I've built 3 trailers of this type in my day, 2 of which were A-frame hitch, and one (the Moose Tail) which is a box frame hitch. All with surge brakes. I vastly prefer the box frame for looks and functionality. The A-frame is very easy though and requires less welding.
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Start with this much of a truck.
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Take sections of the frame ahead of where you made your cuts, and make two pieces that are mirrored. The holes on both sides of the frames are pretty much identical, which makes creating these a snap.
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My tongue on this one is actually designed similar to a reciever hitch and insert. The tongue "floats" in the receiver which is box welded into the front of the frame, and tied to the rear cross member. My original concept was that I was going to have a hydraulic cylinder in there, that was going to allow me to make the tongue longer and shorter on the fly while driving. I even had it worked out so that when I put the turn signal on, the tongue would automatically lengthen to clear the bumper, such that I could back up into a perfect jacknife if I wished. Then, when driving streight, the tongue would shorten, and this would pull the trailer closer to the truck and make it look cooler, and be more aerodynamic.
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Unfortunately in my zeal to do this, I did not give adaquet attention to the balance factor. With a real short tongue, the trailer got ass heavy in a hurry, which led to poor tracking abilities. Consequently, I had to leave the hitch at it's longest setting and I ended up abandoning the very cool hydraulic tongue idea.
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The decision to make it into a dump bed went hand in hand with trying to figure out how to best make the body lines match up.
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I have the guy to thank who helped me build my home addition for donating the hyraulic pump unit.
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I had never built a semi-scissor lift before, and didn't even draw it on paper. I just sort of rednecked my way, and didn't make any real mistakes except one. I should have put the pivot points closer together to take strain off the pivot rod. Other than that, it turned out perfect.
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In addition to the surge brakes, don't forget to make use of the parking brake as well.
Good luck! I look forward to seeing it.