building a removable enclosure for a flatbed trailer

[jesusgatos]

I started a thread over on pirate4x4 about this project, and someone directed me to this site. Really glad to have found this forum, it looks like a great resource.

I've got a small flatbed trailer that I want to turn into an enclosed trailer. The trailer that I'm working with is called an M103A3 trailer (basically just a 10ft flatbed version of the popular M105 military trailer).

I've never built an enclosed trailer before, and I don't have any idea what type of materials I should be using, but I took some measurements and this is what I've come up with. I'm going to be towing this trailer behind an M109 that I'm converting into a motorhome, and plan to spend a great deal of time off-road. So needs to be able to survive the bumps and scrapes you'd expect to encounter when taking a trailer places it wasn't meant to go.

The plan is to mount the enclosure to the deck using the existing stake pockets, so I can remove the enclosure (using an A-frame hoist on the front of my truck). Then I'll have a flatbed trailer and a shed. This is something that I'll probably only do when I'm setting up camp somewhere for an extended period of time, or when I have a specific need for a flatbed trailer, but I like the idea of having the option.



The red tubing is 2 x 2" x .120-wall.
The yellow tubing is 1 x 2" x .120-wall.
The blue tubing is 1 x 2" x .090-wall.
The pink tubing is 1.25" round x .120-wall (fits into a slot at the front of the trailer deck)

SolidWorks tells me that this combination of tubing should weigh about 623lbs. Plus, it's going to have a small door on the front right side for easy access, as well as a fold-down rear door / ramp.

I haven't decided what type of assist-mechanism I'm going to use on the rear door, but I'm leaning towards something like a torsion-type garage door spring. I do know that I want to be able to lock the door into position when it's horizontal, so I can use it as an extension of the trailer deck.

What do you guys think? I'd like to get some input on the overall design and materials before I start cutting and welding anything.

[eamarquardt]

I think I'd start with an approximate weight target for the lift off box and work backawards. Not sure what you are going to "skin" it with but that could be (or not) a significant source of strenght. Is it just going to be the shell of a box or are there going to be any internal structures that might be used to stiffen up the box. Are you going to bow the top a bit to help shed water? Also, I think I'd keep the cross members on the roof whole and cut the longitudinal into sections as I think it would be stronger this way. Less reliance on the long member for strenght and the cross members wouldn't be relying on welds. Most trailers I've seen seem to use just cross members and rely on the skin to separate the ribs/cross members.

I don't see any framework in the floor of the box. If you are going to lift it off is the floor coming also or is there no floor as you'll be using the trailer bed as a floor? I think a floor or some cross members at floor level would help a lot with the rigidity of the structure when off the trailer.

I think, in general, you could use much lighter tubing (half the wall thickness in general) as the trailer frame, being military spec, is very robust but I would add gussets in the corners to beef up those joints and some type of structure on the bottom of the box to stiffen it up.

Given the tow vehicle and basic trailer (military!) you're starting with, weight is not a big factor in your build but the less it weighs the easier it will be to tow, manuver, and lift off.

I certainly have no "expertise" but you asked! Hope this helps.

Cheers,

Gus

[kennyrayandersen]

I think Gus brings up some really good points. Your removable ‘box’ will have posts that insert into your trailer, right? Why not put a floor in your box and the posts would serve to keep the box off the ground when it’s not on the trailer. That way you won’t have to worry about the bottom getting messed up. It looks like you have 4 posts on either side that are lower and at the same level. If you made a minimal frame and a ‘structural box, then it would be HEAPS lighter. You could use lighter gage steel as Gus suggested for the 8 support posts, or frankly you could even use wood. Since you aren’t on some ultimate weight-saving mission (like me ), I think this would be a really good application for the paper-covered foam core used in roofing (there were some previous posts on that stuff but I can’t remember what it’s called – hopefully someone will chime in). It’s not so expensive. It’s around 2 inches thick, and has a paper bonded to both sides. You could build the box out of that foam sheet and then add fiberglass, or you could even bond plywood to the foam and you’d have an incredibly strong sandwich; plus, it would be MUCH lighter than building a steel frame – just something to think about.

[jesusgatos]

Thanks for the replies.

I'm building this trailer to replace the first trailer that I built. That was a different type of military trailer chassis (M353), and I put a 12ft uhaul box on top of it. That trailer chassis weighed about 2,700-2,800lbs and the uhaul box weighed about 1,200lbs.









I decided to build a new trailer because I wanted something a little bit smaller, and if it ends up being any lighter, that would be a bonus. This trailer chassis is already about 1,000lbs lighter than the old one, at about 1,600lbs. If I can keep the total weight of the enclosure under 800lbs, I'm OK with that. I'm sure I could build an enclosure that would weigh less than half that much, but I'd rather err on the side of over-kill. That's sort of a theme with this project...

I don't want to leave any large areas of the roof unsupported, because I want to be able to use that area for storage, and as a rooftop patio. But I'm sure I could lighten it up a bit, like you're suggesting.

I was not planning on making any type of floor for the enclosure. The trailer already has a nice flatbed deck on it that's made out of 3/16" steel plate, and building another floor on top of that just seemed kind of redundant. Although I guess it would be pretty cool if I could remove the enclosure from the trailer without having to empty it first. Would also keep everything inside cleaner and sealed from the elements, which is another plus. I'd have to build wheelwell cutouts into it, but that's no big deal.

I would be very open to the idea of abandoning steel altogether, and building the enclosure out of some other type of materials. It's just that I do a lot of metal fabrication, so know I'm capable of building something like that. But if I should be looking into other (better) options, please just let me know. It would be great if this thing was insulated, because I'll be using it as a sort of mobile workshop.

Oh, and about the posts that will go into the stake pockets. I was planning on using short metal stakes that fit into female stake pockets in the trailer deck AND the enclosure, so that I'd be able to set the enclosure flat on the ground. Interesting idea about keeping it up off the ground though. If I build a floor into it, that sounds like something worth considering.

You've already given me a lot to think about. Please keep it coming.