The goal here is to have a lightweight, retro-looking trailer that is just large enough for a queen sized futon, sleeping two in comfort. This design might need a little stretching if you are pushing two meters tall, or need a lot of headroom. But for us of average stature, it is all we need and no more. I wanted to have solar for off-grid camping anywhere, and lightweight enough to pull with a Toyota car. And the roof absolutely cannot leak - I have poked my head in so many moldy trailers.
Step 1: Modify a Commercial Trailer
I started with a commercial steel trailer from a rural farm supply store. This gets you wheels, a frame, a hitch, and lights. Then I made several modifications, using a side grinder and a welder:
- Stiffen the tongue. These trailers are actually pretty limber, there is no real connection between the tongue and the cross member above the axle. This allows the frame to flex quite a bit, and can cause the trailer to bounce down the road if you hit just the right bump. This bouncing gets real old after about 50 miles of rabbit-hopping. I installed two pieces of 3"X3"X1/4" angle iron from the tongue to the main crossmember at the axle.
- I moved the axle mount point back about a foot.
- I lengthened the tongue by about a foot-and-a half.
- Why make it longer? When you are backing a trailer, it is pretty easy if your vehicle wheelbase is shorter than the trailer-to-ball-hitch. Your vehicle can turn sharper than the trailer. On the other hand if the trailer is shorter than your vehicle, then the trailer can out-turn you. It becomes very touchy backing up - you'll be going straight, suddenly the trailer is turning and you can't turn the vehicle sharp enough to compensate. Very frustrating. A bit longer trailer helps this problem.
- I took some expanded metal material from the oversize tailgate, and welded up a little table on the tongue. This stiffens the tongue, but also has been really handy just for tailgating. Open the trunk, there's your cooler and your folding chair, pop them out and have a feast.
- The large tailgate is now shorter, and becomes a little cargo area and a second tailgate in the rear. I have batteries back here, it is just big enough for a cooler, and is just a handy place for more stuff.
Most of the sections that I welded on are showing up white in the photos, as I painted them with cold galvanizing compound after welding to retard rust. Later I painted them black.
Step 2: Build a Box Inside the Commercial Trailer
This is a watertight box. The outside layer is sheet aluminum. I used aluminum flashing from a hardware store, and that was a mistake. Use thicker aluminum (0.032" or 0.040") than this - I've had some road rocks punch holes in the thin stuff and had to patch them with gutter tape. I bought the aluminum from Airparts, Inc in Kansas City Mo. They supply sheet aluminum for the DIY aircraft and trailer industry.
Outside is aluminum, next is a layer of standard Tyvek housewrap. Why? Aluminum is plenty waterproof, but the joints are not. I do seal the joints (I'll talk about this later) but two waterproof layers are better than one. Another mistake I made was to not use a belly pan of aluminum. I used Tyvek as the bottom layer, over the expanded metal of the stock trailer bed. This was a mistake and if I built it again I'd use an aluminum belly pan.
Next layer is plywood. The sides are 3/4", bottom is 3/8". I did not use treated plywood, as this should never get wet (it hasn't.) A 2X4 frame around the top and bottom, bolted to the steel frame, holds it all in place. The top of the aluminum flashing is designed to lap UNDER the aluminum siding, so water runs off not in.
If I did it again, instead of housewrap I'd use Grace Ice and Water Shield, a 3 foot wide roofing material that is agressively adhesive and remains semi-liquid for a self-sealing surface. I used this stuff on parts of the roof.
I then made a 2X4 frame about 14" above the floor of the box, designed to support the bed. Plywood is fitted to this frame, supported on all edges, and laid loose so that this space can be accessed for a cargo bay under the bed. It is tall enough for an airline carry-on suitcase.
Step 3: Plywood Walls
I used a 3/4" piece of good grade plywood, something that won't warp and will serve as the main frame of the trailer. Put the best side inside where it will show. The CAD printout in the photos show the curves, you can use about any curve that suits. Cut the two halves each out of a 4X8 sheet, careful to make mirror images.
This is where you make cutouts for doors and windows. I used commercially available trailer doors and windows, easy to waterproof, with latches and screens for ventilation. I don't have a record of where I bought the doors, but it was a large commercial RV parts supplier, there are several to choose from. Make the cutout, but don't install the doors and windows yet.
The last picture shows how the frame is simply a ladder of 2X4's between the two plywood sides. Ends are covered in curved 3/8" plywood. Everything is screwed and glued together so it is sturdy.
In this trailer, I happened to have a surplus solar panel what was just exactly as wide as I intended to make my roof. So I designed the trailer with this in mind. Without a panel, the roof could be thin plywood continuing the curves. One would want to reinforce the roof in case something is placed up there, luggage or a foot, but the solar panel needs no bracing, and the roof of this trailer is not for storage.
The photo shows the interior with the solar panel as the roof. This was nice for light, but it let in heat, and worse it sweats terribly at night. When it cools down, and breath creates moisture inside, the humidity condenses on the bottom glass of the solar panel. Later I insulated the panel with foam panels tightly fit, and covered it with plywood.
Step 4: Outside Covering
Ok, this thing absolutely must not leak. I skipped a couple of common details to further this end. There is no roof vent - because roof vents leak, or are left inadvertently open. There is no "Galley Kitchen" hatch, because a door in the top of a roof leaks. I have a cooler and tailgating is just fine. I've used this trailer for five years now, and have yet to worry about rain when I need to make food, it just doesn't rain that often or that long here in the midwest. If it does, you're in a car, go to a restaurant.
The roof is actually two roof systems. Underneath the aluminum siding is a permanently soft self-adhesive flashing material. Two brands are shown - Hardie flashing and Tite Seal window Flash. I used a roll of Grace Ice and Water shield, which is just a wide sheet of the same stuff on big areas. The key is, you can drive a screw through this stuff, and it still won't leak. The goo inside stays moist and self-heals. The aluminum is cut to fit, and roofing screws with rubber gaskets are used to fasten the aluminum to the wood underneath. I was careful not to sink a screw into the thin curved plywood - lines of screws follow 2X4 framing inside. The 3/4" sides are plenty thick to take a screw.
Windows and doors are applied over a layer of non-hardening Butyl Rubber Self Adhesive tape. This is a permanently gooey, aggressively adhesive tape. Once applied, you have to scrape it off with a putty knife - it sticks to a surface better than it sticks to itself. It goes under the edges of the window and door frames, and the fasteners go right through it, sealing the doors to the aluminum skin.
The corners and edges are trimmed with Bendable aluminum roof edge molding over a layer of butyl rubber tape. It flexes around the curves of a teardrop and finishes off the look.
Step 5: Electrical
Lights are legally required. I wired my lights so it works both with a 4 pin and 7 pin tralier harness, as I have one on one vehicle and the other on another. The 7 pin has an adventage that a standard way to wire this brings an extra 12V line on a relay back to the trailer. I wired this to the batteries, so that the batteries in the trailer charge either on solar or when I'm driving. The relay in the truck prevents the trailer batteries from draining the starting battery. I recommend this setup, the solar doesn't quite keep up for two nights. See this link for a good tutorial on how to wire both 4 pin and 7 pin trailer harnesses.
The solar panel is a 42V, 250W panel from Canadian Solar. The juice goes through a Midnight Brat charge controller, to a pair of 12V deep cycle batteries. The Brat is a great little charge controller with a lot of options - just right for a small offgrid system like this. The batteries run a 2500 Watt inverter I bought at a truck stop. The inverter has three 120V outlets, and features a pretty low idle current which is important. 2000W or more is required to start and run my air conditioner, which is the smallest window AC I could find locally. Waterproofing around the AC unit includes an eyebrow shelf, that keeps any leaks out. I had water entry before I put in the eyebrow shelf. On two deep cycle batteries, I can run the AC all night off grid if I'm frugal and don't keep it freezing.
I also installed an electric oil heater. These are very safe heaters, compared to ones with open elements, and the outside never gets too hot to touch, although I would not hold my hand against it for long. It cannot, physically cannot set something on fire, as it doesn't reach the ignition temperature of any material I know of. The electric heater draws too much juice to run on solar, I have to have shore power to run it.
On the side of the trailer I have an AC cord access hatch which allows a cord to go from inside the trailer to an outside source. I use this to plug in if I'm in an RV park, running the heater. The AC I can run on solar for a night, but after than I need to charge up or plug in. I've wanted a voltmeter on the batteries, but never installed one.
Step 6: Interior
We used a standard sized futon as the bed. I put an extra 1" foam pad under it for a little more padding. I didn't insulate anything but under the solar panel - it isn't a winter vehicle. The hatch under the bed allows storage of a suitcase, although it is inconvenient and I wish there was a door on the side instead. I eventually added shelves inside - you need someplace to put your phone, your clothes, a water bottle, a book. I hooked a gooseneck lamp over the bed, with a high efficiency lamp to avoid much heat. There is a little deflector that I made on the air conditioner, so it doesn't blow cold air on my toes. I keep a small fan in there, for nights when simple ventilation is all that is needed or when the batteries are low and the AC won't quite run. That's it! I've had the trailer for about five years now, get a lot of comments and admiration when I'm on the road. It's fun to camp in, easy to pull with a little car, aerodynamic. Total cost was about $2500 including the commercial trailer and the solar, but the solar panel was leftover from a project so it cost "nothing".