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What Are Motion Ratios?

Posted by Dežru Suspension Engineering on Nov 23rd 2025

Introduction

Motion ratios are one of the most important — and most misunderstood — concepts in suspension tuning.

Here’s the reality:

> Your wheel does NOT move the same distance as your spring.
> Your true stiffness at the tire is NOT your spring rate.
> Two cars running the same spring rate can feel completely different.

This guide explains motion ratios in simple, visual language so you can understand how your Dežru spring rates behave in the real world.

What Is a Motion Ratio?

A motion ratio describes the relationship between wheel travel and spring travel.

The formula:

Motion Ratio = (Spring Movement) / (Wheel Movement)

Examples:

  • If the wheel moves 1 inch and the spring moves 1 inch → 1.0 ratio (direct acting)

  • If the wheel moves 1 inch and the spring moves 0.8 inches → 0.8 ratio

  • If the wheel moves 1 inch and the spring moves 0.6 inches → 0.6 ratio

Most modern cars do NOT have a 1:1 motion ratio.

Why?
Because the shock/spring is mounted inboard on a control arm, not directly above the wheel.

This geometry reduces the spring’s “leverage.”

Wheel Rate ≠ Spring Rate

Wheel rate is the stiffness the tire actually “feels.”

The formula:

Wheel Rate = Spring Rate × (Motion Ratio)²

This is why a 450 lb/in spring does NOT produce 450 lb/in at the tire.

Example:

Spring rate: 450 lb/in
Motion ratio: 0.8

450 × (0.8²) = 450 × 0.64 = 288 lb/in wheel rate

So your tire feels 288 lb/in.

This is why copying spring rates from different chassis never works.

Why Motion Ratios Matter So Much

Motion ratios affect:

✔ Ride quality

Lower motion ratio → softer feeling at the wheel
Higher motion ratio → stiffer feeling

✔ Handling

Wheel rate determines:

  • roll stiffness

  • pitch

  • weight transfer

  • mid-corner stability

✔ Spring selection

A 450 lb/in spring on:

  • a Miata

  • a GR Corolla

  • a Subaru WRX

  • a BMW M3

…will NOT behave the same due to different arm geometry.

✔ Damping behavior

Your valving MUST match the wheel rate, not the spring rate.

Dežru valving is developed specifically around each chassis’s motion ratio, ensuring predictable behavior on each platform.

Motion Ratio Examples (Real-World)

(Approximate industry-normal values for educational use)

Subaru WRX / STI (Strut Front)

Motion ratio ≈ 0.9 – 1.0
Very direct → wheel rate is close to spring rate.

Subaru WRX / STI (Rear Multi-Link)

Motion ratio ≈ 0.65 – 0.75
Wheel rate is much lower than spring rate.

Toyota GR Corolla

Ratio ≈ 0.75 front / 0.70 rear
High-travel platform → softer wheel rate vs. spring rate.

Mazda Miata / MX-5 (ND)

Ratio ≈ 0.65 – 0.70
Why Miatas need higher spring rates than expected.

BMW Multi-Link / MacPherson Mix

Front ≈ 0.95
Rear ≈ 0.65 – 0.75

Understanding these ratios is why Dežru does NOT use a one-size-fits-all spring rate philosophy.

How Motion Ratios Work with Dežru’s Fixed-Length Coilovers

Your suspensions use:

✔ fixed shock length
✔ helper springs
✔ linear main springs
✔ perch-only ride height adjustment

This creates a very consistent motion ratio behavior because:

  • The damper stroke is always used correctly

  • Spring position relative to the arm doesn’t change

  • Valving always sits in its intended window

  • No weird leverage changes caused by adjustable hub brackets

Cheap coilovers with adjustable lower mounts change shock length, which changes:

  • bump travel

  • rebound travel

  • valving behavior

  • leverage

  • geometry

  • motion ratio effectiveness

Dežru avoids all of that.

Motion ratio remains consistent — exactly as engineered.

How Motion Ratios Affect Spring Rate Choices

If your motion ratio is low (0.6–0.75)

You need higher spring rates to achieve the same wheel rate.

If your motion ratio is high (0.85–1.0)

You can run softer spring rates while keeping good platform control.

This is why:

  • WRX front rates feel firm even at 350–450 lb/in

  • WRX rear needs more rate to balance

  • Miata needs higher rates than its weight suggests

  • GR Corolla behaves softer than its raw spring rate implies

Wheel rate — not spring rate — determines handling.

How to Calculate Wheel Rate Yourself

If you know your chassis’s motion ratio:

  1. Take your spring rate

  2. Multiply by M.R. squared

Example (450 lb/in spring, 0.7 rear ratio):

Wheel Rate = 450 × 0.49 = 220 lb/in

This explains why cars often feel softer in the rear:
the motion ratio reduces effective stiffness.

Summary: Motion Ratios Simplified

✔ Motion Ratio = spring movement ÷ wheel movement

✔ Wheel Rate = spring rate × (motion ratio)²

✔ Wheel rate determines how stiff the car actually feels

✔ Motion ratios vary widely between platforms

✔ Dežru spring rates are selected based on real wheel rate targets

✔ Fixed-length architecture keeps motion ratio behavior consistent

✔ Adjustable lower mounts can disrupt valving and travel

Understanding motion ratios is key to understanding why your Dežru coilover feels the way it does — balanced, predictable, and engineered for the chassis it’s built for.

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