Light Weight Construction

[Mashurst]

I'm starting the planning stage of a trailer to pull behind my 98 XJ. I don't want it to be any heavier than it needs to be.

I do want it to be completely sealed from the elements. I'm leaning toward a basic box design with a non removable top and a rack to hold RTT=maybe a small gear basket. Then it will have a fold out galley on the sides and slide outs at the rear.

I'm thinking about using a very thin steel 'skin' along the lines of 20 gage just to get the tight seal I want. Then having about 3/8 plywood fitted inside the skin to give it strength and durability. Higher up in the trailer where there would not be gear coming in contact I could go with just an insulation like the blue foam board. I'm thinking the skin protects the wood and the wood protects the skin. This seems like it would yield a very light unit that would have a sold feel to it and be good at protecting it's contents and it's self. Clearly it would not put up with any rock contact and I would want to make sure I had solid frame where contact was likely to occur.

Thanks for your Thoughts?

 

[R_Lefebvre]

I dunno, I don't think it's a great idea.

I think my construction method is actually pretty weight efficient. But if you want to go even lighter I would suggest instead, maybe look at making the box frame out of 2"x1/8" steel angle, and sheet it with 16 or 18 gauge.

What you don't want is to build it, then need to add reinforcement or another frame in order to support the weight of the tent. Don't underestimate the racking forces on the top of a box structure, especially if the back is open.

 

[ntsqd]

Consider making the box also the frame. "Unitbody" or monocoque design. There is strength in the skin if you let it. Point loads will want distribution plates.

 

[Trailpsycho]

IMO, the steel/wood combo layering idea is poor simply from a moisture, corrosion, and inevitably a rust standpoint. I would go with the 16 guage as mentioned above and the angle frame would add alot of additional strenth. If you want to add even more strength to the design that was mentioned, you could have vertical ribs rolled/formed into your sides and longitudinal ones in the floor--prior to their attachemnt to the angle frame. A good sheetmetal shop should be able to do this for you without a crazy amount of cost. Have them roll the beads prior to making the cutting the sheet to your specs. The lid would also benefit...much like a truckbed or older vehicles floorboards.

 

[R_Lefebvre]

Yeah, the only problem is there isn't much strength in flat sheet metal. That's why cars using stampings in complicated strength. I think making a frame out of 1/8"x2" L angle will add a lot of strength without too much weight. It'll be semi-unibody. The frame will take all the point loads, and the sheet will give the light frame some stiffness to diagonal forces.

If you had a 6 sided sheet metal cube, it would be strong. But soon as you open up the back end for loading, you'll allow it to "matchbox". I also don't think large pieces of 20Ga would be strong enough to support the weight of a RTT off-road. And then once you talk about making a subframe to support the weight... why not just build a full box frame?

Or maybe I'm misinterpreting what you're saying. I think the box should be fully welded to the frame, instead of bolted. If you bolt a box to a frame, the frame needs to be very stiff. If the box itself is load bearing, you can lighten up the frame.

ie: 2x2 box tube perimeter frame on the bottom, 2" angle frame uprights and roof frame, 18 Ga sheet fully welded to top and bottom frames.

If you do it the other way, you probably need 2x3 box tube on the frame, then the box itself needs a subframe in order to bolt to the bottom frame, etc.

 

[R_Lefebvre]

Almost forgot, yes, you'll need to do something to stiffen the sheet. If you look at my trailer, I have one HUGE sheet on the front, 14ga. So it's reasonably stiff, but still can "gong" pretty good. I will probably add something like Dynamatt inside to quieten it. Not fully coated, but just a bit to break up the waveform and lower the natural frequency. Also, it's 14ga so it's pretty strong by itself, and supported by 2" box tube.

On the roof, I used 18ga, with one large piece in the middle (4x5 roughly). It was really bad for gonging, had no stiffness at all. (That's why I recommend not going 20). I ended up welding a 1" tube underneath it, and then foaming between the tube and the sheet. The foam really dampens things and it's pretty quiet now.

I'm actually a big fan of using spray foam for structural things like this. (if you couldn't tell ) It has so many uses, and higher end cars are starting to spray it into body cavities for noise, energy absorbing, strength, etc.

 

[Mashurst]

Thanks for the input guys.

I did consider angle but it is much less ridged pound for pound. I'm thinking of using think wall (~14-16 gage) 1" square tube for most of the box and maybe 1.5x2" for the base. I will be welding the skin to the frame. The thin sheet would be plenty to give me a shear plane and get the advantages of the unit body. The racking loads are a big concern and how to deal with them in light of the door openings, the rear door in particular takes the ton of strength out of the picture as noted. Some kind of gusseting or triangulation may be needed and I hate that. But not having doors is not really an and option.

I didn't think about the rust factor. That is something to consider, however it may not be as big a concern for me out here in Caly as it would for you mid westerners.

I love the idea of the corrugated floor that would really add strength without having to add much weight. It would also add a bit of breathing room that would help with the moisture management.

Thanks again for all the input. You guys rock!

 

[ntsqd]

Kinda what I was saying. Flat sheet is plenty strong so long as the span vs. the load doesn't result in distortion. Once distorted you're done. Any internal walls should be structural to the overall trailer, not just a divider. That way they take some load and cut down the size of spanned areas. Openings need continuous reinforcement all the way around (imagine a thicker sheet cut to the same opening size on the ID and then cut on the OD to some particular width - then riveted/welded in place). Corners in openings should have large radii.

Definitely weld it or rivet it all together as one structure. Bolts are for service removal items only.

 

[R_Lefebvre]

On the back, I would just make the door opening smaller than your box size. Leave a few inches of sheet metal "lip" all the way around the back of the box. Say the box is 4' tall, 4' wide, then make the door 3'6" x 3'6" or something like that. That will really help the strength/stiffness.

 

[Mashurst]

internal walls should be structural to the overall trailer, not just a divider

Internal walls! I hadn't really considered putting any in but I could. There will be a water tank at the front that could be walled off as a box. It would take some thinking to get set up so it was not a PITA to accesses for service. I could also do one down the middle front to back but that is not the direction I need strength. Perhaps a right to left bulkhead could be added in the middle but how would I accesses the fount compartments?

More though required... Maybe posts down the mid line and then transverse bulkheads in the upper (above the hips) part that would be between the side compartments. That would resist side to side racking forces very well, and would not cost me much is space or usability. I'm liking it. Thanks guys.

Leave a few inches of sheet metal "lip" all the way around the back of the box.

Ya, I agree that would do a lot. It pains me to do it though as I want to have pull outs at the back and so they would necessitate a lot of wasted space. I think I would rather buttress the walls by triangulating from the fender/taillight area. Maybe the door at the rear will not go full height though.

 

[Mashurst]

After giving this a lot of thought over the last weeks, I feel like I am past the concept phase and I need to start the design phase. This is a link to the construction method I have settled on. http://www.compactcampingconcepts.com/explorerpod.htm
It's a steel frame base with a plywood "glue and screw" box with a durabak coat over the entire thing to seal it from the outside. I will be compartmentalizing the inside to a greater degree than I initially though in order to give the basic box structure. With good joints I think it will be very strong. In the end this just makes a lot of things work for me. It will be light as well as strong. I don't love wood as a medium but I can do a lot with it that I wouldn't be able to in steel. Welding sheet metal has it's problems too.

I'd be going for an end result along these lines http://www.adventuretrailers.com/horizon.html
http://www.campausa.com/att_drs.htm
but smaller.

I'm hoping to able to pull this behind my Subaru Forester as well as the Jeep so I really need to keep it light and small.

My goal with this thing is to be able to decide to go camping at any given point and be on the road in less than 45 min. I want to reduce the hurtles to just leaving. Partly this is because we are living in an extreme fire hazard area and may have to leave, but I also want to camp more and the effort involved in getting out is way too high, with the result being we only go two or three times a year. I would like to up that to 8 to 14. This thing will be stocked with food and camping gear and ready to roll with just maybe some clothes and fresh food added. It will also reduce the stuff that I end up with living in my jeep so that I don't have to carry it all the time costing my gas.

I plan on a rear tail gate, with a slid out in the future for a fridge. The built in water tank will be in the front of this compartment. I'm also wanting a big compartments that opens from the sides and goes across the nose but inside the "main box". It would be for fire wood, trash and the like, dirty stuff that would want to be separate from the rest of the cargo. The kitchen/galley side of this compartment will be walled off to be used as a shallow pantry. Then a side box on each side above the "hips" that has a drop down lid/cover kind of tailgate style such that it makes a work surface. Passenger side would be for a galley the driver for off road gear, and stuff that would be used while underway. These would be deep and go to the mid-line of the trailer. The rooftop tent will open to driver side, away from the kitchen. I would also have a traditional tapered nose box I'm thinking it will have a gas can on ether side and a propane cylinder in front of it. A battery will be in it as well as other gear space. If there is space I am also thinking about making the fenders kind of just a box sticking out the sides and having a compartment in the top of those above the wheels but below the side boxes like the DRS in the link. As far as weight, I would consider under 1000lbs dry to be success but I would love to be more like 800 and I think it's doable.

Thank you all for your input. Am fairly made up about the basic concept here but if you see problems I am certainly open still. I'll start a new thread for the design phase but I'll cross link here.