The 81 Kenworth dump I just purchased has an aluminum dump box. In poking around at areas I likely should have paid more attention to prior to purchase(there should be an acronym for this) I discover the beams under the dump box have an ever so slight sag. Results in the beams not bottoming out at front until a bit of load placed on box. (at this time im talking excavator bucket bearing down . Ive yet to haul a load of anything profitable.)The overall sag in the aluminum beams under the dump box( 16 feet long ) I guess is probably under half an inch but the hinges are pretty tight so it causes the box to come down solid on the truck frame about where the axles attach and results in a gap of an inch or so at front of box,until weight is applied. This seems to cause some undesireable stresses notably a crack in the channel beam across the back of the box that kind of ties everything together. Anyhow im contemplating some things to help reduce those stresses. The easiest and most obvious is to add some shims to the truck frame about where the gap occurs (near front of box). Sort of match the contour of the bowed beams so everything cradles nicely. I could also remove a section of the steel wear strip(1/8 or 1/4 Im not sure) that is in place between frame and box. Could also cut hinges loose and raise them a quarter inch or so. Something tells me thousands have dealt with this issue,on other trucks, before me. Anyone care to share? Id like to repair this in a workmanlike manner that allows recertification etc.(Truck recently passed CVI even with this issue.)
What if you put a 1" (or thicker depending) plank across the truck frame above where the beams are bent down the most and then load the box at the front to try and straighten them. Obviously someone overloaded the box at some point.
im pretty sure this is not straightenable. as i see it the whole box is kind of tied together the sidewalls provide stiffening etc. the whole structure has over time developed this shape. I have seen the shim thing done on an old off highway dumptruck that had developed this sag. (What happened in that instance was the sides were cut and flared out 45 degrees to allow large loads of woody waste to be carried. As soon as the sides were cut free the beams sagged,seemed like there was built in stress in them.) They shimmed the frame so the box touched evenly and it seemed successful..
What do you have to lose by trying? I'd load the front and the back of the box so you're not putting undue strain on the hinges. Use about 2' of planking. And look into flame straightening on aluminum. The Lincoln Arc Welding Manual has a decent explanation of how to do it on steel, aluminum should just be easier since it gets soft at a much lower temp and expands four times what steel does. But find out first... Nothing wrong with some tapered wood bumpers in the front half either.
Today I cut about 3 feet off the centre portion of the 1/8 x 3 wear strips that sit atop the truck frame. The truck frame is humped a bit,right over the axles,especially on the inside edge. Was causing box frame to stop when it got to hump,then flex as load applied. At least thats my take on it. Box frame had been welded(longditudinal cracks in web) where this flexing was going on.Im hoping I may have cured all this by creating a space when I removed the sections of wear strips.