Proper alignment of the front wheels must be maintained in order to ensure ease of steering and satisfactory tire life. The most important factors of front wheel alignment are wheel camber, axle caster, and wheel toe-in.
Wheel toe-in is the distance by which the wheels are closer together at the front than at the rear.
Wheel camber is the amount the top of the wheels incline outward from the vertical.
Front axle caster is the amount in degrees that the steering pivot pins are tilted toward the rear of the vehicle. Positive caster is inclination of the top of the pivot pin toward the rear of the vehicle.
These points should be checked at regular intervals, particularly when the front axle has been subjected to a heavy impact. When checking wheel alignment, it is important that wheel bearings and knuckle bearings be in proper adjustment. Loose bearings will affect instrument readings when checking the camber, pivot pin inclination, and toe-in.
Front wheel camber is preset. Caster can be altered by use of shims between the axle pad and the springs. Wheel toe-in may be adjusted. To measure wheel toe-in, follow the procedure given later on in this section.
See Figure 1
Caster angle is established in the axle design by tilting the top of the kingpin toward the rear, and the bottom of the kingpin forward so that an imaginary line through the center of the kingpin would strike the ground at a point ahead of the point of tire contact.
The purpose of caster is to provide steering stability which will keep the front wheels in the straight ahead position and also assist in straightening the wheels when coming out of a turn. Caster is corrected by installing shims between the axle pad and the springs.
If the camber and toe-in are correct and it is known that the axle is not twisted, a satisfactory check may be made by testing the vehicle on the road. Before road testing, make sure all tires are properly inflated, being particularly careful that both front tires are inflated to exactly the same pressure.
If the vehicle turns easily to either side but is hard to straighten out, insufficient caster for easy handling of the vehicle is indicated. If correction is necessary, it can usually be accomplished by installing shims between the springs and axle pads to secure the desired result.
See Figure 2
The purpose of camber is to more nearly place the weight of the vehicle over the tire contact patch on the road to facilitate ease of steering. The result of excessive camber is irregular wear of the tires on the outside shoulders and is usually caused by bent axle parts.
The result of excessive negative or reverse camber will be hard steering and possibly a wandering condition. Tires will also wear on the inside shoulders. Unequal camber may cause any or a combination of the following conditions: unstable steering, wandering, kickback or road shock, shimmy or excessive tire wear. The cause of unequal camber is usually a bent steering knuckle or axle end.
Correct wheel camber is set in the axle at the time of manufacture and cannot be altered by any adjustment. It is important that the camber be the same on both front wheels. Heating of any parts to facilitate straightening usually destroys the heat treatment given them at the factory. Cold bending may cause a fracture of the steel and is also unsafe. Replacement with new parts is recommended, rather than any straightening of damaged parts.
See Figure 3
First raise the front of the vehicle to free the front wheels. Turn the wheels to the straight ahead position. Use a steady rest to scribe a pencil line in the center of each tire tread as the wheel is turned by hand. A good way to do this is to first run a chalk stripe around the circumference of the tread at the center to form a base for a fine pencil line. Measure the distance between the scribed lines at the front and rear of the wheels using care that both measurements are made at an equal distance from the floor. The distance between the lines should be greater at the rear than at the front by 3 / 64 in. to 3 / 32 in. To adjust, loosen the clamp bolts and turn the tie rod with a small pipe wrench. The tie rod is threaded with right and left-hand threads to provide equal adjustment at both wheels. Do not overlook retightening the clamp bolts to 15-20 ft. lb.
It is common practice to measure between the wheel rims. This is satisfactory providing the wheels run true. By scribing a line on the tire tread, measurement is taken between the road contact points, reducing error by wheel runout.