Spring rate calculator
Find the spring for a target ride frequency, with the motion ratio and the tire included. Or enter a spring you already have and see its ride frequency. Works for any car with coilovers or pushrod springs.
One corner
Spring rate needed
33.3 N/mm
- Wheel rate
- 21.3 N/mm
- Ride rate (with the tire)
- 21.3 N/mm
Check a spring you have
- Ride frequency
- 3.08 Hz
- wheel rate 22.4 N/mm, ride rate 22.4 N/mm
How it's worked out
- The ride rate a frequency needs: ride rate = m × (2π f)², with m the corner's sprung mass.
- Take the tire out. It's a spring in series: 1/ride = 1/wheel + 1/tire.
- Take the motion ratio out: spring rate = wheel rate ÷ MR².
What it leaves out
One corner on its own, with linear springs and a constant motion ratio. Anti-roll bars don't change the ride frequency in heave, but they do add roll stiffness. A rising-rate rocker changes the motion ratio through travel, so check it at ride height. For the formulas on paper, the free Contact Patch cheat sheet has motion ratio, wheel rate and ride frequency on one page.
Questions
What ride frequency should I use?
Common starting points are about 1 to 1.5 Hz for a comfortable road car, 1.5 to 2.5 Hz for a track-day car, and 2.5 to 4 Hz for an FSAE or downforce race car. Higher is stiffer. Many designers run the rear a little higher than the front so the car settles flat over bumps.
What is the motion ratio?
How far the spring compresses for each millimeter the wheel moves. A spring that moves 0.8 mm per 1 mm of wheel travel has a motion ratio of 0.8. The wheel feels the spring rate times the motion ratio squared, so a small change in motion ratio changes the wheel rate a lot.
What's the difference between spring rate, wheel rate and ride rate?
Spring rate is the spring on its own. Wheel rate is what that spring feels like at the wheel, after the motion ratio. Ride rate adds the tire, which acts as a second spring in series, so the ride rate is always a little softer than the wheel rate.
Which mass do I enter?
The sprung mass on that corner: the corner's share of the car's weight, minus that corner's unsprung parts (wheel, tire, upright, brake and about half of the arms, springs and driveshaft). Weigh each corner on scales if you can.
Is my data saved or sent anywhere?
No. The numbers stay in your browser.
More free tools: Roll center calculator · Weight transfer calculator