We've been working with some OEM spec revisions lately and I wanted to clear up a couple of misconceptions about the tolerance built around a goal value.
Many people in the industry believe that the tolerances specified by the OEM have to do with Takt time restraints: sort of like "In the given amount of time, I can only get the alignment this close".
But in reality, time is not a big consideration in specified tolerances. Modern End-of-Line systems are fast and accurate enough to allow the operators to drive the adjustment to the goal values within a very tight window.
One of the drivers is the precision and accuracy of the gauging equipment. The tolerance has to be wide enough so the variation in the measuring equipment does not swallow the entire spread. Alternatively, the equipment needs to be highly accurate. This is the preferred route of most OEMs.
Tire wear and handling are also taken into account. The tolerances must be tight enough to deliver good performance over their entire range. While steer axle toe and tandem axle parallelism (scrub) are kept tight, there is a surprising amount of variation and disagreement on other angles.
Best regards, John Asha