What a shorter or longer spring actually does
A change in spring free length or perch position does not translate one-to-one into ride height, because the suspension's motion ratio changes how much of that spring movement reaches the wheel. Below 1.0, the wheel moves more than the spring does.
| Spring length change | Ride height change at the wheel |
|---|---|
| 0.25" | 0.36" |
| 0.50" | 0.71" |
| 0.75" | 1.07" |
| 1.00" | 1.43" |
| 1.50" | 2.14" |
| 2.00" | 2.86" |
Getting a number you can act on
- 01Determine the spring length change being made
The distance the spring's compressed length changes — from moving a threaded perch, swapping spring free lengths, or any other adjustment at the spring itself.
- 02Establish the suspension's motion ratio
The same figure used for spring rate and wheel travel calculations — wheel travel divided by spring travel, usually below 1.0 on most suspension designs.
- 03Read the actual ride height change at the wheel
Below a motion ratio of 1.0, this is larger than the spring adjustment itself — the leverage that amplifies wheel travel amplifies ride height change by the same factor.
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.
ride height change = spring length change ÷ motion ratioBelow a motion ratio of 1.0, dividing by a fraction amplifies the result — a small spring adjustment produces a larger ride height change.
Why a small perch adjustment can move ride height more than expected
This surprises people setting up coilovers for the first time: a quarter-inch turn on a threaded perch often moves ride height by more than a quarter inch at the fender.
The mechanism is the same leverage relationship behind wheel rate and wheel travel — motion ratio describes how suspension geometry translates movement between the spring and the wheel, and on a design where the spring sits inboard of the wheel's actual pivot point, that geometry amplifies movement rather than transmitting it one-to-one.
This is worth knowing before making large adjustments in one step. A suspension with a 0.7 motion ratio amplifies every spring adjustment by about 1.43 times at the wheel — a half-inch perch adjustment intended to lower the car produces about 0.71 inches of actual ride height change, which can be considerably more than intended if the motion ratio wasn't accounted for.
Ride Height Calculator FAQ
How much does ride height change when I adjust a coilover perch?+
Divide the spring length change by the suspension's motion ratio. With a typical 0.7 motion ratio, a half-inch spring adjustment produces about 0.71 inches of ride height change at the wheel.
Why is ride height change different from the spring adjustment I made?+
Suspension motion ratio scales movement between the spring and the wheel. Below a motion ratio of 1.0, the wheel moves more than the spring, so ride height changes by more than the raw spring adjustment.
What motion ratio do most cars have?+
It varies by design, but commonly somewhere between 0.6 and 0.9 on strut and double-wishbone suspensions — meaning the wheel typically moves somewhat more than the spring itself does.
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.