Thrust Angle and Vehicle Alignment
A vehicles thrust angle is an imaginary line drawn down the center of a vehicle in relation to the direction in which the rear wheels are pointing. The thrust line is an imaginary line created by the direction in which the rear wheels are pointing. The difference between the imaginary line drawn down the center of the vehicle and the thrust line equals the thrust angle.
Since front wheel drive cars are more prominent than rear, four wheel alignments have become the norm. When performing a four wheel alignment the thrust angle must be correct before the front end alignment can be performed. Other than collision damage two conditions will typically cause the thrust angle to be off.
Axle offset: Axle offset is when the rear axle cocks sideways in the vehicle causing both rear tires to point in the wrong direction. Remember just like a forklift if the rear tires point to the left the vehicle will steer to the right. This condition is typically caused by a broken leaf spring center bolt or problems with the shackle. On vehicles with trailing arm suspension worm bushings or a bent arm can cause a pull in the steering wheel.
Rear toe adjustment: Four wheel independent suspension requires the toe of the rear tires to be set. When this adjustment is off the trust angle will be off as well. Here’s an example: the right rear toe is set to 4° toe-in and the left rear toe is set to 0°. This would move the thrust angle to the left 2°. Always adjust the thrust angle to specifications before attempting to set the front toe.
