Spring rate calculator
1 kg/mm equals about 56 lb/in (55.997), so an 8 kg/mm coilover spring is roughly 448 lb/in. This calculator converts between kg/mm, lb/in and N/mm, then uses corner weight and motion ratio to give wheel rate, ride frequency and the spring rate for a target frequency.
Coilover makers often list spring rates in kg/mm, while US spring catalogs use lb/in. The table below covers the common sizes.
- Spring rate
- 8.00 kg/mm · 448 lb/in · 78.5 N/mm
- Wheel rate
- 448 lb/in · 8.00 kg/mm
- Ride frequency
- 2.48 Hz (149 CPM) sports / non-aero road race range (1.5-2.5 Hz)
- Static spring compression
- 40 mm (1.58")
- Spring for 1.6 Hz
- 3.3 kg/mm · 186 lb/in
Spring rate conversion table
1 kg/mm = 9.807 N/mm = 55.997 lb/in. 1 lb/in = 0.1751 N/mm.
| kg/mm | lb/in | N/mm |
|---|---|---|
| 4 | 224 | 39.2 |
| 5 | 280 | 49.0 |
| 6 | 336 | 58.8 |
| 7 | 392 | 68.6 |
| 8 | 448 | 78.5 |
| 9 | 504 | 88.3 |
| 10 | 560 | 98.1 |
| 12 | 672 | 117.7 |
| 14 | 784 | 137.3 |
| 16 | 896 | 156.9 |
Spring rates on coilover brand pages and in our fitment tables are listed the way the manufacturer publishes them, usually in kg/mm.
Spring rate vs wheel rate
The car feels the wheel rate, not the spring rate. Wheel rate equals the spring rate times the motion ratio squared, where the motion ratio is spring travel divided by wheel travel.
When the coilover sits on a strut close to the wheel, the ratio is close to 1 and the two rates are similar. When the spring or coilover mounts inboard on a control arm, the ratio drops and so does the wheel rate: at a ratio of 0.7, only 49% of the spring rate reaches the wheel. If you do not know your ratio, measure how far the spring compresses for a known amount of wheel travel.
Ride frequency
Ride frequency is how many times per second a corner of the car would bounce on its spring. It combines wheel rate and sprung corner weight: frequency (Hz) = 1 / 2π × √(wheel rate / sprung corner mass).
Penske Racing Shocks gives these reference ranges: street cars typically run 1.0 to 1.5 Hz, non-aero road race and sports cars 1.5 to 2.5 Hz, and stock cars on asphalt 2.0 to 3.0 Hz. Higher frequency means a firmer ride and quicker response; lower means more compliance over bumps.
Corner weight and unsprung weight
Corner weight is what one wheel carries on a scale. Sprung weight is that number minus the unsprung parts: wheel, tire, brakes, hub and part of the control arms and coilover. The default values in the calculator are placeholders; replace them with your car's numbers for a meaningful result.
The static compression line shows how far the spring compresses under the car's weight. It helps when setting preload and ride height, covered in how to adjust coilovers.
FAQ
How do I convert kg/mm to lb/in?
How do I convert lb/in to kg/mm?
What is a good ride frequency for a street car?
Why are my front and rear spring rates different?
Related
Sources: Penske Racing Shocks: natural frequency and ride frequency