What Is Scrub Radius?

Scrub radius is the distance between:

• The point where the steering axis (kingpin line) hits the ground
• The center of the tire contact patch

It is measured in millimeters (mm) or inches.

In simple terms, it tells you how far the tire “scrubs” across the ground when you steer.

Small changes in scrub radius can dramatically change:

• Steering weight
• Road feedback
• Torque steer
• Stability under braking
• Wheel bearing load

That’s why a scrub radius calculator is important when changing wheels or suspension parts.

Why Scrub Radius Matters

Scrub radius directly affects how a car feels and behaves.

1. Steering Feel

Positive scrub radius increases steering effort but gives more feedback.
Negative scrub radius can reduce torque steer but may feel less direct.

2. Stability Under Braking

Large scrub values can cause the car to pull during hard braking, especially if one side loses traction.

3. Wheel Bearing Load

Excessive positive scrub radius increases load on wheel bearings and suspension components.

4. Performance Tuning

Race cars often use near-zero or small positive scrub for sharp response.

Types of Scrub Radius

Positive Scrub Radius

The tire contact patch is outside the steering axis.

• Heavier steering
• Stronger road feedback
• More torque steer in FWD cars
• Better self-centering

Common in performance cars and older rear-wheel drive setups.

Negative Scrub Radius

The tire contact patch is inside the steering axis.

• Reduced torque steer
• Better stability during split braking
• Lighter steering
• More kickback over bumps

Common in many modern front-wheel-drive vehicles.

Zero Scrub Radius

The steering axis meets the ground exactly at the tire center.

• Balanced steering
• Minimal kickback
• Requires precise geometry

Often targeted in race setups.

How the Scrub Radius Calculator Works

The calculator uses these inputs:

• Wheel rim offset (ET)
• Wheel width
• Tire section width
• Kingpin inclination (KPI) / Steering axis inclination
• Ball joint or strut center offset
• Vehicle type
• Optional tire pressure

Let’s explain each one clearly.

1. Wheel Rim Offset (ET)

Offset is the distance between the wheel’s mounting face and its centerline.

• Measured in mm or inches
• Positive offset pushes the wheel inward
• Negative offset pushes it outward

Offset changes scrub radius significantly.

2. Wheel Rim Width

The actual width of the wheel.

Wider wheels change where the tire centerline sits, which shifts scrub radius.

3. Tire Section Width

This is the tire’s advertised width (example: 225 in 225/45R17).

The calculator adjusts the tire center relative to the wheel width.

4. Kingpin Inclination (KPI) / Steering Axis Inclination (SAI)

KPI is the angle between vertical and the steering axis.

Typical values: 7° to 14°

This angle influences:

• Mechanical trail
• Self-centering effect
• Steering stability

5. Ball Joint / Strut Center Offset

This measures how far the steering axis sits from the wheel centerline at hub height.

If provided, the calculator uses it directly to compute scrub radius more accurately.

6. Vehicle Application Selection

The calculator includes ideal scrub radius ranges for:

This helps you quickly assess whether your setup is within a safe range.

7. Tire Pressure (Optional)

Tire pressure affects sidewall deflection.

Lower pressure = more deflection = slightly reduced effective scrub radius under load.

The calculator estimates:

• Tire deflection
• Scrub radius under load

This is useful for track tuning.

How to Use the Scrub Radius Calculator (Step-by-Step)

1. Enter your wheel offset
2. Enter wheel width
3. Enter tire section width
4. Input kingpin inclination
5. Enter ball joint offset (if known)
6. Choose your vehicle application
7. Add tire pressure if desired
8. Click Calculate Scrub Radius

You will see:

• Scrub radius (mm and inches)
• Assessment (within range, excessive positive, etc.)
• Mechanical trail
• Kingpin offset
• Tire center offset
• Scrub under load (if pressure entered)

Understanding the Results

Within Ideal Range

Your setup matches the intended vehicle type. Steering feel and stability should be balanced.

Excessive Positive Scrub

• Heavy steering
• Increased torque steer
• Bearing wear
• Tramlining

Consider increasing offset or narrowing the wheel.

Excessive Negative Scrub

• Higher steering effort
• Increased kickback
• Reduced steering feel

Consider reducing offset.

Mechanical Trail Explained

Mechanical trail is calculated using:

Mechanical Trail = Scrub Radius × tan(KPI angle)

More trail increases self-centering and straight-line stability.

Less trail gives lighter steering but less feedback.

Example Calculation

Let’s say:

• Offset: 35 mm
• Wheel width: 8 inches
• Tire width: 225 mm
• KPI: 12°
• Ball joint offset: 20 mm

The calculator determines:

• Tire center location
• Scrub radius
• Mechanical trail
• Ideal range comparison

This removes guesswork when upgrading wheels.

Typical Scrub Radius Values

Most passenger cars fall within:

±25 mm

Extreme values beyond ±50 mm are usually problematic and may indicate incorrect measurements.

When Should You Use a Scrub Radius Calculator?

Use it when:

• Installing aftermarket wheels
• Changing offset
• Converting suspension
• Building a track car
• Diagnosing torque steer
• Adjusting stance setups

It helps prevent expensive mistakes.

Common Mistakes to Avoid

1. Ignoring tire width changes
2. Mixing inches and millimeters incorrectly
3. Guessing KPI angle
4. Using extreme negative offset wheels without geometry correction
5. Forgetting that lowering the car changes geometry