Tire Spring Rate Calculator

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Tire Spring Rate Calculator

Calculate the vertical stiffness of your tires based on load and deflection.

The weight supported by this specific tire.
Distance from hub center to top of tire (lifted).
Distance from hub center to ground (on ground).
Please ensure Loaded Radius is less than Unloaded Radius and all fields are filled.
Tire Deflection: –
Tire Spring Rate (Imperial): –
Tire Spring Rate (Metric): –
Effective Stiffness Rating: –

What is Tire Spring Rate?

Tire Spring Rate refers to the vertical stiffness of a tire. Just like the metal coil springs in your vehicle's suspension, the air-filled tire acts as a spring. It compresses under load and rebounds when the load is removed. This rate is typically measured in pounds per inch (lbs/in) or Newtons per millimeter (N/mm).

Understanding your tire spring rate is crucial for advanced suspension tuning. The tire provides the initial compliance to road surface irregularities. A tire with a high spring rate (stiff) offers sharper handling response but less grip on bumpy surfaces, while a lower spring rate (soft) absorbs bumps better but may feel vague in corners.

How to Calculate Tire Spring Rate

While tire engineers use complex machinery to map spring rates across different camber angles and pressures, you can calculate the Linear Vertical Spring Rate using the formula:

K = Load / (Unloaded Radius – Loaded Radius)

  • Load: The weight resting on that specific tire (Corner Weight).
  • Unloaded Radius: The radius of the tire when the car is jacked up (no weight).
  • Loaded Radius: The distance from the wheel center to the ground when the car is resting on its wheels.

The "Series Spring" Effect

It is important to remember that your suspension spring and your tire act as two springs in a series. This means the Total Wheel Rate is actually softer than the softer of the two springs. The formula for the combined rate is:

1 / Total Rate = 1 / Tire Rate + 1 / Suspension Rate

If you install very stiff coilover springs but run soft tires (low pressure or tall sidewalls), the actual stiffness at the wheel will be significantly dominated by the tire's compliance. Tuning tire pressure is essentially tuning your spring rate; increasing pressure generally increases the tire's spring rate.

Factors Affecting Tire Stiffness

  • Air Pressure: The most significant variable. Higher pressure equals a higher spring rate.
  • Sidewall Height (Aspect Ratio): Lower profile tires (e.g., 30 or 35 series) have shorter, stiffer sidewalls and higher spring rates than tall tires (e.g., 60 or 70 series).
  • Construction: Run-flat tires and high-performance tires with reinforced sidewalls have naturally higher spring rates.
function calculateTireRate() { // 1. Get input values var weightStr = document.getElementById("cornerWeight").value; var unloadedStr = document.getElementById("unloadedRadius").value; var loadedStr = document.getElementById("loadedRadius").value; var errorDiv = document.getElementById("errorMsg"); var resultsDiv = document.getElementById("resultsArea"); // 2. Parse values var weight = parseFloat(weightStr); var unloaded = parseFloat(unloadedStr); var loaded = parseFloat(loadedStr); // 3. Validation if (isNaN(weight) || isNaN(unloaded) || isNaN(loaded)) { errorDiv.style.display = "block"; errorDiv.innerHTML = "Please enter valid numbers in all fields."; resultsDiv.style.display = "none"; return; } if (loaded >= unloaded) { errorDiv.style.display = "block"; errorDiv.innerHTML = "Loaded Radius must be smaller than Unloaded Radius to calculate deflection."; resultsDiv.style.display = "none"; return; } if (weight <= 0) { errorDiv.style.display = "block"; errorDiv.innerHTML = "Corner Weight must be greater than zero."; resultsDiv.style.display = "none"; return; } // Hide error if validation passes errorDiv.style.display = "none"; // 4. Calculation Logic var deflection = unloaded – loaded; var springRateLbs = weight / deflection; // Convert to Metric: 1 lb/in approx 0.175127 N/mm var springRateN = springRateLbs * 0.175127; // 5. Determine Rating (Heuristic) var rating = ""; if (springRateLbs = 1000 && springRateLbs = 2000 && springRateLbs < 3000) { rating = "Stiff / Track Day"; } else { rating = "Very Stiff / Racing Slick"; } // 6. Display Results document.getElementById("resDeflection").innerHTML = deflection.toFixed(3) + " in"; document.getElementById("resRateLbs").innerHTML = Math.round(springRateLbs) + " lbs/in"; document.getElementById("resRateN").innerHTML = springRateN.toFixed(1) + " N/mm"; document.getElementById("resRating").innerHTML = rating; resultsDiv.style.display = "block"; }

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