Scrub Radius
Scrub Radius is the distance between the extended centerline of the steering axis and the centerline of the tire where the tread contacts the road. This distance must be exactly the same from side to side or the vehicle will pull strongly.
If the steering centerline is inboard of the tire centerline, the scrub radius is positive. If the steering centerline is outboard of the tire centerline, the scrub radius is negative.
Rear-wheel drive cars and trucks generally have a positive scrub radius while FWD cars usually have zero or a negative scrub radius because they have a higher Steering Axis Inclination (SAI), angle.
Using different wheels other than stock can alter the scrub radius.
Here is a link for more information, as Scrub Radius alone is NOT the exacting measure for proper setup of your suspension as other measurements of your steering axis and positioning are needed if one were to critique what should and shouldn't be done.....
http://www.anewtoronto.com/wheel%20alignment.html
NOTE: Pay Attention to the two sub-categories for Steering Axis Inclination and Included Angle as these play a signifigant role in what your scrub radius will effectively be, and what is considered a positive or negative alteration of your suspension setup, for the rest of us "poser's" 