My brother worked in trucking in the 70's, 80's, and 90's. The company
he worked for specialized in flat bed trucks. They had one guy in the
shop who was the only one who was 'trusted' to re-camber the trailers.
He did so by using a heat gun or torch and causing the linear beams
of the flat bed trailer to regain the proper amount of camber. When he
was done the trailer had about a 4" to 6" rise in the center (unloaded).
Pretty much the same idea as "tapping the truss rods to see if they
all had the same tone" ... *G* My brother did not say that he tapped
the bed/frame with a hammer and listened (but he might have). What
he did say was that the guy would put the trailer somewhere in the
yard where he could stand back from it and look at the arch. His
skill was in knowing where to apply heat to get the proper amount
of arch and also not have the trailer "twisted" ... and I'm sure he had
to let it cool before saying "it's done".
- Jim B. in Burlington, Wa.