Material recomendation (1 Viewer)

[ExpeditionXJ]

Hello MUD members, could use some guidance on a project.

I'm currently rebuilding a Coleman Colorado pup, some frame upgrades in addition to new suspension and axle. This trailer stock weighs in around 600lbs, I estimate maybe twice that once completed.

My question regards the material for the extended tongue. I want 2x2 with weld on receivers at both ends, making the tow end removable for a little theft deterant. What thickness should I use, will 3/16" be ok or do I need the 1/4" wall?

I don't assume you all would be familiar with these trailers, here's a pic of a similar project that was the inspiration to my build. (photo credit to JetNoise). These are a small light single slide out pop up.

I planned to run and get the material today, but with having to purchase a 24' stick I wanna make sure I don't make the wrong decision. Appreciate the input..

 

[Malleus]

The answer to your question depends on two things:
1. how much weight you plan on loading onto the tongue (at the farthest point of application), and
2. how far from the point of attachment your extension (load) is.

What you're trying to prevent is called an adverse bending moment. A moment is a force, applied at a distance. If you know the force (load) and the distance, you multiply them to determine the moment. If you know the distance but not the force, you can estimate it, just make certain you err on the conservative side of the estimate. Then you consult your handy dandy steel construction handbook (or ask someone who has one) and select the proper size section to resist the load.

Bear in mind that there may not be a standard steel section section which will satisfy your conditions. You may have to select something close, and add/modify material to obtain the strength your application requires.

Post your data and I'll look it up for you.

 

[ExpeditionXJ]

Thank You Malleus, appreciate the response. I don't know the tongue weight as it's stripped to frame now.. but I figure the tongue will be ~6' beyond the front of box. I was planning on adding weight to rear in form of house batt/s and possibly water tank to lighten up tongue if needed.. not very scientific I know..

This is my son's and I first build that involves any fabrication, looking fwd to him learning skills at his younger age that I never did. My priority is to be able to slide out the bed while still being hooked to the vehicle.

I was hoping for an educated guess.. is 3/16" wall generally thick enough for a larger trailer.. or is 1/4" not uncommon? My opinion is that the later is very much overkill on this small of a trailer.. but like I said, I'm not experienced in this sort of thing.

Thanks again..

One other thing, usage will be mainly fire roads here in Pa with the most severe trail being White Rim, Elephant Hill on a trip out west.

 

[Malleus]

Actually, loading behind the trailer axle(s) is a good idea. The only load the tongue sees is forward of the trailer axle. (It's a see-saw) Then the increased weight is only a concern for the hitch pin/ball.

OK, so at 6-ft and estimating a 500-lb tongue load, which is typical for a class 3 hitch (5,000-lb weight class), and 2-in square structural tubing,
1. your moment requirement is 36,000-in·lb [(6)(12)(500)]
2. there a 3 failure conditions for this type of member; the first two don't apply, because of your size, so you're left with M<FZ (yielding)
3. your options are:
s. 1/4-in wall (Z= 0.964), 3/16-in wall (Z=0.797) or 1/8-in wall (Z=0.584)
4. Let's try 3/16-in, using the welding consideration as the selection criteria: the failure limit is (46,000-psi)(0.787)=(0.9)(36,202)=32,581-in·lb
So try 1/4-in wall (which we can see by inspection is enough)
(46,000)(0.964)=(0.9)(44,344)=39,910-in·lb > 36,00-in·lb
5. your minimum wall thickness would be 1/4-in, for 2x2 hollow structural section, having a yield strength (F) of 46,000-psi (ASTM 500, grade B).

This is a very simplified version of the complete calculation, which I haven't shown here, for brevity. I have assumed a uniformly increasing load for a simply supported beam (since you specified joints at both ends). It is my understanding that this type of calculation is typical for trailer design, although I'm not a trailer designer, by any stretch of the imagination.

One additional note, this is a static load calculation. Impact loads, which exist during transit, can be up to 10x static loads. These are momentary loads and are considered differently from the estimate I've shown above. I mention this only for completeness; I do believe your complete loading will be greater off-road (including what are considered "improved" roads, such as firebreaks) than on paved roads. Speed is a load factor here as well, so with the lower speeds on more difficult trails, impact becomes less of a concern.

 

[Malleus]

While I'm at it, one more note about the yield failure criteria. Steel is a unique material in that it has two distinct load profiles, elastic and inelastic. In the elastic region, a steel section can be loaded as many times as you wish, and it will return to its previous shape/profile. In the inelastic region, however, permanent deformation will occur. This doesn't mean that the section will fail catastrophically (break), just that it will be "bent out of shape".

A separate failure criteria exists for ultimate failure. The limits for this criteria far exceed yielding. Common practice is to design for yielding, assuming that if a section doesn't yield, it won't fail. There are other failure criteria, such as tensile (stretching) and torsion (twisting), but as I mentioned in my previous post, these don't apply to your section in your situation.

HTH

 

[LAMBCRUSHER]

I used receiver tube. It's about 2 1/2" square x .25". Then I made my hitch bar using a long piece of 1/4" 2x2. The hitch bar can slide in or out for adjustable tongue length, removal and storage, theft prevention

 

[ntsqd]

Strongly suggest buying M.M. Smith's book "Trailers; How to Design and Build, vol. II" It covers all of this in layman's terms. I seem to reference my copy several times a year and my trailers are already built. I know it's out of print and not as inexpensive as it used to be. I still strongly suggest it.

I think putting the batteries at the rear is exceptionally bad advice unless there is a permanent and offsetting weight being added to the front. Classically it is far too easy for a trailer to have too little tongue weight than it is for it to have too much tongue weight. I'd venture too much tongue weight is almost impossible in a trailer of this size.