Glossary
What is Spring Rate? Units, How to Calculate It & Typical Values
Spring rate is the force required to compress a spring by one unit of distance, quoted in pounds per inch (lb/in), newtons per millimetre (N/mm) or kilograms per millimetre (kg/mm). It is the single biggest influence on how much a car rolls, pitches and moves on its suspension. Race cars choose spring rate to hit a target ride frequency, typically 1.5–2.5 Hz for non-aero cars and 3–5 Hz for high-downforce cars.
| From | To | Multiply by |
|---|---|---|
| lb/in | N/mm | 0.1751 |
| N/mm | lb/in | 5.710 |
| kg/mm | N/mm | 9.807 |
| kg/mm | lb/in | 56.0 |
| lb/in | kg/mm | 0.01786 |
Choosing a spring rate
The usual method is to pick a target ride frequency and work backwards. Ride frequency f = (1 / 2π) × √(wheel rate / sprung corner mass). Non-aero circuit cars sit around 1.5–2.5 Hz, with the rear 10–20% higher than the front so the car settles flat after a bump. Aero cars go much stiffer to hold ride height.
Once you have a wheel rate target, divide by the motion ratio squared to get the spring rate to order. Setup Genius includes a spring rate converter and dual spring calculator in its racing calculators.
Effects of changing spring rate
Stiffer front springs increase front roll stiffness and shift balance towards understeer; stiffer rear springs do the opposite. Stiffer springs everywhere reduce pitch under braking and squat under power, which helps aero cars, but let the tyre lose contact over bumps and kerbs.
Frequently asked questions
What is the difference between spring rate and wheel rate?
Spring rate is measured at the spring. Wheel rate is the effective rate at the tyre contact patch after the suspension leverage (motion ratio) is taken into account. Wheel rate is what the car actually feels.
Should the front or rear springs be stiffer?
In frequency terms the rear is usually 10–20% higher so the rear catches up with the front after a bump. In absolute rate terms it depends on weight distribution and motion ratios.
How does spring rate interact with anti-roll bars?
Both contribute to roll stiffness. Springs also control pitch and heave; bars only act in roll. Many engineers use springs for ride and aero platform, then trim balance with bars.
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