forkDiscAdj = 0.01;
shockDiscAdj = 0.1;
break;
}
// 7. Calculate Skill Adjustments
var forkSkillAdj = 0;
var shockSkillAdj = 0;
switch(skill) {
case "novice":
// Slower riders generally need softer suspension
forkSkillAdj = -0.01;
shockSkillAdj = -0.1;
break;
case "amateur":
forkSkillAdj = 0;
shockSkillAdj = 0;
break;
case "expert":
// Faster riders hit bumps harder
forkSkillAdj = 0.01;
shockSkillAdj = 0.1;
break;
case "vet":
// Vets often prefer comfort over performance
forkSkillAdj = -0.005;
shockSkillAdj = -0.05;
break;
}
// 8. Final Calculation
var finalFork = baseFork + forkWeightAdj + forkDispAdj + forkDiscAdj + forkSkillAdj;
var finalShock = baseShock + shockWeightAdj + shockDispAdj + shockDiscAdj + shockSkillAdj;
// Edge case handling: Ensure no negative numbers for weird inputs
if (finalFork < 0.2) finalFork = 0.2;
if (finalShock < 2.0) finalShock = 2.0;
// 9. Conversions (kg/mm to N/mm)
// 1 kg/mm approx 9.807 N/mm
var forkNewtons = finalFork * 9.807;
var shockNewtons = finalShock * 9.807;
// 10. Display Results
document.getElementById('result').style.display = 'block';
document.getElementById('forkResult').innerHTML = finalFork.toFixed(2) + " kg/mm";
document.getElementById('shockResult').innerHTML = finalShock.toFixed(2) + " kg/mm";
document.getElementById('forkNmm').innerHTML = "(" + forkNewtons.toFixed(1) + " N/mm)";
document.getElementById('shockNmm').innerHTML = "(" + shockNewtons.toFixed(1) + " N/mm)";
}
Mastering MX Suspension: Understanding Spring Rates
In the world of motocross and off-road riding, few modifications impact your bike's handling as profoundly as installing the correct spring rates. Whether you are riding a KTM, Yamaha, Honda, or Kawasaki, the springs support the combined weight of the rider and the motorcycle, maintaining the correct chassis geometry while riding.
Using an MX Tech Spring Rate Calculator helps eliminate the guesswork, ensuring that your suspension works within its optimal range. If your springs are too soft, the bike will dive excessively under braking and bottom out on jump faces. If they are too stiff, the bike will feel harsh, deflect off rocks, and fail to settle into corners.
Why Rider Weight Includes Gear
When inputting your data into the calculator, it is critical to use your "ready-to-ride" weight. A common mistake is entering body weight in street clothes. Full motocross gear—including helmet, boots, chest protector, knee braces, and hydration packs—can easily add 15 to 25 pounds to your total mass. Since suspension systems are sensitive engineering components, this difference is significant enough to change the recommended spring rate by a full step.
Pro Tip: Weigh yourself on a bathroom scale while wearing all your riding gear to get the most accurate input for the calculator.
Static Sag vs. Race Sag
Once you install the recommended springs, the next step is verifying the fitment by measuring "Sag".
Race Sag (Rider Sag): The amount the suspension compresses with the rider on the bike. For most full-size bikes (125cc-450cc), the target is typically between 100mm and 105mm.
Static Sag (Free Sag): The amount the suspension compresses under the bike's own weight (without the rider). The target is usually between 30mm and 40mm.
If you achieve the correct Race Sag but your Static Sag is less than 30mm (or zero), your spring is too soft (you had to preload it too much). If your Static Sag is greater than 45mm, your spring is likely too stiff.
Factors Influencing Spring Selection
While weight is the primary variable, the discipline and skill level play crucial roles:
Discipline: Supercross (SX) requires significantly stiff springs to handle massive G-outs and landing impacts. Enduro and Woods riding require softer springs to absorb roots, rocks, and provide traction on slippery surfaces.
Bike Size: A 125cc 2-stroke is lighter than a 450cc 4-stroke, meaning it requires lighter springs not just because of the bike's weight, but because the chassis dynamics and inertia are different.
Skill Level: Faster riders hit obstacles with more force. A Pro rider will need stiffer suspension than a Novice rider of the same weight to prevent bottoming out at high speeds.
Understanding Units: kg/mm vs N/mm
Spring rates are generally measured in two units:
kg/mm (Kilograms per millimeter): The most common unit in the aftermarket industry (e.g., 5.4 kg/mm). It indicates how many kilograms of force are required to compress the spring by 1 millimeter.
N/mm (Newtons per millimeter): Often used by European manufacturers like WP (KTM/Husqvarna). To convert kg/mm to N/mm, multiply by roughly 9.8.