"This image illustrates what is called scrub radius.  Scrub radius is very important on the front wheels  because it affects handling and steering. Scrub  radius is the distance between the center of the tire  contact patch touching the ground while mounted  on the front hub at ride height and the imaginary  line passing thought the upper and lower ball joint  balls that intersect with the ground under the tire.  It is measured in positive or negative inches or  millimeters. Negative scrub radius is where the  contact patch center is outboard the virtual pivot  axis intersection with the ground. ""0"" is when they  coincide, and positive is when the contact patch  center is inboard the virtual pivot axis intersection. A  slight positive scrub radius is preferable for steering  feedback and reduction of tram lining and ridge  nibbling. It is required on front drive vehicles and  vehicles with ABS to prevent excessive steering pull.  A zero scrub vehicle has somewhat ""numb"" steering,  but braking and steering loads are equalized.  Negative scrub is common with wide front tires as it  is diffi cult to build a wheel and suspension that will  deliver this with a wide tire. Try to minimize scrub  as much as possible to prevent a vehicle that follows  every seam in the road and gives negative steering  feedback, in addition to braking instability and  excessive steering component wear."
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