Scrub Radius
The distance on the ground between the steering axis projection and the center of the tire contact patch.
Scrub radius is an important suspension alignment geometry parameter. It is the distance measured at the ground level between two lines: the centerline of the tire tread width (contact patch) and the projected line of the steering axis (the angle of the steering knuckle pivots, or Kingpin Axis). Scrub radius can be positive, zero, or negative.
A positive scrub radius occurs when the steering axis intersects the ground inside the tire centerline, while a negative scrub radius occurs when it intersects outside the tire centerline. Scrub radius creates leverage when braking or cornering. Most modern front-wheel-drive cars use a negative scrub radius to provide stability in the event of a tire blowout or brake failure, while rear-wheel-drive vehicles often use positive or near-zero scrub for better steering feedback.
Installing aftermarket wheels with incorrect offset or using thick wheel spacers will change the scrub radius, typically pushing it in a positive direction. This can cause the vehicle to pull hard to one side when braking, increase steering effort, and cause instability over bumps (known as bump steer). Technicians inspect suspension geometry and alignment to ensure the scrub radius remains within safe design limits.