Originally Posted by HRSprings

To understand scrub, you must first know about Steering Axis Inclination (SAI). The steering axis is the line between the top pivot point of your hub and the lower ball joint of your hub. On a MacPherson strut, the top pivot point is the strut bearing, and the bottom point is the lower ball joint. On a suspension using upper and lower control arms, the pivot points are where the upright connects to the control arms. The inclination of the steering axis is measured as the angle between the steering axis and the centerline of the wheel, so if your camber is adjustable within the pivot points (i.e. Volkswagen) you can change the SAI.

Now back to the subject at hand. The scrub radius is the distance on the ground between the centerline of the tire contact patch and the point at which the SAI intersects the ground. If these two lines intersect at ground level, then you are said to have zero scrub. If the SAI intersects the ground at a point inside or outside of the centerline of the contact patch, you are said to have positive or negative scrub respectively.

You now know what elements make up scrub, now you may ask what it has to do with the price of tea in China? Well, the point at which the steering axis line contacts the ground is the fulcrum pivot point on which the tire turns. The location of this point within the contact patch has a great effect on steering effort, feel, and stability. If you have not already guessed, the easiest way to change scrub is by changing your offset with either new wheels, or hub centric wheel spacers.

If the scrub is zero, the scrubbing action of the contact patch is equal on either side of the pivot point causing the tire to act like a car with a welded differential, inducing a condition called 'squirm'. In a straight line the tire tends to be stable and tracks well. As you turn though, the portion of the contact patch on the outside of the pivot point moves faster than the portion on the inside of the contact patch. Since the scrubbing area is equal on each side of the pivot point, yet the forces are different, the tire tends to fight itself and it becomes 'grabby' causing tire wear to increase and the steering to become unstable.

Positive and negative scrub radii have benefits in different types of suspension. A MacPherson strut assembly typically performs well with a lot of SAI and caster, a system negative scrub works well in. Because both SAI and caster increase the amount of camber on the outside wheel when steering, the fulcrum pivot point is at a point that has more leverage, requiring less steering effort. Negative scrub also helps reduce torque steer in front wheel drive cars. Positive scrub radius works well with suspensions that use dual control arms that use less caster and SAI to optimize geometry.

As with anything else, a little of a good thing is great, but lot of a good thing is not necessarily better. When you have excessive scrub, whether it be positive or negative, steering effort increases and road 'feel' increases, as the steering is more susceptible to road shock. Additionally, if you plan on doing some homework on, and modifying your scrub radius, you must take into account the amount of sidewall flex your tire will encounter under hard cornering. When the sidewall flexes, the contact patch moves in relation to the SAI and can make a slightly negative scrub radius become zero.

Originally Posted by l8apex

oh, so it feels like when i have slicks on the front od my teggy, and i squrms allover the road.
couldent i defeate this by going negitive ofset and keeping the tire centered?