If the thrust line corresponds to the vehicle centerline, the vehicle will steer and track straight. But if the line angles off to either side of center, rear axle steer will cause the vehicle to lead or pull in the opposite direction of the thrust line. A thrust angle to the right of center is said to be positive. A thrust angle to the left of center is negative. A positive thrust angle will steer the vehicle towards the left, and a negative thrust angle will steer it towards the right.
HOW A THRUST ANGLE AFFECTS STEERING A thrust angle can cause "dog tracking" and off-center steering because the rear wheels do not follow the ones up front. It can also contribute to poor directional stability on ice, snow or wet pavement. If the thrust angle is due to incorrect toe settings on the rear wheels, it can cause diagonal wear on the rear tires, and increase front tire wear because front toe alignment changes any time the wheels are steered off-center.
The underlying cause of a thrust angle may be rear axle shift, a bent axle housing or anything that causes rear toe misalignment (bent trailing arm or lateral link, worn trailing arm bushings, bent rear axle spindle on a FWD car or minivan, or incorrect or unequal rear toe adjustment.
COMPENSATING FOR A THRUST ANGLE To eliminate the thrust angle, realigning the rear axle or rear toe is necessary. If bent or damaged parts are responsible, these must be replaced. Realigning the rear axle on a rear-wheel drive car is usually not possible without structural repairs (frame straightening) though offset trailing arm or leaf spring bushings may be available for some applications. The other alternative is to perform a "thrust angle" alignment and align front toe to the thrust angle rather than centerline of the vehicle. This will allow center steering but will not restore proper tracking.