Individual rear wheel angles are measured, then adjusted so the thrust line
coincides with the geometric centerline. The front wheels are then aligned to
the geomenic centerline/thrust line. This sets all wheels straight ahead and
parallel, and centers the steering wheel.
Why Is 4-Wheel Alignment Recommended?
Your front wheels "steer" the vehicle, but your rear
wheels "direct" the vehicle. Say your rear wheels are tilted to the
right. Even if your steering wheel is pointed straight, your vehicle will travel
left. Four-wheel alignment solves this problem by referencing all four wheels to
a common centerline.