Scrub Radius Explained

What is Scrub Radius?

Scrub radius is the distance on the road surface between the point where the steering axis would theoretically intersect the road and the center of the tire's contact patch. The steering axis is an imaginary line drawn through the upper and lower pivot points of the suspension (e.g., top of the strut and the lower ball joint). It's a critical suspension geometry parameter that influences steering effort, feedback, and stability.

Types of Scrub Radius

• Positive Scrub Radius: The steering axis intersects the road surface inboard (towards the center of the vehicle) of the tire contact patch centerline. This can increase steering effort and make the car more sensitive to road imperfections or differing grip levels (e.g., one tire on a wet patch).
• Negative Scrub Radius: The steering axis intersects the road surface outboard (away from the center of the vehicle) of the tire contact patch centerline. This is common in many modern FWD cars and can help maintain stability during braking if one side has less grip by creating a self-correcting force.
• Zero Scrub Radius: The steering axis intersects the road surface directly at the center of the tire contact patch. This can provide light steering but may reduce feedback from the road.

How is Scrub Radius Affected?

Scrub radius is primarily determined by the vehicle's suspension design (steering axis inclination and knuckle design). However, it can be significantly altered by:

• Wheel Offset: Changing wheel offset is the most common way drivers inadvertently or intentionally alter scrub radius. A lower offset (wheels pushed further out) generally moves the scrub radius in a positive direction (more positive or less negative). A higher offset (wheels pulled further in) does the opposite.
• Wheel Spacers: These have the same effect as lowering the offset, pushing the wheels out and altering the scrub radius.
• Tire Diameter: While less direct, very significant changes in tire diameter can slightly influence the geometry.
• Suspension Modifications: Changes to suspension components or ride height can also impact the steering axis inclination and thus the scrub radius.

Why Does It Matter?

• Steering Effort & Feedback: Directly influences how heavy or light the steering feels and how much information from the road is transmitted to the driver.
• Stability: Particularly during braking or when one tire loses traction. Negative scrub radius can help counteract pulling to one side.
• Tire Wear: Extreme changes in scrub radius can potentially contribute to unusual tire wear patterns if combined with other alignment issues.
• Handling Characteristics: Can affect how the car behaves during cornering and its overall responsiveness.