Rear Coilover Spring Rate Calculator
Calculate ideal spring rates based on vehicle weight and suspension geometry
Exclude wheel/tire weight (unsprung weight).
Distance pivot-to-shock / pivot-to-wheel.
Degrees from vertical (0 is perfectly vertical).
1.2 – Soft Street
1.5 – Performance Street
2.0 – Club Racing
2.5 – Dedicated Race
Typical range: 1.0Hz to 3.0Hz.
Recommended Spring Rate:
0 lb/in
Resulting Wheel Rate: 0 lb/in
Understanding Rear Spring Rates
Choosing the correct rear coilover spring rate is critical for maintaining traction, managing weight transfer, and ensuring ride comfort. Unlike front springs, rear rates must account for specific leverage ratios often found in Independent Rear Suspension (IRS) or trailing arm setups.
Key Variables Explained:
- Sprung Corner Weight: The actual weight the spring supports. This is the total corner weight minus the weight of the wheel, tire, and half the suspension arms.
- Motion Ratio: This describes the leverage the wheel has over the spring. In a setup where the spring is mounted halfway along the control arm, the motion ratio would be 0.5.
- Shock Angle: As a shock leans over, it loses effectiveness. A 15-degree angle reduces the effective spring rate significantly compared to a vertical mount.
- Ride Frequency: Measured in Hertz (cycles per second), this determines the "stiffness" feel. Most passenger cars are 1.0-1.5Hz, while track cars usually exceed 2.0Hz.
Calculation Example
If your rear corner weight is 800 lbs, your motion ratio is 0.7, your shock is at a 10-degree angle, and you want a 1.5Hz frequency:
- Determine Wheel Rate: ((1.5 / 3.13)² * 800) = 183.7 lb/in.
- Adjust for Motion Ratio: 183.7 / (0.7 * 0.7) = 374.9 lb/in.
- Adjust for Angle: 374.9 / cos(10°) = 380.6 lb/in.
- Recommended Spring: 375 lb/in or 400 lb/in.