Based on a linear Hooke's Law approximation at the sag point.
function calculateSpringRate() {
// Get Input Values
var w = parseFloat(document.getElementById('riderWeight').value);
var t = parseFloat(document.getElementById('wheelTravel').value);
var s = parseFloat(document.getElementById('shockStroke').value);
var sag = parseFloat(document.getElementById('desiredSag').value);
var bias = parseFloat(document.getElementById('weightBias').value);
// Validation
if (isNaN(w) || isNaN(t) || isNaN(s) || isNaN(sag) || isNaN(bias)) {
alert("Please enter valid numbers in all fields.");
return;
}
if (s === 0 || sag === 0) {
alert("Stroke and Sag cannot be zero.");
return;
}
// Logic Implementation
// 1. Calculate Leverage Ratio (Wheel Travel / Shock Stroke)
var leverageRatio = t / s;
// 2. Calculate Weight on Rear Wheel (Rider Weight * Bias %)
var rearWeight = w * (bias / 100);
// 3. Calculate Force applied to Shock shaft (Rear Weight * Leverage Ratio)
// Note: This assumes a constant leverage ratio for simplicity, which is standard for basic calculators.
var forceOnShock = rearWeight * leverageRatio;
// 4. Calculate Displacement at Sag (Shock Stroke * Sag %)
var sagDisplacement = s * (sag / 100);
// 5. Calculate Spring Rate (Force / Displacement) via Hooke's Law (F = kx)
// k = F / x
var exactSpringRate = forceOnShock / sagDisplacement;
// 6. Determine Recommended Spring (Nearest 50lbs step)
// Standard springs come in 50lb increments (e.g. 350, 400, 450)
var recommendedRate = Math.round(exactSpringRate / 50) * 50;
// Display Results
document.getElementById('displayRatio').innerHTML = leverageRatio.toFixed(2) + ":1″;
document.getElementById('displayExactRate').innerHTML = Math.round(exactSpringRate) + " lbs/in";
document.getElementById('displayRecommended').innerHTML = recommendedRate + " lbs/in";
// Show Results Area
document.getElementById('resultsArea').style.display = 'block';
}
Mastering Your Suspension: The Shock Spring Rate Calculator
Finding the correct spring rate for your coil shock is arguably the most critical step in setting up a mountain bike's suspension. If your spring is too soft, you will bottom out frequently and wallow in the travel. If it is too hard, you will lose traction and experience a harsh ride. This Shock Spring Rate Calculator uses physical kinematics to determine the ideal spring stiffness based on your weight, your bike's geometry, and your preferred feel.
Note on Units: This calculator uses Imperial units (lbs and inches) as these are the industry standard for marking coil springs (e.g., a "450×2.75" spring indicates a 450 lbs/in rate). If your travel is in millimeters, divide by 25.4 to get inches.
How to Calculate Spring Rate
The calculation relies on the relationship between the force applied to the rear wheel and the mechanical advantage the bike frame has over the shock (Leverage Ratio). Here is a breakdown of the inputs required:
Rider Weight: Do not use your "morning bathroom scale" weight. You must include your riding kit: helmet, shoes, hydration pack, and protective gear. This typically adds 10-15 lbs to your base body weight.
Rear Wheel Travel: The total vertical distance the rear axle moves. This can be found in your bike's manual.
Shock Stroke: The distance the shock shaft compresses. Be careful not to confuse this with eye-to-eye length. A 210x55mm shock has a 55mm (2.16 in) stroke.
Desired Sag: This is the percentage of travel compressed by just your body weight when sitting on the bike in a neutral position.
Downhill/Enduro: Typically 30% – 33%.
Trail/XC: Typically 25% – 30%.
Rear Weight Bias: This accounts for how your weight is distributed between the front and rear wheels. In a standing "attack position," about 60-70% of the rider's weight is carried by the rear suspension. We default to 65% for a balanced setup.
Understanding Leverage Ratio
The Leverage Ratio is the mechanical advantage of the rear wheel over the shock. It is calculated as Wheel Travel ÷ Shock Stroke.
For example, if a bike has 6 inches of travel and a 2-inch shock stroke, the leverage ratio is 3:1. This means for every 1 pound of force the shock pushes back with, it supports 3 pounds at the wheel. Higher leverage ratios require heavier springs to support the same rider weight.
Why Sag Matters
Sag allows the suspension to extend into depressions in the trail (like potholes or gaps between roots), maintaining traction. Without proper sag, the wheel would skip over holes rather than tracking through them. Using this calculator ensures you purchase a spring that lands you right in the "sweet spot" of your suspension's kinematics.
Next Steps
Once you have your calculated spring rate (e.g., 425 lbs/in), you will likely need to buy the nearest available spring increment. Most manufacturers sell springs in 50 lb steps (400, 450, 500). If your result is 425 lbs/in, you can choose:
400 lbs/in: For a softer, more plush feel (more sag).
450 lbs/in: For a supportive, racier feel (less sag).
Use the preload collar on your shock to fine-tune the sag once the spring is installed. However, preload should strictly be used for minor adjustments (1-3 turns max) and not to compensate for the wrong spring rate.