Modified Widget RE-build - sleeps 4 + potty

[nelsonrx]

LOL, it's not barf at all. In fact, that last post actually made it much clearer for me. Okay, so based on your recommendations, here's the plan:

1. Remove tie plates and don't replace them
2. Re-weld cracks on xmember flanges, and add fish plate. I like the idea of using angle iron instead of just a flat bar. From the sound of your explanation it looks like I'd want the vertical section of the angle iron to be above the horizontal plane of the angle iron and not below, hence your comment about grinding it down to pass under the tongue (and to not weld it to the tongue)
3. If I want to reinforce the tongue, use a truss method. Speaking of which....

I found this interesting pic from one of Andrew's from a while back that surprised me (http://tnttt.com/viewtopic.php?f=35&t=39508&start=15). I'm surprised that the middle design would be less effective than the first design. Just seems as though you'd want to spread the tension all along the arm from end to end, rather than terminating part way down - seems like there would be a lot of stress placed where the forward rod/flat bar terminates.

[KCStudly]

You got it.

If you look at a moment diagram for a simply loaded beam supported at both ends, it looks almost exactly like the top arrangement (except the tips of the triangles extend all the way to the bearing points at the ends of the beam), so I guess that makes sense. Triangles are always more stable than rectangles. I would also think that tying the bar in closer to the coupler would be better, but gussets are most effective at 45 deg, so maybe that has something to do with Andrew's intention.

In my description about the two posts being on either side of the xmbr, my assumption was that they would be relatively close to the xmbr, and by using two posts, instead of just passing the bar directly onto the underside of the xmbr, you would avoid the issues of the athwartship stress cracking; so the way I had envisioned it would be closer to the top sketch, but with the straps ending a little closer to the coupler.

[KCStudly]

In the link^ Andrew mentioned the importance of eliminating springiness from the truss. Pulling the slack out is important, too. The best way I know of to do this is to weld the strap or rod to the post(s) first, then weld the tips, then clamp a portion adjacent to the tip of the strap down flat to the tongue, pinching some tension in to it, and weld along the side to hold it down flat. For thinner material at shallow angles this shouldn't be an issue. For thicker material or steeper angles you can pre-bend the strap slightly, but keep it less than the desired final angle so that there is still a little pull down (er' ... up) when you clamp it.

[nelsonrx]

Okay, even more clear now. I guess now it comes down to the decision whether to add the support to the tongue itself or not. When I run Andrew's spreadsheet, I get a needed Z-value of 0.379 for a 1500 # camper with a 46.5 inch tongue, unbraked.

I looked up the section modulus calculation for channel and came up with the following:




My channel is 1/8" thick all around, with a web (H) that is 2.5 inches high, and flanges (B) that are 1.5 inches long. When I run the numbers I get a z-value of 0.518. If I'm doing the math correctly that means that my channel material is 137% of what it needs to be. One interesting thing I noticed from Andrew's spreadsheet is that you need more strength for a braked trailer. In my head, that's almost backwards as I would think that a braked trailer would prevent as much momentum being transferred through the tongue and to the tow vehicle since the camper brakes would absorb some of that energy, but I'm sure there's a more technical answer

[pchast]

Actually I think there is a simpler answer....

Braking imparts a circular motion around the wheel pivot.
This seems to create greater stresses on the tow bar.

[KCStudly]

Concur. Braking adds a torque to a the moment calculation.

[nelsonrx]

See, I knew there was an answer

Okay, enough about tongues for now. I'm going to try to get that all figured out over the next week or so and then I'll post some more pix. We have an Asheville trip coming up in October so I need to get this sucker done! Thanks everyone for all the advice!