WARNING
To avoid personal injury and/or vehicle damage, refer to the service precautions at the beginning of this section.
Looking at a vehicle from the side, caster angle describes the steering axis from front to rear. The steering knuckle is attached to a control arm or strut at the top and a control arm at the bottom. The wheel pivots around the line between these points to steer the vehicle. When the upper point is tilted back, this is described as positive caster. Having positive caster tends to make the wheels self-centering increasing directional stability. Excessive positive caster makes the wheels hard to steer, while an uneven caster will cause a pull to one side. Overloading the vehicle or sagging rear springs will affect caster, as it will raise the rear of the vehicle. If the rear of the vehicle is lower than normal, the caster becomes more positive. Caster affects straight-line stability. Caster wheels used on shopping carts, for example, employ negative caster.
| Caster
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Looking from the front of the vehicle, camber is the inward or outward tilt of the top of wheels. When the tops of the wheels are tilted in, this is negative camber; if they are tilted out, positive. In a turn a slight amount of negative camber helps maximize contact of the tire with the road. Too much negative camber compromises straight-line stability, increases bump steer and torque steer. Camber influences tire contact with the road.
| Camber
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Looking down at the wheels from above the vehicle, toe angle is the difference in distance between the front of the wheels, to the back of the wheels. If the wheels are closer at the front, they are said to be toed-in or to have negative toe. A small amount of negative toe enhances directional stability and provides a smoother ride on the highway.
| Toe-in
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| Frame misalignment
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There are two types of frames commonly used, the ladder frame or full length frame and the subframe. Both frames are isolated from the body using rubber isolators. The subframe does not run the full length of the vehicle. The subframe supports the engine and transaxle. The subframe provides the mounting point for the front suspension lower control arms. Any misalignment of the subframe (accident damage, improperly performed heavy engine work where the subframe is loosened, lowered and/or removed, etc.) causes a misalignment of the front wheels. Movement of the subframe usually causes an increase in caster on one side of the vehicle and decrease in caster on the other side. This can cause the exhaust system to bind up, problems with control cables and unacceptable noise. Check the frame/subframe for any obvious damage, especially on a used vehicle with an unknown history.
| Subframe setback
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Setback applies to both the front and the rear wheels. Setback is the amount that one wheel spindle may be aligned behind the other wheel spindle. Setback may be the result of a road hazard (heavily hit pothole, for example) or a collision. The first clue is a caster difference from side-to-side of more than one degree.
| Thrust angle
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The front wheels aim or steer the vehicle. The rear wheels control tracking. This tracking action relates to the thrust angle. The thrust angle is the path that the rear wheels take. Ideally the thrust angle is geometrically aligned with the body centerline. If, for example, the toe-in on the left rear wheel is out of specification, it moves the thrust line off center. The resulting deviation from the centerline is the thrust angle.
| Torque steer
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Lead is the deviation of the vehicle from a straight path on a level road, without hand pressure on the steering wheel. Lead is usually the result of tire construction, uneven parking brake adjustment or the wheel alignment. The way in which a tire is built may produce lead. Rear tires do not cause lead.
A vehicle pulls or leads in one direction during hard acceleration. A vehicle pulls or leads in the other direction during deceleration. The following factors may cause torque steer to be more apparent on a particular vehicle:
Memory steer is when the vehicle wants to lead or pull in the direction the driver previously turned the vehicle. Additionally, after turning in the opposite direction, the vehicle will want to lead or pull in that direction.
Wander is the undesirable drifting or deviation of a vehicle toward either side from a straight path with hand pressure on the steering wheel. Wander is a symptom of a vehicle's sensitivity to external disturbances, such as road crown and crosswind. A poor, on-center steering feel accentuates a wander condition.
