Fig. 5: Illustrating Thrust Angles
The front wheels aim or steer the vehicle. The rear wheels control tracking. This tracking action relates to the thrust angle (3). The thrust angle is the path that the rear wheels take. Ideally, the thrust angle is geometrically aligned with the body centerline (2).
In the illustration, toe-in is shown on the left rear wheel, moving the thrust line (1) off center. The resulting deviation from the centerline is the thrust angle.
If the thrust angle is not set properly the vehicle may "dog track", the steering wheel may not be centered or it could be perceived as a bent axle. Thrust angle can be checked during a wheel alignment.
Positive thrust angle means the thrust line is pointing to the right hand side (RHS) of the vehicle.
Negative thrust angle means the thrust line is pointing to the left hand side (LHS) of the vehicle.
If the thrust angle is out of specification, moving the axle to body relationship will change the thrust angle reading.
If the vehicle is out in the Positive (+) direction-moving the RHS forward and/or LHS rearward will move the thrust angle towards zero degrees.
If the vehicle is out in the Negative (-) direction-moving the RHS rearward and/or LHS forward will move the thrust angle towards zero degrees.
TOE DESCRIPTION
Fig. 6: Illustrating Toe Angle
Toe is a measurement of how much the front and/or rear wheels are turned in or out from a straight-ahead position. When the wheels are turned in, toe is positive (+). When the wheels are turned out, toe is negative (-).
The actual amount of toe is normally only a fraction of a degree. The purpose of toe is to ensure that the wheels roll parallel.
Toe also offsets the small deflections of the wheel support system that occur when the vehicle is rolling forward. In other words, with the vehicle standing still and the wheels set with toe-in, the wheels tend to roll parallel on the road when the vehicle is moving.
Improper toe adjustment will cause premature tire wear and cause steering instability.
TORQUE STEER DESCRIPTION
Fig. 7: Identifying 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:
WANDER DESCRIPTION
Wander is the undesired drifting or deviation of a vehicle to either side from a straight path with hand pressure on the steering wheel. Wander is a symptom of the vehicle's sensitivity to external disturbances, such as road crown and crosswind, and accentuated by poor on-center steering feel.