Include helmet, shoes, hydration pack, and protective gear.
Standard is 25-30% for Trail/Enduro, 30-35% for DH.
Calculated Ideal Rate:– lbs/in
Recommended Spring (Soft):– lbs/in
Recommended Spring (Firm):– lbs/in
Leverage Ratio:–
function calculateSpringRate() {
// 1. Get Inputs
var weight = parseFloat(document.getElementById('riderWeight').value);
var travelMm = parseFloat(document.getElementById('frameTravel').value);
var strokeMm = parseFloat(document.getElementById('shockStroke').value);
var sagPercent = parseFloat(document.getElementById('targetSag').value);
// 2. Validation
if (isNaN(weight) || isNaN(travelMm) || isNaN(strokeMm) || isNaN(sagPercent) || strokeMm === 0 || sagPercent === 0) {
alert("Please enter valid numbers greater than zero for all fields.");
return;
}
// 3. Unit Conversions
// Spring rates are typically Lbs/Inch.
// Travel and Stroke are input in MM, so convert to Inches.
var travelIn = travelMm / 25.4;
var strokeIn = strokeMm / 25.4;
// 4. Calculate Leverage Ratio
var leverageRatio = travelIn / strokeIn;
// 5. Calculate Spring Rate
// Formula Logic:
// Force at Wheel = Rider Weight * Rear Bias (approx 65% for static sag)
// Force at Shock = Force at Wheel * Leverage Ratio
// Spring Displacement at Sag = Shock Stroke * (SagPercent / 100)
// Rate (k) = Force at Shock / Spring Displacement
var rearBias = 0.65; // Assumes 65% of weight on rear wheel in seated/neutral position
var forceAtWheel = weight * rearBias;
var forceAtShock = forceAtWheel * leverageRatio;
var compressionDistance = strokeIn * (sagPercent / 100);
var exactK = forceAtShock / compressionDistance;
// 6. Calculate Recommended Springs (Nearest 50 lbs increments)
var lowerSpring = Math.floor(exactK / 50) * 50;
var upperSpring = Math.ceil(exactK / 50) * 50;
// If the exact rate is very close to the lower, suggest lower and lower-50, etc.
// But simpler logic: just show the bracketing springs.
// 7. Display Results
document.getElementById('exactRate').innerHTML = Math.round(exactK) + " lbs/in";
document.getElementById('softSpring').innerHTML = lowerSpring + " lbs/in";
document.getElementById('firmSpring').innerHTML = upperSpring + " lbs/in";
document.getElementById('leverageRatioDisplay').innerHTML = leverageRatio.toFixed(2) + ":1";
document.getElementById('resultsArea').style.display = "block";
}
Understanding Rear Shock Spring Rates
Choosing the correct coil spring for your mountain bike is the most critical step in setting up your suspension. Unlike air shocks, which can be adjusted with a shock pump, a coil shock requires the physical spring to match your body weight, riding gear, and the bike's kinematics. If your spring rate is incorrect, no amount of compression or rebound damping adjustment will fix the ride quality.
How This Calculator Works
This calculator determines the spring stiffness (measured in pounds per inch, or lbs/in) required to support your weight at a specific sag point. The calculation relies on three main variables:
Rider Weight: The total mass the bike must support. This must include your helmet, shoes, water, tools, and protective gear. A standard riding kit adds 10-15 lbs to your body weight.
Leverage Ratio: This is the relationship between how much the rear wheel moves compared to how much the shock compresses. High-leverage bikes require stiffer springs to support the same rider weight compared to low-leverage bikes.
Desired Sag: The amount the suspension compresses under your static weight. Standard sag is usually 25-30% for trail/enduro and up to 35% for downhill applications.
Why "Rear Bias" Matters
When you sit on a bike, your weight is not distributed 50/50 between the wheels. Typically, about 60-70% of a rider's weight is over the rear axle. This calculator uses a standard rear-weight bias factor to estimate the actual force applied to the shock, providing a more accurate starting point than simple linear equations.
Rounding to Standard Spring Sizes
Springs are manufactured in increments, usually 25 lbs/in or 50 lbs/in (e.g., 400, 450, 500). It is rare for your calculated ideal rate to match a manufactured spring exactly. You will need to choose between a softer or firmer option:
Choose the Softer Spring (Lower Number): If you prefer a plush, active feel, ride technical terrain where traction is paramount, or if your bike has a highly progressive linkage.
Choose the Firmer Spring (Higher Number): If you ride jump lines, prefer a supportive/poppy feel, are a heavier rider concerned with bottoming out, or if your bike has a linear leverage curve.
Preload Notes
Once you install your spring, you should achieve your target sag with minimal preload (1 to 2 turns of the collar). If you have to turn the preload collar more than 3-4 times to get the correct sag, your spring rate is too soft. If the spring is loose even with the collar tightened, the rate is too stiff.