First we need to seperate the two functions of "Steering axis" and "Scrub radius". The Steering axis is the setting that causes the wheel to point forward. As the steering is turned from side to side the 'SA' causes the front end to rise as the wheel reaches full lock. It is the weight of the car, acting on the 'SA', that forces the wheel to return to the straight ahead position.
The Scrub Radius is as you say, the distance that the center of the tyres contact patch, is located from the point, that the Steering axis meets the ground.
Ideally they should be on the same spot. But as nothing is perfict
they are placed at different points to overcome some other problem. Wide tyres are one, along with torque steer, how solid the suspension is mounted, and diagonal brake connectons. All these cause the wheel to move from the straight ahead position.
In the SVX we have all those, the cross connecting of the brakes, require negitive scrub, to keep the car from swerving across the road, when a brake failure causes only one front and one rear brake to work. Without it the car would swerve to that side, so we have negitive scrub to cause that wheel to steer in to the center of the car. This allows the car to hold a straight course, under these conditions.
Front wheel drive, calls for a positive scrub. When cornering the weight is removed from the inside wheel to the outside wheel. If the inside wheel loses traction, the outside wheel will try to steer out, If it has positive scrub the wheel will pull around the axis, to steer in the same direction, as the wheel is turned.
Using a scrub offset, is fine while both tyres have the same grip. If the traction is unever, like braking or accelerating on uneven road surface, the car will go in the direction of the wheel with the most grip. This can cause the car to dive around under brakes, or to torque steer under acceleration.
So there is allways a compromise used. With the SVX it is to use a negitive scrub to nutrailse the effect of the cross braking, and rely on the AWD to cancel the need to use positive, to hold the front wheel drive force.
Harvey.
