The scrub radius is a distance between Steering Axis Inclination and center of the wheel where both of them would theoretically be touching the road. Steering axis is a line between the lower ball joint and the top most pivot of the hub. On the McPherson strut, the top most pivot point is the strut bearing whereas the bottom point consists of the lower ball joint. Making use of suspension using the lower and the upper control arms, pivot points are placed where the upright connects the control arms. The Inclination to the steering axis could be measured as the angle between the centerline and the steering axis. This means, if camber is found adjustable at the pivot points then SAI can be changed. Therefore, altering the scrub radius by altering the width of the offsets of the wheels could allow you to run the car safely.
If the centerline is inboard then the scrub radius is positive ad if it is outboard of tire centerline then the scrub radius is a negative one. The rear wheel drive trucks and cars mostly have a positive scrub radius and the FWD cars have negative or a zero scrub radius as they are having a higher level SAI angle. In order to alter the scrub radius, usage of the wheels having different offset than the stock could prove helpful. The main purpose of the scrub radius is to provide help center wheels after the cornering also for retaining the stability of the wheel.
Net effect of the running of an offset LESS in positive is a poor steering control that develops tram lining seen over the surfaces. The wheels would pull off on either of the sides which would encourage instability at high speed. Such sorts of effects could be lessened simply by setting the front wheel alignment to the neutral. However, in many of the cases it would simply be unwise for the usage of wheels that alters scrub radius of cars. This is the main reason why many of the insurance companies don’t pay out on accidents that involve cars with after market wheels.
A larger scrub radius, four inches or maybe about 100mm were used earlier in most of cars. Its advantage is that the tire rolls just as wheel is steered that reduces on the efforts while parking the car. This also helps in increasing the width of the engine bay which is quite an important point in most of the sports cars. The scrub radius is of very much importance in regards with the steering stability and the wheel alignment. In case of a small scrub radius, contact patch would be spun in the place while parking the vehicle and it takes a lot of effort. An advantage of the scrub radius is that the small scrub radius allows the steering to become bit less of sensitive for braking inputs, particularly.