Scrub radius is the distance between the center of the tire where it hits the ground and the rotational axis of the tire, which is where a line drawn through the kingpin hits the ground (see diagram below.)
More kingpin inclination (KPI) results in a lower or more negative scrub radius, and less KPI gives higher or more positive scrub radius.
Positive scrub radius (kingpin axis inside tire center):
- Increased on-power steering
- Less off-power stability
Negative scrub radius (kingpin axis outside tire center)
- Less on-power steering
- More off-power stability