Understanding Spring Rate for Your Ohlins Suspension
Choosing the correct spring rate for your motorcycle's Ohlins suspension is crucial for optimal performance, comfort, and safety. The spring rate determines how much force is required to compress the spring by a certain distance. An incorrect spring rate can lead to a bike that feels too soft (bottoms out easily) or too stiff (doesn't absorb bumps effectively), negatively impacting handling and rider fatigue.
Factors Influencing Spring Rate Choice
Several factors come into play when determining the ideal spring rate for your setup:
Rider Weight: This is the most significant factor. A heavier rider will require a stiffer spring than a lighter rider.
Bike Weight: The overall weight of the motorcycle also contributes to the forces acting on the suspension.
Suspension Travel: The amount of travel your suspension is designed to handle influences the spring rate needed to achieve proper sag.
Leverage Ratio: This is a critical factor in motorcycle suspension. It's the ratio between the amount the rear wheel travels and the amount the shock compresses. A higher leverage ratio means the shock compresses more for a given wheel movement, requiring a softer spring. Ohlins and other manufacturers often provide this ratio.
Riding Style and Conditions: Aggressive riders or those who frequently ride on rough terrain might opt for slightly stiffer springs, while track day riders might prefer a setup that allows for more precise adjustments.
Preload: While preload adjusts the initial sag of the suspension, it doesn't change the spring's stiffness (spring rate). However, it's an important adjustment to make *after* selecting the correct spring rate.
The Calculation Explained
This calculator uses a common formula to estimate the required spring rate. It takes into account the combined weight of the rider and bike, then factors in the suspension's leverage ratio and travel to determine the necessary spring stiffness. The formula is generally:
Note that some formulas may add a small percentage for dynamic load or include preload considerations indirectly. This calculator provides a good starting point.
Why Use This Calculator?
This Ohlins Spring Rate Calculator aims to simplify the process of finding a suitable spring rate. By inputting your specific details, you can get a recommended spring rate to discuss with your suspension tuner or to use as a basis for your own research. Remember that this is a guideline, and fine-tuning by a professional suspension technician is always recommended for the best results.
function calculateSpringRate() {
var riderWeight = parseFloat(document.getElementById("riderWeight").value);
var bikeWeight = parseFloat(document.getElementById("bikeWeight").value);
var springLength = parseFloat(document.getElementById("springLength").value); // Not directly used in basic rate calc, but good for context
var travel = parseFloat(document.getElementById("travel").value);
var preload = parseFloat(document.getElementById("preload").value); // Not directly used in basic rate calc, but good for context
var leverageRatio = parseFloat(document.getElementById("leverageRatio").value);
var resultDiv = document.getElementById("result");
resultDiv.innerHTML = ""; // Clear previous results
if (isNaN(riderWeight) || isNaN(bikeWeight) || isNaN(travel) || isNaN(leverageRatio) ||
riderWeight <= 0 || bikeWeight <= 0 || travel <= 0 || leverageRatio <= 0) {
resultDiv.innerHTML = "Please enter valid positive numbers for all fields.";
return;
}
// Constants
var gravity = 9.81; // m/s²
// Calculate total weight in Newtons
var totalWeightKg = riderWeight + bikeWeight;
var totalWeightNewtons = totalWeightKg * gravity;
// Calculate the required spring rate in N/mm
// The formula effectively considers how much force is applied to the spring
// Total Weight needs to be distributed over the travel at the shock, considering the leverage ratio.
// Force at the shock = Total Weight * Gravity
// Effective travel at the shock = Wheel Travel / Leverage Ratio. This is NOT how it's typically calculated for spring rate.
// A more direct approach:
// The goal is to achieve a certain amount of sag (e.g., 30-33% of total travel) under rider weight.
// However, this calculator aims for a general recommendation based on total load and leverage.
// A common simplified calculation focuses on the force needed to compress the spring through the intended travel.
// Let's re-evaluate the common approach for spring rate which is about static sag under rider weight.
// Sag Percentage is usually 30-33% of total travel for most bikes.
// Let's assume static sag is desired at ~1/3 of total travel.
// Force required = Spring Rate * Sag
// We want to find Spring Rate.
// A simplified model: Spring Rate (N/mm) = (Total Weight * Gravity) / (Desired Sag in mm)
// Desired Sag is often a percentage of total travel, and also influenced by leverage ratio.
// A common way to relate spring rate to bike weight and travel is:
// Spring Rate (N/mm) = (Bike + Rider Weight in kg) * Gravity / (Total Suspension Travel in mm) * (some factor, often related to leverage ratio or desired sag)
// Let's use a widely cited formula that relates rider weight + bike weight to suspension characteristics:
// Spring Rate (lbs/in) = (Rider Weight + Bike Weight) / (Desired Sag Percentage * Suspension Travel)
// To convert to N/mm: lbs/in * (0.175127) = N/mm
// Let's use a direct N/mm calculation aiming for a certain sag percentage.
// Assume desired static sag is 30% of total travel.
// Required Force = (riderWeight + bikeWeight) * gravity
// Effective Force at the Wheel = (riderWeight + bikeWeight) * gravity
// Force at the Shock = Effective Force at the Wheel * Leverage Ratio
// Let's assume we want a certain sag percentage (e.g. 30%) of the SHOCK travel.
// Target Shock Compression (Sag) = Travel * 0.30
// The core idea is that the spring must support the rider+bike weight distributed across the suspension.
// A simpler approach often used for initial estimation:
// Spring Rate (N/mm) = (Total Weight (kg) * Gravity) / (Total Suspension Travel (mm) * A Factor)
// The factor is often empirically derived or related to leverage.
// Another common method:
// Spring Rate (N/mm) = (Total Weight (kg) * Gravity) / (Desired Sag in mm)
// Desired Sag for a rear shock is often 10-15mm for ~60mm travel, or 30-33% of shock travel.
// Let's use a simplified but common approach that incorporates leverage ratio:
// The force acting on the spring is effectively the rider/bike weight multiplied by the leverage ratio,
// and this force needs to compress the spring through a portion of its travel (sag).
// A commonly used formula by suspension tuners is based on the load and the desired sag:
// Load at the shock = (riderWeight + bikeWeight) * gravity
// For a desired sag (e.g., 12mm for a 63mm travel shock), the spring rate would be:
// Spring Rate (N/mm) = Load at the shock / Desired Sag (mm)
// However, the leverage ratio is key. It dictates how much the shock compresses for a given wheel movement.
// So, if total wheel travel is 210mm and leverage is 2.5, shock travel is ~84mm.
// If rider+bike weight is 275kg, force is 275 * 9.81 = 2697.75 N.
// For 30% sag on shock travel: 0.30 * 84mm = 25.2mm.
// Spring Rate = 2697.75 N / 25.2 mm = ~107 N/mm.
// Let's refine the calculation to use the provided inputs more directly and commonly.
// The "travel" input is typically the total shock travel. The "spring length" isn't directly used in the rate calculation.
// Let's assume the "travel" input IS the total SHOCK travel.
// And we are aiming for a sag percentage. A common target is 30-33% of shock travel for sag.
var desiredSagMm = travel * 0.33; // Using 33% as a common target for sag
// The force applied to the spring is the total weight.
// We need to convert kg to Newtons for force.
var totalForceKg = riderWeight + bikeWeight;
var totalForceNewtons = totalForceKg * gravity;
// Spring Rate = Force / Displacement (Sag)
// However, the leverage ratio affects how much force the spring experiences at the shock for a given wheel load.
// A simpler way often presented:
// Spring Rate (N/mm) = (Total Weight * Gravity) / (Leverage Ratio * Desired Sag in mm)
// This formula feels more intuitive for a motorcycle setup where leverage is prominent.
// But the 'travel' input should be the total available shock travel.
// Let's reconsider the common method: focus on desired sag in mm.
// Static Sag desired is usually 30-33% of the total shock travel.
// Load on the spring at static conditions = (Rider Weight + Bike Weight) * Gravity.
// Spring Rate = Force / Deflection
// So, Spring Rate (N/mm) = [(Rider Weight + Bike Weight) * Gravity] / Desired Static Sag (mm)
// Where Desired Static Sag (mm) is often related to the *total shock travel* and rider weight distribution.
// The leverage ratio affects the *range* of motion and *feel*, but the fundamental rate is about supporting the weight.
// Let's use a calculation that is commonly cited for motorcycle suspension:
// It aims to achieve a certain amount of sag under the rider's weight.
// The spring rate is calculated based on the force required to compress the spring.
// Force (N) = Weight (kg) * Gravity (m/s²)
// Spring Rate (N/mm) = Force (N) / Sag (mm)
// The key is determining the "Sag" that is appropriate.
// For rear suspension, 30-33% of total shock travel is a common target for rider sag.
// So, Sag (mm) = Total Shock Travel (mm) * 0.33
// The "Total Weight" used is typically Rider Weight + Bike Weight.
// Let's use the provided 'travel' as total shock travel.
var targetSagMm = travel * 0.33; // Assuming 33% sag of total shock travel
if (targetSagMm <= 0) { // Ensure target sag is a positive value
resultDiv.innerHTML = "Error: Calculated target sag is zero or negative. Check travel input.";
return;
}
var calculatedSpringRate = totalForceNewtons / targetSagMm;
// Round to a reasonable number of decimal places
calculatedSpringRate = parseFloat(calculatedSpringRate.toFixed(2));
var resultHTML = "Estimated Spring Rate: " + calculatedSpringRate + " N/mm";
resultHTML += "This is an estimated spring rate. It's recommended to consult with a professional suspension tuner for precise setup.";
resultDiv.innerHTML = resultHTML;
}
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