Wheel Centre line (1) is parallel to the long axis of the wheel viewed side on (ie. looking at the tyre tread). It divides the wheel into halves. By definition the wheel centre line is perpendicular to the rotational axis of the wheel. (see picture)
The wheel contact patch (see picture- (3)) is the intersection between the wheel centre line and the road. (Or that point where the wheels centre line meets the road. This point could be virtual depending on extreme wheel width )
#3 is a virtual point depending on the wheels width
|(1) Wheel Centre line
(2) Wheel Rotation axle
(3) Wheel Contact patch
(4) Geometric Driving axis
The driving axis (see picture(4)) is determined by the angle of toe of both
rear wheels.The driving axis is the line drawn between the centre of the rear
wheels center line (or track?) to the point
of intersection of the toe angles extrapolated to that point.
The total angle (which can be toe-in or toe-out) is the sum of right hand and left hand side toe angles. The angles do not intersect in the centre of the vehicle, nor, necessarily, to the long axis of the vehicle.
The Rear axle is in every case the track-giving axle of the vehicle !
|Vehicle longitudinal centre level|
The vehicle longitudinal axis (or also called symmetry axis) represents
the line between the centres of the front and rear axle tracks.
A case of misalignment. The left hand side toe is 0° and the right hand side toe is negative. The total toe angle is toe in, but the longitudinal axis (blue) is not congruent with the drive direction (geometrical driving axis) (red).
The "straight ahead" is the auxiliary position of the steering element and must be used when the toe and Camber values for the rear Wheels are measured. For this "help position" the front wheel individual toe angles are aligned to the same value, with respect to the vehicle longitudinal axis. (Hard stuff, isn´t it? [Yes!- Rob]) The adjustment is made with the assistance of modern computer aided axle measurement by operator control; the exact position of the steering wheel is not evaluated here yet. In the auxiliary position "straight ahead", the track and camber values of the rear axle can be measured.