The axis around which the wheel assembly swivels as it turns to the right or left is called the steering axis. It is formed by drawing a line through the upper and lower pivot points of the suspension assembly. Seen from the front of the car, it is tilted inward.
The angle formed between this line and the vertical, provides steering axis inclination angles.
Steering axis inclination acts, with caster, to provide a self-centering of the front wheels.
When the wheels are in the straight-ahead position, the ends of the stub axles are almost horizontal.
When the wheels turn to either side, the effect of steering axis inclination is to make the ends of the stub axle tend to move downward, but this is prevented by the wheel. The stub axle carrier then must move up, which raises the front of the vehicle.
When the steering wheel is released, the mass of the vehicle forces the stub carrier back down, which pushes the wheels back to a central position.
With a vertical steering axis, no self-centering would occur. The wheel would pivot on a radius with the steering axis as its center. This would introduce a turning moment on the wheel, road shocks would be transmitted back to the steering wheel, and steering would be difficult to control.
Steering axis inclination brings the pivot point close to the center of the tire contact patch at the road surface. It intersects with the camber line drawn through the tire and the wheel.
The angle between the steering axis inclination and the camber line is called the included angle. It is a diagnostic angle.
Since the steering axis inclination is not adjustable, if the camber angle is correct, then the steering axis inclination should also be correct, that is it should match the specification.
