Spring Rate Calculator
Estimate the spring rate your car needs from its weight and how you drive it, then convert between kg/mm, lb/in and N/mm. Most coilover listings, including every SF Racing kit we carry, quote spring rates in kg/mm.
How the spring rate calculator works
A car bounces on its springs at a natural rate called the ride frequency, measured in hertz (Hz). Factory cars are usually soft, around 1 to 1.5 Hz. Street coilovers usually run firmer, around 1.5 to 2 Hz, and track and drift cars run higher again. The calculator works backwards from the frequency you pick to the spring rate that gives it.
- Sprung corner weight. Curb weight times the front or rear share, divided by two, minus the unsprung weight at that corner.
- Wheel rate. The stiffness you need at the tire: sprung corner mass times (2π × frequency)².
- Spring rate. The wheel rate divided by the motion ratio squared. A spring mounted partway along an arm moves less than the wheel, so it has to be stiffer than the wheel rate.
Motion ratio, simply
Motion ratio is how far the spring compresses for each inch the wheel moves. A strut that sits right over the hub is close to 1:1. A coilover bolted halfway along a lower arm might be about 0.7, which means the spring needs to be about twice as stiff (1 ÷ 0.7²) to give the same wheel rate. If you are unsure, measure: jack the wheel up 1 inch and see how far the spring moves.
Using the result
- Every SF Racing coilover listing we carry shows the front and rear spring rates in kg/mm in its spec table. Compare them with the range the calculator gives.
- On many SF Racing kits you can choose different spring rates as an option on the product page.
- Rates a little above the calculated range are normal for coilovers, since they are usually set lower than stock with less travel to work with.
- Very stiff springs on a bumpy road can make the car skip and lose grip. If it is a street car, err on the softer side.
Frequently asked questions
How do I convert kg/mm to lb/in?
Multiply by 55.997. For example, 8 kg/mm is about 448 lb/in. To go from lb/in to kg/mm, divide by 55.997.
What spring rate do I need for a daily driver?
Pick "Sporty street car" in the calculator. For most cars that lands well below the rates on track-focused kits. If your car is a daily, look at street kits and the softer spring options.
Are higher spring rates always better for handling?
No. Stiffer springs reduce body roll, but too stiff for the road surface and the tires lose contact over bumps. Dampers also need to match the springs.
Is the calculator exact?
No. It is an estimate from typical values. Real corner weights, motion ratios, tires and how the car is driven all change the ideal rate. Contact us if you want help choosing a kit.