Understand the OE's perspective here: They have just a few minutes to install an axle or set of axles on a frame, and something less than 10 minutes to verify alignment later in the assembly process. When building 200 trucks a day, time is a very expensive luxury.
A little tolerance on the spec isn't necessarily bad. It gets the truck out the door in reasonably good shape at reasonable cost at a level of accuracy engineers, assemblers and warranty people can live with. But is it good enough for your truck in your application?
"The OEM specification is a manufacturing tolerance," says Mike McCoy, national and special accounts manager at Bee Line. "If I was a fleet owner, I wouldn't be satisfied with someone setting my alignment to within OE tolerance."
McCoy points to camber as an example. Some OE specifications allow as much as 7/16 of a degree positive or negative. The ATA Technology & Maintenance Council's Recommended Practice 642 (now amended to RP 642A) calls for less than 1/4 of a degree.
While drafting RP 642, OEs and axle makers, tire companies and alignment providers submitted preferred alignment specifications, and they varied widely. McCoy was on that committee. He says the exercise produced two Excel spreadsheets of specs plus another page of footnotes.
Mike Beckett of Iowa-based M.D.Alignment, who also was on the committee, says that in analyzing why the specs were so different, the group determined "that the development of the specifications were for goals and priorities that were not necessarily the optimum performance of the tire."
In the end, the committee came up with Appendix 9, "Recommended Alignment Targets." As you'd guess, they are a compromise, middle-of-the-road generic numbers.
So, why should one assume that "factory specs" are good for your tires? RP 642 does recommend doing a "vehicle-in-service" alignment as well as a first PM alignment. If the factory specs aren't to your tire's liking, it's best they be changed early before the tires self destruct.
For more information on the ideal tire alignment specs, click here.
From the December 2011 issue of HDT.