Scrub Radius

Ideally, this measurement is as small as possible. Normally, the Steering Axis Inclination (SAI) angle and the centerline of the wheel and tire intersect below the road surface, causing a positive scrub radius. With struts, the SAI angle is much larger than with a long arm/short arm-type suspension. This allows the SAI angle to intersect the camber angle above the road surface, forming a negative scrub radius. The smaller the scrub radius, the better the directional stability. Scrub radius is dramatically increased when aftermarket wheels and tires, that have additional offset, are installed. They may cause the centerline of the tires to move further away from spindle. This will increase the scrub radius, which in turn reduces directional stability.

A large amount of scrub radius can cause severe shimmy after hitting a bump. That is why four-wheel drive vehicles with large tires use a steering damper to compensate for an increased scrub radius. Scrub radius is not directly measurable by the conventional methods. It is projected geometrically by engineers during the design phase of the suspension.