Wheel Alignment

In addition to allowing the vehicle to be turned, the steering system must be set up to allow the vehicle to track straight ahead without steering input from the driver. Thus, an important design factor for the vehicle is the wheel alignment. Four parameters are set by the designer, and these must be checked regularly to ensure they are within the original vehicle specifications. The four parameters discussed here are as follows:

1. Camber

2. Steering axis inclination (SAI)

3. Toe

4. Caster

Camber

Camber is the angle of the tire/wheel with respect to the vertical as viewed from the front of the vehicle. Camber angles usually are very small, on the order of 1 "; the camber angles shown in Fig.  are exaggerated. Positive camber is defined as the top of the wheel being tilted away from the vehicle, whereas negative camber tilts the top of the wheel toward the vehicle. Most vehicles use a small amount of positive camber, for reasons that will be discussed in the next section. However, some off-road vehicles and race cars have zero or slightly negative camber.

Steering Axis Inclination (SAI)

Steering axis inclination (SAI) is the angle from the vertical defined by the centerline passing through the upper and lower ball joints. Usually, the upper ball joint is closer to the vehicle centerline than the lower.

the advantage of combining positive camber with an inclined steering axis. If a vertical steering axis is combined with zero camber (left side of Fig.), any steering input requires the wheel to scrub in an arc around the steering axis. In addition to increasing driver effort, it causes increased tire wear. The combination of SAI and positive camber reduces the scrub radius (right side of Fig.). This reduces driver effort under low-speed turning conditions and minimizes tire wear. An additional benefit of this system is that the wheel arc is no longer parallel to the ground. Any turning of the wheel away from straight ahead causes it to arc toward the ground. Because the ground is not movable, this causes the front of the vehicle to be raised. This is not the minimum potential energy position for the vehicle; thus, the weight of the vehicle tends to turn the wheel back to the straight ahead position. This phenomenon is very evident on most vehicles-merely turning the steering wheel to full lock while the vehicle is standing still will make the front end of the vehicle rise visibly. Although the stationary the weight of the vehicle may not be sufficient to rotate the wheels back to the straight-ahead position, as soon as the vehicle begins to move, the wheels will return to the straight-ahead position without driver input. Caster angle also contributes to this self-aligning torque. Note that the diagrams in the preceding figures have been simplified to facilitate discussion. In practice, the wheel is dished so that the scrub radius is further reduced.