Scrub Radius
Scrub radius is the lateral distance on the road plane between the steering axis intersection and a defined center of the tire contact area.
Positive, zero, or negative
The sign depends on the adopted coordinate convention and on which side of the tire center the steering axis meets the road. Wheel offset, tire dimensions, ride height, camber, steering-axis inclination, and suspension geometry all affect the value.
Scrub radius creates a lever arm through which tire forces and brake or drive-force differences can generate steering torque. It can influence feedback, brake stability, torque steer, parking effort, and how road disturbances reach the steering wheel.
The static geometric value is not a complete description because the contact patch, tire forces, wheel travel, steering angle, compliance, and split-friction braking change the operating response. Changing wheel offset or tire size can alter scrub radius even when alignment angles are unchanged.
For steering-axis geometry and compliant wheel motion, see Suspension Kinematics and Vehicle Dynamics.