Understanding Bike Spring Rate and Sag
Proper suspension setup is crucial for optimizing your mountain bike's performance, comfort, and control on the trails. A key element of this setup is achieving the correct "sag" in your suspension. Sag refers to the amount the suspension compresses under your weight when you are seated on the bike in a riding position.
The spring rate of your shock or fork is the stiffness of the spring itself. It dictates how much force is required to compress the spring a certain distance. A spring with a higher spring rate is stiffer, while a lower spring rate is softer.
The goal is to match the spring rate to your riding weight and the intended use of your bike. This calculator helps you determine the ideal spring rate based on your rider weight, bike weight, desired sag percentage, and the total travel of your suspension.
How it works:
- Rider Weight (kg): Your body weight in kilograms.
- Bike Weight (kg): The weight of your bicycle.
- Desired Sag Percentage (%): This is typically between 15% and 35% for mountain bikes, with 25% being a common starting point. Higher sag means a softer feel, while lower sag provides a firmer ride.
- Wheel Travel (mm): The maximum amount your rear wheel can travel upwards in millimeters (e.g., for a 160mm travel fork or shock).
The calculator uses these inputs to determine the required spring rate. The formula typically involves calculating the total weight the suspension needs to support, converting the desired sag into a distance, and then using that to find the spring rate (force per distance). A common simplified formula to derive the necessary spring force and then infer spring rate is:
Spring Force (N) = (Rider Weight (kg) + Bike Weight (kg)) * 9.81 m/s²
Sag Distance (mm) = Wheel Travel (mm) * (Desired Sag Percentage / 100)
Then, the spring rate (N/mm) would be Spring Force / Sag Distance.
By inputting your specific details, you can get a recommended spring rate that will help you achieve optimal suspension performance for your riding style. Remember that this is a starting point, and fine-tuning may be required based on personal preference and trail conditions.
Example: For a rider weighing 75 kg on a 15 kg bike, wanting 25% sag on a 160 mm travel suspension, the calculator will provide a recommended spring rate.
function calculateSpringRate() {
var riderWeight = parseFloat(document.getElementById("riderWeight").value);
var bikeWeight = parseFloat(document.getElementById("bikeWeight").value);
var sagPercentage = parseFloat(document.getElementById("sagPercentage").value);
var wheelTravel = parseFloat(document.getElementById("wheelTravel").value);
var resultDiv = document.getElementById("result");
resultDiv.innerHTML = ""; // Clear previous results
if (isNaN(riderWeight) || isNaN(bikeWeight) || isNaN(sagPercentage) || isNaN(wheelTravel) ||
riderWeight <= 0 || bikeWeight <= 0 || sagPercentage <= 0 || wheelTravel 100) {
resultDiv.innerHTML = "Please enter valid positive numbers for all fields. Sag percentage must be between 1 and 100.";
return;
}
var gravity = 9.81; // Acceleration due to gravity (m/s^2)
// Calculate total weight acting on the suspension
var totalWeightKg = riderWeight + bikeWeight;
var totalWeightNewtons = totalWeightKg * gravity;
// Calculate the desired sag distance in millimeters
var sagDistanceMm = wheelTravel * (sagPercentage / 100);
// Calculate the required spring rate (Newtons per millimeter)
// Ensure sagDistanceMm is not zero to avoid division by zero
if (sagDistanceMm === 0) {
resultDiv.innerHTML = "Sag distance cannot be zero. Please check Wheel Travel and Sag Percentage.";
return;
}
var springRateNperMm = totalWeightNewtons / sagDistanceMm;
// Often springs are rated in lbs/inch. Conversion: 1 N/mm = 5.71015 lbs/inch
var springRateLbsPerInch = springRateNperMm * 5.71015;
resultDiv.innerHTML =
"
" +
"Total Weight Supported: " + totalWeightKg.toFixed(2) + " kg" +
"Desired Sag Distance: " + sagDistanceMm.toFixed(2) + " mm" +
"Recommended Spring Rate: