Spring Rate Converter
Convert spring rates between pounds per inch, newtons per millimetre and kilograms per millimetre. Add a motion ratio to get the rate at the wheel.
Convert a spring rate
- Motion ratio is squared when converting spring rate to wheel rate. Halving the ratio quarters the rate at the wheel.
- Ride frequency ignores the anti-roll bar and any tire stiffness, so it is a comparison tool rather than a prediction.
- Progressive springs have no single rate. These conversions assume a linear spring.
The conversions
One pound per inch is 0.1751 newtons per millimetre. One kilogram per millimetre is 56 pounds per inch, which is why Japanese coilover rates look so small next to American ones. An 8 kg/mm spring is 448 lb/in.
Motion ratio, and why it gets squared
A spring mounted inboard on a control arm moves less than the wheel does. Motion ratio is how much spring travel you get per unit of wheel travel. A coilover sitting almost on the hub is near 1.0. A spring halfway along a long arm might be 0.6.
The ratio applies twice. Once because the spring compresses less, and again because of the leverage on the force. So wheel rate is spring rate times the ratio squared. At 0.6 motion ratio a 400 lb/in spring gives you 144 lb/in at the wheel, which catches people out constantly.
Ride frequency is the number that compares setups
Spring rate on its own tells you nothing without knowing what weight it is holding up. Ride frequency folds both together, which is why suspension people talk in hertz.
Roughly: under 1.0 Hz is a comfortable road car. 1.0 to 1.5 is a firm street setup. 1.5 to 2.0 is sport. Above 2.0 belongs on a track, and above 2.5 belongs on a smooth one.
Frequently asked questions
How many lb/in is 8 kg/mm?
448 lb/in. Multiply kg/mm by 56 for a quick approximation, or use 55.997 if you want to be exact.
Are stiffer springs always faster?
No. A spring's job is keeping the tire on the road. Too much rate and the tire skips over surface irregularities instead of following them, which costs grip. On a rough public road a softer, well damped setup beats a stiff one almost every time.
Do I need new shocks if I change spring rates?
Usually yes. Damping has to match the rate it is controlling. Fitting much stiffer springs to stock shocks leaves the suspension underdamped and bouncing, and it destroys the shocks quickly. Springs and dampers get chosen together.
How do I find my motion ratio?
Measure it. Jack the car up, move the wheel through a known amount of travel, and measure how far the spring perch moves over that same range. Spring travel divided by wheel travel is your ratio. Estimating it from a photo is how people end up hundreds of pounds off.
Everything on this page is an estimate worked out from the numbers you entered and typical figures for the conditions you picked. Your vehicle, your parts and your conditions will differ. Check anything you intend to act on against the manufacturer's specification, the service manual, or a qualified technician. Verifying the result before relying on it is up to you.