Spring Rate Calculator

How Spring Rate is Calculated

Spring rate (measured in lbs/in or N/mm) determines how much force is needed to compress a spring by one unit of distance. A 500 lb/in spring requires 500 pounds of force to compress it one inch. The right spring rate for your car depends on the corner weight, the suspension's motion ratio, and the target ride frequency — the natural oscillation speed of the sprung mass.

The Formulas

Wheel Rate = (4π² × Frequency² × Corner Weight) / 386.4

Spring Rate = Wheel Rate / Motion Ratio²

386.4 is gravitational acceleration in inches per second squared (32.2 ft/s² × 12). Wheel rate is the effective rate at the tire contact patch. The spring rate is always higher than the wheel rate because the motion ratio is less than 1.0 — the spring moves less than the wheel.

For example, if a car has 750 lb corner weight, a target frequency of 2.0 Hz, and a motion ratio of 0.75: Wheel Rate = (4 × 9.87 × 4.0 × 750) / 386.4 = 306 lb/in. Spring Rate = 306 / 0.75² = 306 / 0.5625 = 544 lb/in. You would buy a 550 lb/in spring (the nearest standard size).

Ride Frequency Guide

Ride frequency is the most reliable way to select spring rates because it normalizes for vehicle weight. A 2.0 Hz spring on a 2,200 lb Miata and a 2.0 Hz spring on a 4,000 lb Camaro will feel similarly controlled, even though the actual spring rates are very different.

• 1.0-1.5 Hz: Comfort/luxury — soft ride, more body roll, excellent bump absorption. Typical for OEM sedans and touring cars.
• 1.5-2.0 Hz: Street/sport — good balance of comfort and body control. Popular for daily-driven cars with coilovers.
• 2.0-2.5 Hz: Autocross/track day — firm, responsive, quick transient response. Requires matched dampers to avoid harshness.
• 2.5-3.0 Hz: Dedicated track car — stiff, minimal body roll, very fast weight transfer. Not street-friendly on rough roads.
• 3.0+ Hz: Full race — very stiff. Requires a smooth track surface and professional-grade dampers. Common in formula cars and tube-frame race cars.

Front/Rear Frequency Split

Most well-handling cars run the rear frequency 5-15% higher than the front. This gives the rear axle a slightly faster natural response, which means the rear of the car settles before the front in a corner entry — making the car feel stable and predictable. A typical street/track setup might run 1.8 Hz front and 2.0 Hz rear. Running equal frequencies front and rear is acceptable, but running the front stiffer than the rear tends to create nervous, unpredictable handling.

Motion Ratio

The motion ratio is how far the spring moves relative to the wheel. If the wheel moves 1 inch and the spring compresses 0.8 inches, the motion ratio is 0.8. This ratio squared converts wheel rate to spring rate, so even small differences in motion ratio have a large effect on the required spring rate.

• MacPherson strut: 0.90-1.0 — the spring is mounted directly on the strut, so motion ratio is nearly 1:1.
• Double wishbone / multi-link: 0.60-0.80 — the spring is mounted inboard on the lower arm, so it moves less than the wheel.
• Solid axle with coils: 0.85-1.0 — depends on where the spring sits on the axle tube.

To measure your motion ratio: jack the car up so the suspension hangs free, measure the distance between two points on the spring and two points on the wheel center, then compress the suspension 1 inch at the wheel and re-measure. The spring compression divided by the wheel compression is your motion ratio.

Linear vs. Progressive Springs

A linear spring has the same rate throughout its travel — 500 lb/in at 1 inch of compression and 500 lb/in at 3 inches. A progressive spring starts soft and gets stiffer as it compresses, usually achieved by varying the coil spacing. Progressive springs offer a more comfortable ride over small bumps while still resisting body roll under hard cornering. However, they are harder to tune because the effective rate changes with load. For track and autocross use, most serious setups use linear springs because the handling characteristics are more consistent and predictable.

Matching Springs to Dampers

Springs and dampers (shocks) must work together. The damper controls how fast the spring compresses and rebounds — without adequate damping, the car bounces and oscillates. A common mistake is installing much stiffer springs without upgrading the dampers. Stock dampers are valved for stock spring rates and will be overwhelmed by a significantly stiffer spring, resulting in a harsh ride that never settles. If you increase spring rate by more than 20-30% over stock, plan to upgrade your dampers to match.