Coilover Spring Rate Calculator
Calculated Results:
Required Wheel Rate: lbs/in
Required Spring Rate: lbs/in
Recommended Spring (Nearest Standard): lbs/in
How to Calculate Spring Rate for Your Coilovers
Choosing the correct spring rate is the most critical step in tuning a suspension system. It determines how your car handles bumps, weight transfer during cornering, and the overall "feel" of the chassis. To calculate the correct rate, you must look beyond the spring itself and understand how the suspension geometry affects the force applied to that spring.
The Core Formula: From Wheel Rate to Spring Rate
The "Wheel Rate" is the effective stiffness at the tire contact patch. The "Spring Rate" is the actual stiffness of the coil spring. Because springs are usually mounted inboard on a control arm, they have a mechanical disadvantage (leverage). We calculate this using the Motion Ratio.
Step 1: Determine Sprung Weight
Subtract the unsprung weight (wheels, tires, brakes, half the suspension arm weight) from the corner weight of the car.
Sprung Weight = Corner Weight – Unsprung Weight
Step 2: Calculate Desired Wheel Rate
Using your target natural frequency (how fast the suspension oscillates), calculate the wheel rate:
Wheel Rate (WR) = (Sprung Weight × (2 × π × Frequency)²) / 386.4
Step 3: Calculate Spring Rate
Account for the motion ratio and the installation angle of the shock:
Spring Rate = WR / (Motion Ratio² × cos(Angle))
Motion Ratio and Angle Correction
The Motion Ratio (MR) is the ratio of shock travel to wheel travel. If the wheel moves 2 inches while the shock moves only 1 inch, your MR is 0.5. Since force is squared through a lever, an MR of 0.5 means you need a spring four times stiffer than the desired wheel rate.
The Shock Angle also reduces effectiveness. If a shock is mounted at an angle, only a portion of the force acts vertically. We use the cosine of the angle to correct the math.
Typical Natural Frequency Ranges
| Vehicle Application | Target Frequency (Hz) |
|---|---|
| Passenger Cars (Comfort) | 1.0 – 1.2 Hz |
| Sporty Street / Track Day | 1.5 – 2.0 Hz |
| Dedicated Race Car (High Downforce) | 2.5 – 3.5+ Hz |
Example Calculation
Imagine a car with an 800 lb corner weight and 80 lbs of unsprung mass (720 lbs sprung weight). The motion ratio is 0.7, and the shock is mounted at a 10-degree angle. If we want a 1.8 Hz natural frequency (stiff street/track setup):
- Wheel Rate: (720 * (2 * 3.14 * 1.8)^2) / 386.4 = Approx 238 lbs/in.
- Spring Rate: 238 / (0.7^2 * cos(10°)) = Approx 493 lbs/in.
- Selection: You would likely choose a 500 lbs/in spring.