How to Calculate Spring Rate for Car

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Automotive Spring Rate Calculator

Weight on one wheel minus unsprung components.
Street: 1.0-1.5 | Track: 2.0-3.0+
Leverage ratio of suspension arm.
Angle from vertical (usually 0 for struts).

Calculation Results

Required Wheel Rate: lb/in

Calculated Spring Rate: lb/in

Note: This is the theoretical rate. Round to the nearest available commercial spring rate.

function calculateSpringRate() { var sprungWeight = parseFloat(document.getElementById('sprungWeight').value); var targetFreq = parseFloat(document.getElementById('targetFreq').value); var motionRatio = parseFloat(document.getElementById('motionRatio').value); var springAngle = parseFloat(document.getElementById('springAngle').value) || 0; if (isNaN(sprungWeight) || isNaN(targetFreq) || isNaN(motionRatio)) { alert("Please fill in all required fields with valid numbers."); return; } // Constant for gravity in inches/sec^2 is 386.4 // Wheel Rate = (4 * PI^2 * Freq^2 * SprungWeight) / 386.4 var wheelRate = (Math.pow(2 * Math.PI * targetFreq, 2) * sprungWeight) / 386.4; // Correct for Angle: Spring Rate = Wheel Rate / (MR^2 * cos(angle)) var angleRad = springAngle * (Math.PI / 180); var angleCorrection = Math.cos(angleRad); var springRate = wheelRate / (Math.pow(motionRatio, 2) * angleCorrection); document.getElementById('wheelRateOut').innerHTML = wheelRate.toFixed(2); document.getElementById('springRateOut').innerHTML = springRate.toFixed(2); document.getElementById('springResult').style.display = 'block'; }

How to Calculate Spring Rate for Your Car

Calculating the correct spring rate is the foundation of automotive suspension tuning. Whether you are building a track car or improving a street vehicle, the spring rate determines how the car handles bumps, weight transfer, and body roll. To calculate the spring rate, you must first understand the relationship between the wheel rate and the mechanical leverage of your suspension system.

Key Definitions

  • Sprung Weight: This is the weight supported by the springs. It includes the frame, engine, and body, but excludes "unsprung" parts like the wheels, tires, and half of the control arm weight.
  • Natural Frequency (Hz): This measures how many times the suspension oscillates per second. High-performance cars use higher frequencies (stiffer) to control aerodynamic loads and body movement.
  • Motion Ratio: This is the leverage the wheel has on the spring. In most suspension designs (like double wishbones), the spring is mounted part-way down the arm, meaning the spring moves less than the wheel.
  • Wheel Rate: The actual stiffness felt at the tire contact patch. This is the value that truly dictates handling.

The Spring Rate Formula

The mathematical path to finding your required spring rate involves two steps:

  1. Find the Wheel Rate: Wheel Rate = (Sprung Weight × (2 × π × Frequency)²) / Gravity
  2. Convert to Spring Rate: Spring Rate = Wheel Rate / (Motion Ratio² × cos(Angle))

Example Calculation

Let's say you have a track-oriented car with the following specs:

  • Corner Sprung Weight: 750 lbs
  • Desired Frequency: 2.2 Hz (Track setting)
  • Motion Ratio: 0.6 (Spring is mounted 60% of the way out the control arm)
  • Spring Angle: 15 degrees

First, we calculate the Wheel Rate. Using the physics of harmonic motion, 750 lbs at 2.2 Hz requires a wheel rate of approximately 371 lb/in. Next, we account for the leverage. Since the Motion Ratio is 0.6, the spring must be much stiffer than the wheel rate. Dividing 371 by (0.6² × cos(15°)) gives a required Spring Rate of 1,067 lb/in.

Choosing the Right Frequency

If you aren't sure what frequency to target, use these general guidelines:

Application Target Frequency (Hz)
Typical Passenger Car 1.0 Hz – 1.2 Hz
Sporty Street Car 1.3 Hz – 1.6 Hz
Non-Aero Race Car 1.8 Hz – 2.5 Hz
High Downforce Race Car 3.0 Hz – 5.0+ Hz

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