The Trailer and Cabin

The Trailer Portion of the Teardrop

Most teardrops, whether commercially produced or home-built, start with either a custom welded frame or a bolt-together frame, such as this one from Northern Tool.  The drawbacks are a) the frame is steel, which can corrode, b) the underside of the plywood floor must be carefully waterproofed, and c) the trailer has to be inspected and assigned a serial number by the state's DMV or DOT before it can be registered.

I opted instead to purchase a pre-built, all-aluminum trailer from Aluma.

The version I purchased omits the gate on the rear of the trailer; I also added a spare wheel and mounting bracket.  The 6310H includes a 3000# torsion axle.  It is 63" wide and 10' long.

The advantages here, in my thinking, are that the trailer is already road-legal, is highly corrosion resistant, and provides a solid platform on which I can build my cabin.

Cabin Shape - Profile

Before beginning to design the profile I spent a lot of time looking at what others have done.  None of the profiles I found were exactly what I had in mind.  They were either too "eggy" - almost an egg on wheels - or too angular.  None of them had the dimensions I wanted - a 10' long platform that was 4'6" to 5' high.  And so I embarked on learning a new drafting tool to design my profile.

Alibre Design is the program I settled on, mostly because it appeared capable and was relatively affordable in the consumer version.  I have to say, though, that it supports a workflow that was completely foreign to me.  It assumes you will begin with solid modeling and proceed to detailed drawings.  I just wanted to start with the detailed drawing.  I give Alibre's customer service high marks for helping me get there.

It took eight iterations (I call this one "Take 8", a tongue-in-cheek reference to Dave Brubeck) before I arrived at this profile:

This profile is a combination of two different ellipses with their horizontal axes aligned;  the ellipses join at the topmost point.  Keeping the horizontal axis of the front ellipse shorter provides for better headroom in the front, while extending the horizontal axis of the rear ellipse creates a very nice reflex in the curve which will be part of the hatch/galley.  With the ellipses extending ahead of and behind the limits of the 10' trailer bed, the overall length of the cabin grows to approximately 11' 1-5/8".

The dotted line is the floor of the trailer, but that will not be the floor of the cabin.  I will use 3 2"x6" running front-to-back, boxed in the front but open in the back and covered with 3/4" birch plywood for the base of the cabin as seen in the section below:

One side effect of this floor construction is that I will have two cavities, each about 29" wide by the 10' length of the trailer to store items such as folding tables, chairs, tools, and even (potentially) a screen room.  These cavities will be accessible from the open hatch since I will not box in the rear.

I wanted to build the side walls and front/top using a sandwich construction method.  For the side walls the inner and outer layers will be 1/4" Baltic Birch plywood with 3/4" x 1-1/2" oak structural components as appropriate.  These oak components will primarily serve to create the cavities that will be filled with rigid foam, and to provide solid attachment for doors, windows, galley partitions and other items as required.  With the entire surface of the plywood glued to the rigid foam and the oak members, the result will be a lightweight, rigid panel.  By running the side wall construction to the top of the curve I will be able to deduct 4" from the 63" width of the trailer, and thereby use slightly cut down 5'x5' Baltic Birch for the inside and outside of the front/top curve.

The front/top curve will be similarly constructed.  This presents a challenge - how to structure the curved front and top while achieving the same rigid panel result.  I decided to laminate six ribs to form the basis of the structure.  The two ribs at the walls will be 1-1/2" x 2", laminated from eight layers of 1/4" thick white oak.  The four inner ribs will initially be laminated as two ribs 1-1/2" x 1-1/2" in section from 6 layers of 1/4" white oak.  These will then be sawn along their length to create four ribs 3/4" x 1-1/2".  The ribs will be connected across the width of the cabin with 3/4" oak stock in a box pattern, with the voids being filled with two layers of 3/4" rigid foam.  (The 3/4" foam will conform to the curve without needing to be kerfed, whereas the 1-1/2" foam will not make the bends.)  The inside of the curved panel will be covered with a single layer of 1/8" Baltic Birch plywood, while the outside will receive two layers of 1/8".

All connections will be made with glue and screws, with the screw holes being pre-drilled to avoid splitting the oak.

Why white oak for the ribs?  I want to use oak for strength, and the supplies of white oak can provide somewhat more regular grain than red oak.  In a subsequent post I'll show how we set up a proof-of-concept using red oak, and the red oak worked great.  I don't know that the additional investment in white oak will pay for itself in the ease of working the laminated ribs, but I do know that when I leave it exposed and varnished under the hatch it will look very, very nice.

What the section above does not show is that the entire outside will be clad in pre-painted aluminum.  I'm thinking on a two-tone scheme using red and white that highlights the waistline on which the ellipses are based.  The front and top may be either white or natural aluminum in color.


In the next post I'll add some photos of the facility where the trailer will be built, the worktable we constructed for the purpose, and the clamping stations we have laid out for laminating the ribs.