# Truck wheel alignment

Parameters relative to wheel alignment in trucks, buses, trailers and semi-trailers:

• CHASSIS FRAME AXIS OF SYMMETRY is a line running along the centres of the chassis frame cross-beams.
• GEOMETRICAL DRIVE AXIS is a line which divides total rear wheel toe angle into two even angles and which the vehicle follows during straight ahead drive.
• CAMBER ANGLE is the angle between the wheel’s side-plane and the perpendicular. When viewed from the front or rear, the wheel leans outwards from the perpendicular, it is said to have positive camber (fig. 1). This measurement needs to be made with the front wheels set to the straight-ahead position.
• KING-PIN INCLINATION is the angle between the perpendicular and the projection of the king-pin axis onto a vertical surface perpendicular to the vehicle’s symmetry axis (when viewed from the front or rear) (fig. 3). The angle is positive when the upper part of the king-pin projection is inclined towards the vehicle’s symmetry axis (as shown in the figure).
• CASTER ANGLE is the angle between the perpendicular and the projection of the king-pin axis onto a vertical surface parallel to the vehicle’s symmetry axis (the side view) (fig. 4). The angle is positive when the kin-pin’s projection is leaning towards the front of the car (as shown in the figure).    • TOTAL TOE (FRONT OR REAR WHEELS) is the angle between the side planes of both wheels of the same axle, measured horizontally (fig. 5). This parameter may also be given a linear measure. Then, it is the difference between the distances separating the wheel rims of the same axle (front or rear), which is measured at the front and back of rim flanges horizontally. The total toe value is positive if the intersection of the side planes is in front of the tested wheels (towards the driving direction), and negative when the intersection occurs behind the axle.
• INDIVIDUAL TOE (FRONT OR REAR WHEELS) is an angle between the side plane of the wheel and the symmetry axis (fig. 6) when the wheels are set to straight-ahead drive. Individual front wheel toe should be calculated from “Toe partial” results read from the measuring units.
• THRUST ANGLE is the angle between the geometrical drive axis and the vehicle symmetry axis (fig.2). This angle is positive when the geometrical drive axis is leaning right from the vehicle symmetry axis.
• CHASSIS/AXLE UNPERPENDICULARITY is the angle between the wheels axis and a line perpendicular to the chassis symmetry axis  (fig.7).    • AXLE OFFSET is the angle between the bisector of track difference angle and geometrical drive axis. If the standard value of vehicle tread axis is known, this parameter can be described in linear measure.
• WHEELBASE DIFFERENCE is the angle between the first axle and each another axle (fig. 9).
• TOE OUT ON TURNS is the toe difference angle between the in and the out wheels when turning the out wheel by 20° (fig. 10).
• LOCK ANGLES is the maximum possible in and out turn with the right or left wheel (fig. 11 and 12).
• INCLUDED ANGLE is the sum of the front camber angle and the corresponding king-pin inclination angle.    