Stance & Suspension
Coilover Spring Rate, Wheel Rate & Ride Frequency
Turn a coilover spring rate into wheel rate and ride frequency using motion ratio, install angle, and corner sprung weight — with lb/in, N/mm, and kg/mm units and a spring-swap ride height estimate.
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How to Use This Tool
- Enter the spring rate and pick lb/in, N/mm, or kg/mm (the unit most JDM coilovers are sold in).
- Enter the motion ratio directly, or measure the spring mount and wheel distances from the control-arm pivot.
- Add the damper install angle from vertical and the corner sprung weight (corner weight minus unsprung wheel, tire, brake, and arm weight).
- Read wheel rate and ride frequency against the street, sport, and track ranges, and optionally compare a new spring rate.
The Formula & Physics Behind the Math
Motion ratio (MR) is how far the spring moves per unit of wheel travel — spring mount distance from the pivot divided by wheel distance from the pivot. Because the spring both moves less and has less leverage, wheel rate = spring rate × MR² × cos(install angle), with the angle measured from vertical (a common approximation; some references apply a cos² correction for large angles). Ride frequency treats the corner as a simple spring-mass system: f = (1 ÷ 2π) × √(k ÷ m), with wheel rate k in newtons per meter and corner sprung mass m in kilograms (1 lb/in = 175.13 N/m, 1 lb = 0.4536 kg). Static deflection at the wheel is sprung weight ÷ wheel rate; swapping to a stiffer spring with the same free length and perch position reduces that sag, so the car sits higher by the difference. Preload does not change spring rate: the loaded spring length is set by corner load, so threading a perch up to add preload raises the car by the distance the perch moved.
Frequently Asked Questions
What is the difference between spring rate and wheel rate?
Spring rate is the stiffness of the spring itself. Wheel rate is the effective stiffness felt at the tire contact patch after suspension leverage. Most double-wishbone and multi-link cars have a motion ratio below 1, so wheel rate is noticeably softer than the spring rate — a 0.7 motion ratio cuts it roughly in half.
What ride frequency should I aim for?
As general guidance, comfort-oriented road cars sit around 1.0–1.5 Hz, sporty street cars 1.5–2.0 Hz, non-aero track cars 2.0–3.0 Hz, and high-downforce race cars 3.0 Hz and up. Many setups run the rear slightly higher than the front for a flatter ride over bumps.
How do I convert kg/mm spring rates to lb/in?
One kgf/mm equals about 56 lb/in (55.997), and one N/mm equals about 5.71 lb/in. So an 8 kg/mm spring is roughly 448 lb/in, and a 10 kg/mm spring is about 560 lb/in.
Does coilover preload make the suspension stiffer or lower the car?
Neither. Preload does not change spring rate, and the spring's loaded length is set by the corner load, not the preload. On a threaded-perch coilover, winding the perch up to add preload raises the car by the same distance the perch moved — it acts as a height adjustment. Full-body coilovers let you keep preload fixed and set height with the lower mount. Stiffer springs, by contrast, compress less under the same weight, so the car sits slightly higher unless you readjust height.
How do I measure motion ratio?
With the car on stands, measure from the lower control-arm inner pivot to the spring or damper mount, and from the pivot to the ball joint or wheel center. Divide spring distance by wheel distance. For MacPherson struts the ratio is often close to 0.9–1.0; for double wishbones it is commonly 0.6–0.8.