32 00... Description of front axle and wheel alignment

General overview:

Fig 1: Identifying General Chassis And Suspension Definitions

Courtesy of BMW OF NORTH AMERICA, INC.

Toe angle difference:

a = Toe difference angle

D = Center point of operating circle

The toe angle difference is the angle adjustment of the inner cornering wheel relative to the outer cornering wheel when negotiating a curve. Steering is designed in such a way that angular position of wheels changes as steering lock progresses.

A correctly adjusted toe angle difference produces equal values for left and right lock with consideration of factory tolerances.

Toe angle difference provides information on corresponding operation of steering trapezoid for left or right steering lock from center position.

Fig 2: Identifying Toe-Differential Angle

Courtesy of BMW OF NORTH AMERICA, INC.

Camber:

This is the inclination of the wheel from the perpendicular.

Fig 3: Identifying Camber

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Toe:

This is the reduction in distance of front of front wheels to rear of front wheels. The toe-in prevents the wheels from moving apart during driving and thus:

the wheels from vibrating and grinding
excessive tire wear
excessive strain on the steering linkage and its links/joints
heavy vehicle steering

Measurement is performed in "straight-ahead mode".

Fig 4: Identifying Toe-In

Courtesy of BMW OF NORTH AMERICA, INC.

Caster:

This is the inclination of the kingpin in the direction of travel viewed from the side. The line through the center point of the spring strut support bearing and the control arm ball joint corresponds to the "kingpin".

Thanks to caster, wheels are pulled and not pushed. In a similar manner to king pin inclination, when driving in curves or around corners, returning forces are reproduced to help return wheels to straight-ahead position.

Fig 5: Identifying Caster

Courtesy of BMW OF NORTH AMERICA, INC.

Geometrical driving axis (1):

This is the angle bisector from the total rear-wheel toe.

Front-wheel measurements are taken in reference to this axis.

Symmetrical axis (2):

This is the center line running through front and rear axles.

Fig 6: Identifying Geometrical Driving Axis And Symmetrical Axis

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Wheel misalignment:

This is the angle by which one front wheel is displaced more towards front or rear than the other front wheel. The wheel offset angle is positive when the right wheel is displaced towards the front and negative when it is displaced towards the rear.

Fig 7: Identifying Wheel Displacement Angle

Courtesy of BMW OF NORTH AMERICA, INC.

Kingpin offset/scrub radius:

This is the distance from the center of the wheel contact face to the intersection point of the kingpin extension. The line through the center point of the spring strut support bearing and the control arm ball joint corresponds to the "kingpin".

The scrub radius is influenced by camber, kingpin angle and wheel offset of the wheel rim.

Fig 8: Identifying Kingpin Offset

Courtesy of BMW OF NORTH AMERICA, INC.