Wheel alignment refers to the angular relationship between the wheels, the suspension attaching points and the ground.
On the W-Body vehicles covered by this guide, GM specifies that Four Wheel Alignment be used, since the rear suspension is independent and adjustable, not a solid beam axle used on some other Front Wheel Drive vehicles. A complete four wheel alignment check is recommended whenever a service check is deemed necessary (uneven tire wear, the vehicle pulls to one side or a major suspension component has been replaced). This check includes the measurement of all four tires. The fuel economy and tire life increases when the vehicle is geometrically aligned. Additionally, the steering and the vehicle performance is maximized.
Because four wheel alignment is critical to the vehicle's performance and safety, it should be performed by a qualified technician with the proper equipment. The latest alignment equipment uses lasers for accurate sighting and a computer for precise calculations. Below is a description of the various angles used in wheel alignment on these vehicles.Caster
Caster is the tilting of the uppermost point of the steering axis, either forward or backward from vertical, when viewed from the side of the vehicle. A backward tilt at the top is called positive caster (+) and a forward tilt is called negative caster (-). Caster influences the directional control of the steering, but caster does not affect tire wear. One wheel with more positive caster than the other wheel causes that wheel to pull toward the center of the vehicle. The vehicle will move or lead toward that side with the least amount of caster.Camber
Camber is the tilting of the wheels from the vertical when viewed from the front of the vehicle. When the wheels tilt outward at the top, the camber is said to be positive (+). When the wheels tilt inward, the camber is said to be negative (-). The amount of tilt, measured in degrees, from the vertical is known as camber angle. Camber influences both directional control and tire wear. Excessive camber results in tire wear and causes the vehicle to pull or lead to the side with the most positive camber. On GM W-Body vehicles, camber adjustment is available at both the front and rear wheels.Toe
Toe-in is the turning-in of the wheels, while toe-out is the turning-out of the wheels from the geometric centerline. The purpose of toe is to ensure parallel rolling of the wheels. Toe also serves to offset the small deflections of the wheel support system which occur whenever the vehicle is rolling forward. Even when the wheels are set to toe-in or toe-out, the wheels tend to roll parallel on the road when the vehicle is moving.Frame Misalignment
The frame is a rubber isolated subframe (so-called on these vehicles because it does not run the full length of the vehicle), in the front of the vehicle. The subframe supports the engine and transaxle. The frame 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 frame 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 subframe for any obvious damage, especially on a used vehicle with an unknown history.Setback
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 Angles
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.Lead/Pull
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.Torque Steer
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
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.
PRELIMINARY ALIGNMENT INSPECTION
A knowledgeable and competent professional alignment shop will make a number of checks before attempting a vehicle alignment. Loose or worn suspension parts prevents an accurate setting of alignment angles. Checks should include:
The alignment should be checked with a full tank of fuel. The alignment shop should then check the alignment in the following order: