Thrust Angle

The front wheels aim or steer the vehicle, but the rear wheels control tracking. This tracking action is relative to thrust angle. Thrust angle is defined as the path that the rear wheels will take with the vehicle center line. Ideally, the thrust angle is geometrically aligned. Front wheel drive vehicles introduce another dimension of thrust angles. Due to minor accidents, hitting chuck holes and other related occurrences, one of the rear wheels may change. This directly affects the thrust angle, by keeping the rear wheels from tracking properly. Other obvious problems may also occur, such as severe wheel scrubbing, causing unusual and accelerated tire wear, a decrease in fuel economy, and less than optimum handling stability. No matter what is done to the front end of a vehicle, with the rear thrust angle altered as just described, it may not cure handling and rear tire wear problems. Many vehicles leave the alignment machine with perfect front wheel alignments, but the rear wheels are literally dragged down the road because one rear wheel has altered the thrust angle.