Scrub radius is also known as steering offset, and scrub geometry. It is the distance between 2 imaginary points on the road surface - the point of center contact between the road surface and the tire, and the point where the steering-axis center-line contacts the road surface.
The effect of scrub radius - positive or negative - is to provide a turning moment which attempts to turn the wheel away from the central position, when the vehicle is in motion.
On a rear-wheel-drive vehicle with positive scrub radius, the vehicle’s forward motion and the friction between the tire and the road causes a force which tends to move the front wheels back. This would cause the wheels to toe-out.
If it has negative scrub radius, the front wheels again tend to move back, but this time, they toe-in.
On front-wheel-drive vehicles, the opposite occurs. Positive scrub radius causes toe-in, and negative causes toe-out.
During braking, on any type of drive, if braking effort is greater on one side of the vehicle than the other, positive scrub radius will cause the vehicle to veer towards the side with the greater effort.
Negative scrub radius will cause the vehicle to veer away from the side of greatest effort. How much it veers depends on the size of the scrub radius.
This is why, vehicles with a diagonal-split brake system have negative scrub radius built into the steering geometry. If one half of the brake system fails, then the vehicle will tend to pull up in a straight line.
Since the offset of the wheel rim determines where the centerline of the tire meets the road surface, it is important that the offset is not changed if wheels are being replaced.
Changing the rim offset changes the scrub radius, and also the predictability of the vehicle handling, if brakes should fail.
