Suspension Setup Results
Understanding 4-Link Rear Suspension Spring Rates
Tuning a 4-link rear suspension requires precise calculation of spring rates to ensure optimal traction, handling, and ride quality. Unlike a simple beam axle on leaf springs, a 4-link setup with coilovers introduces variables like motion ratios and shock angles that significantly alter how the spring acts on the wheel.
1. Sprung Corner Weight
This is the foundation of your calculation. It represents the actual weight resting on the spring at ride height. To get an accurate number, you should weigh the rear of the car and subtract the "unsprung weight" (the weight of the rear end housing, axles, brakes, wheels, and tires). The remaining figure is the sprung weight that the springs must support.
2. Motion Ratio (The Leverage Effect)
In many 4-link setups, the coilover is not mounted directly over the axle centerline. It is often mounted on the lower link, somewhere between the frame pivot and the axle. This creates a lever arm.
The Motion Ratio (MR) is calculated as:
If your shock is mounted halfway down the link, your motion ratio is 0.5. This means for every 1 inch the wheel moves, the shock only moves 0.5 inches. Consequently, the spring needs to be significantly stiffer to control the wheel leverage.
3. Shock Angle Correction
Springs work most efficiently when compressed vertically. As you angle a shock (often done for packaging constraints in drag cars or street rods), its effective rate decreases. The calculator applies a correction factor based on the cosine of the angle. A shock mounted at a 30-degree angle requires a stiffer spring than one mounted vertically to achieve the same wheel rate.
4. Natural Frequency Targets
The "feel" of the suspension is mathematically defined by its Natural Frequency (measured in Hz). This determines how quickly the suspension settles after a bump.
- 1.0 – 1.2 Hz: Comfortable street ride, typical for cruisers.
- 1.3 – 1.6 Hz: Sporty handling, Pro-Touring, firmer ride.
- 1.8 – 2.5 Hz: Dedicated track cars, drag racing launch stability, or high-downforce applications.
Use this calculator to find the ideal spring rate (lbs/in) for your specific 4-link geometry and intended use.