In an automobile's suspension system, the scrub radius is the distance in front view between the king pin axis and the center of the contact patch of the wheel, where both would theoretically touch the road. It can be positive, negative or zero.
The kingpin axis also known as steering inclination is the line between the upper and lower pivot points of the steering knuckle.
If the kingpin axis intersection point is outboard of the center of the contact patch, it is negative; if inside the contact patch, it is positive. The term scrub radius derives from the fact that either in the positive or negative mode, the tire does not turn on its centerline (it scrubs the road in a turn) and due to the increased friction, more effort is needed to turn the wheel.
Large positive values of scrub radius, 4 inches/100 mm or so, were used in cars for many years. The advantage of this is that the tire rolls as the wheel is steered, which reduces the effort when parking, provided you're not on the brake.
The advantage of a small scrub radius is that the steering becomes less sensitive to braking inputs. More scrub radius adds to road feel by pushing the inside wheel into the ground.
An advantage of a negative scrub radius is that the geometry naturally compensates for split μ (mu) braking, or failure in one of the brake circuits. It also provides center point steering in the event of a tire deflation, which provides greater stability and steering control in this emergency.