function calculateSpringRate() {
// 1. Get input values
var weight = parseFloat(document.getElementById("riderWeight").value);
var travelMm = parseFloat(document.getElementById("wheelTravel").value);
var strokeMm = parseFloat(document.getElementById("shockStroke").value);
var sagPercent = parseFloat(document.getElementById("desiredSag").value);
// 2. Validate inputs
if (isNaN(weight) || isNaN(travelMm) || isNaN(strokeMm) || weight <= 0 || travelMm <= 0 || strokeMm <= 0) {
alert("Please enter valid positive numbers for Weight, Travel, and Stroke.");
return;
}
// 3. Convert MM to Inches for calculation (Springs are rated in lbs/inch)
var travelIn = travelMm / 25.4;
var strokeIn = strokeMm / 25.4;
// 4. Calculate Leverage Ratio
var leverageRatio = travelIn / strokeIn;
// 5. Logic Constants
// Weight Bias: Approx 68% of rider weight is on the rear wheel for modern Enduro/DH geometry in attack position.
var rearBias = 0.68;
// Sag Decimal
var sagDecimal = sagPercent / 100;
// 6. Calculate Required Spring Rate
// Formula: Force on Shock / Compression Amount
// Force on Shock = (Rider Weight * Rear Bias) * Leverage Ratio
// Compression Amount at Sag = Stroke * SagDecimal
// Rate = ((Weight * Bias) * Ratio) / (Stroke * Sag)
var sprungWeight = weight * rearBias;
var forceOnShock = sprungWeight * leverageRatio;
var compressionAtSag = strokeIn * sagDecimal;
var idealRate = forceOnShock / compressionAtSag;
// 7. Determine Recommended Fox Spring (Rounding)
// Fox springs generally come in 25lb increments for SLS (Super Light Steel) and Standard steel.
// We round to the nearest 25.
var remainder = idealRate % 25;
var recommendedRate;
if (remainder < 12.5) {
recommendedRate = idealRate – remainder;
} else {
recommendedRate = idealRate + (25 – remainder);
}
// 8. Display Results
var resultsDiv = document.getElementById("resultsArea");
resultsDiv.style.display = "block";
document.getElementById("calcRate").innerHTML = Math.round(idealRate) + " lbs/in";
document.getElementById("recSpring").innerHTML = recommendedRate + " lbs";
document.getElementById("levRatio").innerHTML = leverageRatio.toFixed(2) + ":1";
}
Optimizing Your Fox DHX2 Performance
The Fox DHX2 is one of the premier coil shocks on the market, offering unparalleled sensitivity and adjustability for downhill and enduro riding. However, unlike air shocks where you can simply grab a pump to adjust the sag, a coil shock requires the correct physical spring to function correctly. Using this calculator helps you identify the correct spring rate (measured in pounds per inch) to achieve your desired sag.
Why Spring Rate Matters
The spring rate determines how much force is required to compress the shock. If your spring rate is too soft, the bike will sit too deep in its travel (excessive sag), causing pedal strikes, poor geometry, and frequent bottom-outs. If the spring is too stiff, you will lose traction, the ride will feel harsh, and you won't utilize the full travel of your bike.
For the Fox DHX2, getting the spring rate right is the foundation of your setup. The damping adjustments (HSC, LSC, HSR, LSR) are designed to fine-tune the ride, but they cannot compensate for an incorrect spring rate.
How to Use This Calculator
Rider Weight: Enter your weight fully geared up. This includes your helmet, shoes, hydration pack, and protective gear. A typical riding kit adds 10-15 lbs to your body weight.
Rear Wheel Travel: This is the total vertical travel of your rear wheel, found in your bike frame's manual. Enter this in millimeters (e.g., 160mm).
Shock Stroke: This is the distance the shock shaft compresses. It is critical to get this number exactly right. It is the second number in metric shock sizing (e.g., in a 230×60 shock, 60 is the stroke).
Desired Sag: For most coil applications, 30% is the standard starting point. Less sag (25%) provides a firmer platform for jumping and climbing, while more sag (33%) offers maximum traction for steep downhill tracks.
Fox SLS vs. Standard Steel Springs
Once you have your calculated rate, you will need to purchase a spring. Fox offers two main types:
Standard Steel: Durable and cost-effective, typically black. These are usually available in 50lb increments (e.g., 400, 450, 500).
SLS (Super Light Steel): These orange (or sometimes black) springs use a higher grade steel that allows for a thinner wire diameter and fewer coils, significantly reducing weight. SLS springs are often available in 25lb increments (e.g., 425, 475), allowing for a more precise setup.
Preload and Fine Tuning
After installing your recommended spring, you should check your sag physically. You can adjust the preload collar on the DHX2 to fine-tune the sag, but you should never use more than 2 full turns of preload. If you need more than 2 turns to achieve proper sag, you need a stiffer spring. If you have zero preload and less than your desired sag, you need a softer spring.