Rear Spring Rate Calculator

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Rear Spring Rate Calculator

Optimize your bike's suspension for your specific weight and riding style.

Recommended Spring Rate — lb/in
Leverage Ratio — :1

Metric Equivalent: — N/mm

How to Calculate Rear Spring Rate

Spring rate is the amount of force required to compress a spring by one inch. For mountain bikes and motorcycles with rear coil shocks, selecting the correct spring rate is critical for maintaining traction, preventing bottom-outs, and ensuring a comfortable ride.

The Physics of the Calculation

The calculation relies on the Leverage Ratio of your bike's frame. This is the relationship between how much the rear wheel moves versus how much the shock compresses. Most modern enduro bikes have a ratio between 2.3:1 and 3.0:1.

  • Rider Weight: Use your full weight including helmet, shoes, hydration pack, and tools. This typically adds 10-15 lbs to your body weight.
  • Wheel Travel: The vertical distance the rear axle moves from top to bottom (found in your bike's specs).
  • Shock Stroke: The distance the shock shaft actually travels into the body (different from the shock's eye-to-eye length).
  • Sag: The amount the suspension compresses under your static weight. Generally, 28% – 33% is standard for gravity-oriented riding.

Example Calculation

If you weigh 180 lbs (fully geared), have 150mm of wheel travel and a 50mm shock stroke (a 3:1 leverage ratio), and want 30% sag:

  1. Determine weight bias (usually 60% of weight is on the rear wheel): 180 lbs * 0.6 = 108 lbs.
  2. Multiply by leverage ratio: 108 lbs * 3 = 324 lbs of force at the shock.
  3. Calculate sag in inches: 50mm * 0.30 = 15mm (approx 0.59 inches).
  4. Spring Rate = Force / Sag Distance: 324 / 0.59 = 549 lb/in (Rounding to the nearest 50lb spring, you'd choose 550 lbs).
function calculateSpringRate() { var weight = parseFloat(document.getElementById('riderWeight').value); var sagPercent = parseFloat(document.getElementById('desiredSag').value); var travel = parseFloat(document.getElementById('wheelTravel').value); var stroke = parseFloat(document.getElementById('shockStroke').value); if (isNaN(weight) || isNaN(sagPercent) || isNaN(travel) || isNaN(stroke) || stroke === 0) { alert("Please enter valid numerical values for all fields."); return; } // 1. Calculate Leverage Ratio var leverageRatio = travel / stroke; // 2. Estimate Rear Weight Bias (Standard 60% for MTB/Moto) var rearWeightForce = weight * 0.60; // 3. Convert Sag % to Inches of stroke compression // Convert stroke mm to inches (mm / 25.4) var strokeInches = stroke / 25.4; var sagDistanceInches = strokeInches * (sagPercent / 100); // 4. Calculate Spring Rate (lb/in) // Formula: (Rear Weight Force * Leverage Ratio) / Sag Distance var rawRate = (rearWeightForce * leverageRatio) / sagDistanceInches; // Round to nearest whole number for display var finalRate = Math.round(rawRate); // Convert to N/mm (1 lb/in = 0.175127 N/mm) var metricRate = (rawRate * 0.175127).toFixed(2); // Display Results document.getElementById('rateResult').innerHTML = finalRate; document.getElementById('ratioResult').innerHTML = leverageRatio.toFixed(2); document.getElementById('metricResult').innerHTML = metricRate; document.getElementById('resultArea').style.display = 'block'; }

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