Suspension Ride Frequency & Spring Rate Calculator
Enter corner weights, unsprung weight, spring rates, motion ratios, and tire rates to compare front and rear ride frequency. Set target frequencies to see the spring rates needed to reach them.
Required front spring rate (lb/in)318.01
- Required rear spring rate (lb/in)
- 343.08
- Current front ride frequency (Hz)
- 1.75
- Current rear ride frequency (Hz)
- 2.01
- Rear-to-front frequency ratio
- 1.14%
Front and rear suspension worksheet
| Measure | Front | Rear |
|---|---|---|
| Sprung corner weight (lb) | 810 | 620 |
| Wheel rate (lb/in) | 324 | 325.13 |
| Ride rate including tire (lb/in) | 255.12 | 255.82 |
| Static suspension deflection (in) | 2.5 | 1.91 |
Motion ratio is spring travel divided by vertical wheel travel, so it is squared when converting spring rate to wheel rate.
The tire-corrected ride rate treats the tire and suspension as springs in series.
How to use this calculator
- Choose the unit system that matches your corner weights and spring-rate data.
- Enter loaded front and rear corner weights, then subtract the unsprung weight for each corner.
- Add the current spring rates, motion ratios, and tire vertical rates for the front and rear suspension.
- Set target ride frequencies to size the front and rear springs for the setup you want.
How the suspension numbers are calculated
This calculator follows the quarter-car worksheet method commonly used for initial spring selection. For each end of the car, it starts with loaded corner weight Wc, unsprung weight Wu, spring rate Ks, motion ratio r, and tire vertical rate Kt. The sprung corner weight is Ws = Wc - Wu.
Wheel rate is lower than spring rate when the spring moves less than the wheel:
Kw = Ks × r²
The tire and suspension act like two springs in series, so the body sees a tire-corrected ride rate:
Kr = (Kw × Kt) ÷ (Kw + Kt)
In US customary units, with rates in lb/in and sprung weight in lb, ride frequency is:
f = 3.127 × sqrt(Kr ÷ Ws)
In metric units, with rates in N/mm and sprung mass in kg, it is:
f = sqrt((1000 × Kr) ÷ Ms) ÷ (2π)
Reverse sizing for target frequency
To find the spring rate for a target frequency ft, the calculator first solves the ride rate needed at the body. In US units, Krt = Ws × (ft ÷ 3.127)². In metric units, Krt = Ms × (2πft)² ÷ 1000. It then backs out the required wheel rate from the tire spring in series:
Kwt = (Krt × Kt) ÷ (Kt - Krt)
Finally, it converts wheel rate back to spring rate with Kst = Kwt ÷ r². If the target ride rate is equal to or higher than the tire rate, no finite spring rate can reach it because the tire limits the combined rate.
What moves the result most
Motion ratio has a large effect because it is squared. A spring mounted farther inboard may need a much higher spring rate than the wheel-rate target suggests. Tire rate also matters: very stiff springs make ride rate approach the tire vertical rate, but the ride rate never exceeds the tire rate in this linear model.
What this leaves out
The result is a heave-mode estimate for one corner at a time. It does not model damping, suspension friction, chassis flexibility, aero load, bump stops, nonlinear motion ratio, or anti-roll-bar contribution in roll and one-wheel bump. Use the result as a spring-selection worksheet, then check travel, damper stroke, coil-bind clearance, and manufacturer limits.
Worked example
For the default US customary setup, the front sprung corner weight is 900 lb - 90 lb = 810 lb. A 400 lb/in front spring at a 0.90 motion ratio gives a wheel rate of 324 lb/in, and a 1200 lb/in tire puts the tire-corrected front ride rate at about 255.12 lb/in.
The current front ride frequency is about 1.75 Hz. At the rear, 700 lb - 80 lb leaves 620 lb sprung, the 450 lb/in spring and 0.85 motion ratio give 325.13 lb/in wheel rate, and the current rear ride frequency is about 2.01 Hz. The rear-to-front frequency ratio is about 114%.
With target frequencies of 1.60 Hz front and 1.80 Hz rear, the reverse calculation asks for about 318 lb/in at the front spring and 343 lb/in at the rear spring.
Common questions
What is a good ride frequency for a street, autocross, or track car?
Street cars are often near 1.0 to 1.5 Hz, performance street and autocross setups often land around 1.5 to 2.2 Hz, and dedicated track cars can be higher. The right number depends on tire grip, aero load, suspension travel, surface roughness, and driver preference.
Should ride frequency use total corner weight or sprung corner weight?
Use sprung corner weight for body-mode ride frequency. The wheel, tire, brake, hub, and part of the links move mostly with the road input, so including all of that unsprung weight makes the body mode look softer than it is.
How do I measure suspension motion ratio?
Measure vertical wheel movement and spring or damper movement near ride height, then divide spring travel by wheel travel. Use small movements around ride height because many suspensions have changing motion ratio through travel.
Why is motion ratio squared when calculating wheel rate?
The spring moves a fraction of the wheel distance, and the force leverage changes by the same fraction. Those two effects multiply, so wheel rate is spring rate times motion ratio squared.
Should rear ride frequency be higher than front ride frequency?
Many setups run the rear ride frequency higher than the front to help the car settle after a bump and to support handling balance. That is not a universal rule; weight distribution, aero balance, tire stagger, suspension travel, and intended use can justify a different split.
Do coilover preload and helper springs change ride frequency?
Preload alone does not change ride frequency after the car is sitting at ride height unless it changes the active spring rate or available droop. Helper springs usually collapse at ride height, so the main spring rate controls the calculation once the helper is fully bound.