Scrub radius

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The scrub radius is distance between the Steering Axis Inclination (SAI)and the center of the wheel where both would theoretically touch the road. 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. This means that if your camber is adjustable within the pivot points (i.e. Volkswagen) you can change the SAI, thus alter your scrub radius, that alter the width and offset of tyres you can safely run on your car.

If the steer axis intersection point is outboard of the centre of the contact patch it is Negative, if inside the contact patch it is positive. The purpose of the scrub radius is to help center the wheels after cornering, and to retain wheel stability at speed. EG: In a Porsche 928 there is a +57mm positive scrub radius which means the correct wheel for this car is ideally an ET65, or a 65mm positive offset.

The net effect of running an offset LESS in the positive is poor steering control and it develops what is known as "tram lining" over uneven surfaces (The wheels pull to either side. This also encourages instability at high speed. Such effects can be lessened by setting front wheel alignment to neutral, but in many cases it is simply unwise to use a wheel that alters the scrub radius of a car. This is why some insurance companies will not pay out on an accident involving a car that has after market wheels.

Large positive values of scrub radius, 4 inches or 100 mm or so, were used in cars for many years. The advantage of this is that the tire rolls as the wheel is steered, which reduces the effort when parking. This also allows greater width in the engine bay, which is very important in some compact sports cars. Scrub Radius is very important in regards Wheel Alignment and steering stability.

If the scrub radius is small then the contact patch is spun in place when parking, which takes a lot more effort. The advantage of a small scrub radius is that the steering becomes less sensitive to braking inputs, in particular.

An advantage of a negative scrub radius is that the geometry naturally compensates for split mu braking, or failure in one of the brake circuits.