General information
The front axle requires periodic servicing to maintain accurate wheel alignment. Correct front axle wheel alignment is needed to ensure long tire life, ease of handling, and steering stability. When aligning the front axle, it is essential that the rear axle(s) be checked for correct alignment at the same time. Alignment of the rear axle(s) has a direct impact on how the vehicle tracks. All axles, front and rear, should be aligned whenever alignment is done.
Three factors are involved in wheel alignment: camber angle, caster angle, and wheel toe-in.
Camber angle (Fig.) is the vertical tilt of the wheel as viewed from the front of the vehicle. Camber angle is measured in degrees, and is not adjustable. Positive camber is the outward tilt of the wheel at the top. Excessive positive camber in one wheel causes the vehicle to pull in the opposite direction, rapidly wearing the outboard side of the tire tread. Negative camber is the inward tilt of the wheel at the top. Excessive negative camber in one wheel causes the vehicle to pull in the same direction that the negative-camber wheel is on, wearing the inboard side of the tire tread. If camber angles are not correct, the tires will wear smooth around the edge on one side. See Fig.. If the front axle is damaged enough to affect the camber angle it must be replaced.
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Camber (Positive)
Fig. 1, Camber Angle (front view)
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One side of the tread is worn excessively.
Fig. 2, Tire Damage Due to Excessive Camber
Caster angle (Fig.) is the tilt of the knuckle pin (or kingpin) as viewed from the side. Caster angle is measured in degrees and is adjustable. A positive caster angle is the tilt of the top of the knuckle pin toward the rear of the vehicle. A negative caster angle is the tilt of the top of the knuckle pin toward the front of the vehicle. Caster angles are based on the design load of the vehicle. An incorrect caster angle does not cause tire wear. However, a positive caster angle that exceeds specifications could cause vehicle shimmy, road shock, and an increased steering effort. A negative caster angle that does not meet specifications could cause unstable steering. The vehicle may wander and weave, and extra steering effort may be necessary. After leaving a turn, the tendency to return to and maintain a straight-ahead position is reduced. Too much or too little caster in one wheel can cause erratic steering when the service brakes are applied to stop the vehicle.
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Knuckle Pin (King Pin)
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Camber (Positive)
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Positive Caster
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Zero Caster
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Negative Caster
Fig. 3, Camber Angle (front view)
Wheel toe-in (Fig.) is the distance in inches that the front of the wheels are closer together than the rear of the wheels, as viewed from the top. Wheel toe-in is adjustable. If it is not adjusted correctly, the vehicle could pull to one side while driving. Wheel shimmy and cupped tire treads (indentations on the road contact surface of the treads) could occur. Also, rapid or severe tire wear on the steering axle could occur, usually in a feather-edged pattern. See Fig..
Note: B minus A equals toe-in.
Fig. 4, Wheel Toe-In (overhead view)
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Feathered Edges