943 Front to rear wheel misalignment

An imaginary line projected longitudinally between the centre of the front and rear wheel tracks is known as the vehicle's centre line or the axis of symmetry (Fig. 9.27(a)). If the vehicle's body and suspension alignment is correct, the vehicle will travel in the same direction as the axis of symmetry. When the wheel axles at the front and rear are misaligned, the vehicle will move forward in a skewed line relative to the axis of symmetry. This second directional line is known as the thrust axis or driving axis. The angle between the axis of symmetry and the thrust axis is referred to as the thrust axis deviation which will cause the front and rear wheels to be laterally offset to each other when the vehicle moves in the straight ahead direction.

If the vehicle has been constructed and assembled correctly the thrust axis will coincide with the axis of symmetry, but variation in the rear wheel toe angles relative to the axis of symmetry will cause the vehicle to be steered by the rear wheels. As a result, the vehicle will tend to move in a forward direction and partially in a sideway direction. The vehicle will therefore tend to pull or steer to one side and when driving round a bend the steering will oversteer on one lock and understeer on the opposite lock. In the case of Fig. 9.27(a), with a right hand lateral offset the vehicle will understeer on left hand bends and oversteer on right hand turns.

The self-steer effect of the rear wheels due to track or axle misalignment will conflict with the suspension geometry such as the kingpin inclination and castor which will therefore attempt to direct the vehicle along the axis of symmetry. Consequently, the tyres will be subjected to excessive scrub.

Thrust axis deviation may be produced by body damage displacing the rear suspension mounting points, rear suspension worn bushes, poorly located leaf springs and distorted or incorrectly assembled suspension members.

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