When a friend asked me how often he should align his trucks, I responded that there were some other questions that should be answered first: How do you know when a truck needs to be re-aligned? What is the best procedure for measuring alignment? Perhaps the best way to answer these is to make sure we understand exactly what we're trying to accomplish through “proper” alignment.
The two primary goals are assuring optimum vehicle handling and minimizing premature tire wear. Questionable truck handling or stability conditions are typically found by careful review of driver write-ups and can usually be confirmed in a short test drive. Load, speed and road type should be documented, as these variables can accentuate or mask the condition and are often key in making an accurate diagnosis.
Road walk (directional instability), and wander often can be traced to a toe-out condition on the steer axle. Consistent pull to one side is normally caused by a drive axle that is not positioned square to the chassis centerline. With today's high-torque drivelines, drive axle alignment should be checked under power to observe whether or not the axle changes position relative to the chassis centerline compared to its at-rest position.
Worn bushings in radius rods or other locating brackets can make static alignment efforts fruitless. Significant differences in left-to-right side steer axle camber can also cause a right or left side pull. The truck will pull to the side having the more positive (or less negative) camber.
Premature tire wear, on the other hand, is nearly always related to excessive tire side scrub and most often appears as a steer tire symptom. Improper steer axle toe setting is the most frequent culprit, with toe-in wearing the outside shoulders of steer tires and toe-out wearing the inside shoulders.
However, if fast wear is observed on only the outside or only the inside steer tire shoulders, a chassis thrust angle resulting from drive axle misalignment is the likely culprit. A careful and knowledgeable look at both steer tires will allow accurate diagnosis of the offending alignment condition needing correction.
There is normally little or no need to perform a complete truck alignment procedure beyond the specific correction required, provided that a technician with proper training and experience in interpreting driver write-ups and “reading” steer tires directs correction of the offending condition.
It's also important to understand the several different approaches used by the companies that make alignment equipment. The first is to align axles and other wheel end components to a chassis centerline, either physically measured or established theoretically.
A second, and newer, approach aligns all axle wheel ends to one another, so that tire scrub is minimized and straight ahead travel is assured with a minimum of tire scrub. This requires fitting sensors to each axle end and checking these points relative to one another.
A third approach is the tried-and-true but labor intensive procedure of physical measurements, such as plumb bob drops and frame based cross measuring referenced to a documented level surface.
I would suggest choosing one approach and accumulating experience on that type of alignment equipment. In terms of frequency, arbitrary mileage or time frame guidelines don't make sense. Rather, align your trucks whenever they need it, based on knowledgeable observation of your equipment and careful review of service records.