What the tire actually feels
Spring rate is what is printed on the spring. Wheel rate is what the tire actually feels, once the suspension's leverage — the motion ratio — is accounted for. Natural frequency turns that wheel rate into a number you can compare against known handling targets.
Wheel rate works out to 196 lb/in — the spring rate multiplied by the motion ratio squared, because leverage affects both the force and the distance the linkage passes through. Natural frequency ranges are context-dependent: a soft street setup and a high-downforce race car target completely different numbers, both correctly.
| Use case | Natural frequency range |
|---|---|
| Soft street / comfort-tuned | 1.0–1.5 Hz |
| Sport street / mild track | 1.5–2.0 Hz |
| Track-focused, non-aero | 2.0–2.5 Hz |
| Racing, non-aero | 2.5–3.0 Hz |
| High-downforce racing | 3.0–4.0 Hz |
Getting a number you can act on
- 01Get the spring's rated stiffness
Printed on the spring or in its part specification — the raw lb/in or N/mm figure before any suspension geometry is applied.
- 02Establish the suspension's motion ratio
The ratio of wheel travel to spring travel over a small range of motion, usually below 1.0 on most double-wishbone and strut designs — measured directly, or taken from published suspension geometry for the vehicle.
- 03Enter corner weight
The sprung weight that specific corner supports — from a corner-weight scale reading for real accuracy, since it varies with driver, fuel load and ballast placement.
- 04Compare natural frequency against a target range for the intended use
The guidance table below gives broad ranges by use case, from soft street comfort through to high-downforce racing — the right target depends entirely on what the vehicle is built for.
What the calculator is actually doing
Nothing here is proprietary. If you would rather check it by hand, or explain it to someone at a counter, these are the same expressions the tool evaluates.
wheel rate = spring rate × motion ratio²Squared because motion ratio scales both the force multiplication and the distance the linkage passes through.
frequency (Hz) = 3.13 × √(wheel rate ÷ corner weight)The 3.13 constant comes from (1 ÷ 2π) × √386.4, where 386.4 is standard gravity in inches per second squared — the unit conversion that lets lb/in and lb produce Hz directly.
Why motion ratio is squared, not applied once
Motion ratio describes how much the spring compresses for a given amount of wheel travel — a lever-arm relationship, and like any lever it affects both force and distance simultaneously, in opposite directions.
A motion ratio below 1.0 means the wheel moves more than the spring does for the same suspension travel. That geometry multiplies the force reaching the spring (mechanical advantage works both ways), but it also means the spring's own stiffness gets divided down by the same ratio when felt at the wheel — and those two effects compound rather than cancel, which is why the relationship comes out squared.
This is a genuinely counterintuitive result the first time you encounter it: a suspension with a 0.7 motion ratio needs a spring roughly twice as stiff as the desired wheel rate would suggest at first glance (1 ÷ 0.7² ≈ 2.04), not 1.43 times stiffer as a linear guess would produce.
Why natural frequency is the number worth comparing, not raw spring rate
Spring rate alone means very little without knowing what it's supporting. A 400 lb/in spring on a lightweight formula car and the same 400 lb/in spring on a heavy sedan produce completely different ride characteristics, because the mass being controlled is entirely different.
Natural frequency accounts for that by folding in corner weight, producing a figure — cycles per second the suspension would oscillate at if disturbed — that is directly comparable across completely different vehicles. This is why race engineers talk in Hz rather than lb/in when discussing suspension stiffness targets.
The published ranges vary enormously by intended use, and that variation is deliberate rather than arbitrary: a soft street car targets low frequency for ride comfort, while a high-downforce race car targets high frequency to control aerodynamic platform movement, at the direct cost of ride quality neither car is built to prioritize the same way.
Suspension Spring Rate Calculator FAQ
What is the difference between spring rate and wheel rate?+
Spring rate is what's printed on the spring itself. Wheel rate is what the tire actually experiences, after the suspension's motion ratio has scaled it — wheel rate equals spring rate times motion ratio squared.
Why is motion ratio squared in the wheel rate formula?+
Because it's a lever-arm relationship that affects both force and distance simultaneously in opposite directions, and those two effects compound multiplicatively rather than adding — producing a squared relationship.
How do I calculate suspension natural frequency?+
Multiply 3.13 by the square root of wheel rate divided by corner weight, with wheel rate in lb/in and corner weight in lb. The 3.13 constant converts those units into a result in Hz.
What natural frequency should my suspension have?+
It depends entirely on the intended use — roughly 1.0-1.5 Hz for soft street comfort, 1.5-2.5 Hz for sport street or mild track use, and up toward 3.0-4.0 Hz for high-downforce racing where platform control matters more than ride comfort.
Standards and references behind these figures
The arithmetic on this page is fixed, but the boundaries and conventions around it come from published standards and manufacturer guidance. These are the documents they come from, so you can check them rather than take them on trust.
01Ford Performance — dynamometer testing and engine performance tech tipsManufacturer guidance on dyno correction and how quoted power figures are arrived at.↗02The Tire and Rim Association — standards filing (NHTSA docket)TRA has been the US standardising body for tire and rim interchangeability since 1903; this filing sets out dimensional practice.↗Geometry is exact; your specific suspension may not match the assumption. Motion ratio, in particular, changes through the suspension's travel on most real cars rather than staying constant — these formulas use a single fixed value, which is a reasonable approximation near ride height and a worse one at the extremes of travel. Measure your own motion ratio directly where precision matters.