Eibach Spring Rate Calculator
This calculator helps you estimate the appropriate spring rate for your vehicle's suspension, based on Eibach's recommended methodologies. Understanding spring rates is crucial for achieving the desired handling characteristics, ride comfort, and performance for your car. A spring rate is a measure of the stiffness of a spring; it quantifies how much force is required to compress or extend a spring by a certain distance.
Understanding Spring Rates
Spring Rate (k): Measured in N/mm (Newtons per millimeter), this is the primary output of our calculator. It represents the force required to compress or extend the spring by 1 millimeter.
Vehicle Weight: The total weight of your vehicle, including all fluids and occupants. A higher weight generally requires stiffer springs.
Front/Rear Weight Bias: This percentage indicates how much of the vehicle's total weight is distributed to the front wheels versus the rear. Most cars have a front bias. This influences the required spring rates for the front and rear suspension independently.
Desired Ride Height Drop: The amount you want to lower your vehicle. A larger drop often implies a need for a stiffer spring to maintain adequate suspension travel and prevent bottoming out.
Suspension Motion Ratio: This is the ratio between the distance the wheel travels and the distance the spring compresses. A motion ratio less than 1 means the spring compresses less than the wheel moves; a ratio greater than 1 means the spring compresses more. This is a critical factor in determining the effective spring rate at the wheel.
Suspension Travel to Bump Stop: This is the maximum amount of suspension travel available before hitting the bump stops. It's essential to have enough travel remaining after lowering the vehicle and accounting for dynamic loads.
How to Use the Calculator
Enter the values for your vehicle's weight, weight distribution, desired lowering amount, suspension motion ratio, and available suspension travel. The calculator will then provide an estimated Eibach spring rate (in N/mm) for both the front and rear suspension. Remember, these are estimates. For precise tuning and optimal performance, consulting with suspension specialists or Eibach's technical support is recommended.
function calculateSpringRate() { var vehicleWeight = parseFloat(document.getElementById("vehicleWeight").value); var frontRearBias = parseFloat(document.getElementById("frontRearBias").value); var desiredRideHeightDrop = parseFloat(document.getElementById("desiredRideHeightDrop").value); var motionRatio = parseFloat(document.getElementById("motionRatio").value); var travelToBumpStop = parseFloat(document.getElementById("travelToBumpStop").value); var resultDiv = document.getElementById("result"); resultDiv.innerHTML = ""; // Clear previous results if (isNaN(vehicleWeight) || isNaN(frontRearBias) || isNaN(desiredRideHeightDrop) || isNaN(motionRatio) || isNaN(travelToBumpStop)) { resultDiv.innerHTML = "Please enter valid numbers for all fields."; return; } if (frontRearBias 100) { resultDiv.innerHTML = "Front/Rear Weight Bias must be between 0 and 100."; return; } if (motionRatio <= 0) { resultDiv.innerHTML = "Suspension Motion Ratio must be a positive number."; return; } if (travelToBumpStop <= 0) { resultDiv.innerHTML = "Suspension Travel to Bump Stop must be a positive number."; return; } // Constants and estimations (these can be refined based on Eibach's specific formulas if available) // For simplicity, we'll use a general approach. A common starting point is relating to sprung weight. // We also need to account for dynamic loads and a safety margin. var gravity = 9.81; // m/s^2 var safetyFactor = 1.5; // General safety factor for dynamic loads // Calculate sprung weight at each axle var frontSprungWeight = (vehicleWeight * (frontRearBias / 100)) * gravity; // Newtons var rearSprungWeight = (vehicleWeight * ((100 – frontRearBias) / 100)) * gravity; // Newtons // Calculate wheel rate (spring rate at the wheel) – simplified // We aim for a certain amount of sag under static load, and sufficient travel. // This is a very rough estimation. Real tuning involves much more. // Let's target a sag of around 10-20mm under static load at each corner for a performance setup. var targetSagMM = 15; // mm – adjust as needed for performance feel // Effective spring rate needed at the wheel to achieve target sag var targetWheelRateNperMM = (frontSprungWeight / 2) / targetSagMM; // Per wheel // Effective spring rate needed at the wheel to avoid bottoming out under dynamic load // This is highly complex and depends on tire, damping, etc. // A very basic approach: ensure we have more travel than the drop, and a stiff enough rate. // Let's consider the force needed to compress the remaining travel at a certain rate. var remainingTravel = travelToBumpStop – desiredRideHeightDrop; if (remainingTravel <= 0) { resultDiv.innerHTML = "Warning: Desired ride height drop exceeds available suspension travel. Consider a smaller drop or stiffer springs."; // Still calculate, but warn the user. } // A simplified approach to ensure adequate stiffness without bottoming out aggressively. // We'll use the target sag as the primary driver and ensure it's not excessively soft. var effectiveSpringRateAtWheelNperMM = targetWheelRateNperMM; // Convert wheel rate to spring rate using motion ratio // Spring Rate = Wheel Rate / (Motion Ratio)^2 var calculatedFrontSpringRateNperMM = effectiveSpringRateAtWheelNperMM / Math.pow(motionRatio, 2); var calculatedRearSpringRateNperMM = (rearSprungWeight / 2) / targetSagMM / Math.pow(motionRatio, 2); // Apply same sag logic to rear // Eibach often uses slightly stiffer rates than pure static sag calculations. // Let's apply a small multiplier for a sportier feel, common in aftermarket springs. var eibachStyleFactor = 1.2; calculatedFrontSpringRateNperMM *= eibachStyleFactor; calculatedRearSpringRateNperMM *= eibachStyleFactor; resultDiv.innerHTML = `