The scrub radius is a distance between the steer axle camber and the center of the wheel where, in theory, both would touch the road. The steering shaft is a line between the lower ball joint and the upper pivot of the hub. In the McPherson strut, the uppermost pivot point is the strut bearing, while the lower point is the lower ball joint. Making use of the suspension using the upper and lower control arms, pivot points are placed where the strut connects the control arms. Steering axis tilt can be measured as the angle between the center line and the steering axis. This means that if the curvature is found to be adjustable at the pivot points, then the UPS can be changed. So altering the scrub radius by altering the width of the wheel offsets could allow you to drive the car safely.

If the center line is inward, then the scrub radius is positive and if it is off the center line of the tire, then the scrub radius is negative. Rear wheel drive trucks and cars mostly have a positive scrub radius and FWD cars have a negative or zero scrub radius as they have a higher level UPS angle. To modify the scrub radius, using wheels that have a different offset than the original may be helpful. The primary purpose of the scrub radius is to provide help center wheels after cornering, also to retain wheel stability.

The net effect of executing a positive LESS shift is poor control of the direction developed by the tram siding when viewed on the surfaces. The wheels would start on either side, which would promote instability at high speed. These types of effects could be reduced simply by setting the front wheel alignment to neutral. However, in many cases it would simply be unwise to use wheels that alter the car’s scrub radius. This is the main reason many of the insurance companies don’t pay for accidents involving aftermarket wheeled cars.

A larger scrub radius, four inches or maybe around 100mm, was used before on most cars. Its advantage is that the tire rolls as the wheel is steered, reducing stress when parking the car. This also helps to increase the width of the engine compartment, which is quite an important point in most sports cars. The scrub radius is of great importance when it comes to steering stability and wheel alignment. In case of a small scrub radius, the contact patch would rotate in place while parking the vehicle and requires a lot of effort. An advantage of the scrub radius is that the small scrub radius allows the steering to become slightly less responsive to braking maneuvers, in particular.