How To Build A Teardrop Trailer/Trailer Bed
The Trailer Bed
The next step after the trailer frame is assembled is to make a trailer bed.
At this point you need to make some decisions about how you are going to attach the walls to the trailer. You need to make a choice because it will affect the width of the wooden bed you are about to make.
Just a brief aside: Keeping with the main theme of building a simple trailer we are going to be using 3/4 inch thick plywood sidewalls. Another factor is that the Harbor Freight trailer frame is actually 48.25 inches wide.
A few options
Two of these are common and the last one is what the author did.
1. Sidewalls sit on top of the trailer frame
If you decide to use this method then the Sidewalls sit directly on top of the trailer frame with the outside of the wall flush with the side rails. This means that the wall thickness infringes on available interior width. So for the 48.25" (122,56 cm) wide trailer, you have only 46.25 inches (117,48 cm) of interior width. On the plus side, you have the sidewalls fully vertically supported by the trailer frame.
2. Sidewalls are bolted to the side of the trailer frame
In this configuration, the sidewalls are attached to the side of the trailer frame's side rails. This provides a full 48.25 inch interior width. On the downside, it will reduce the interior height by 4 inches and the sidewalls are only supported by the bolts through to the trailer and the glue attached to the trailer bed. Also you'll have to create reliefs in the sidewalls for all the bolt heads on the side rail and have the wooden frame of the bed compensate for the .25 inch gap you'll have.
3. Sidewalls are half sitting on the trailer frame
The problem with option 1 is that you can't use the existing holes in the trailer frame to bolt down the bed because the bed will be too narrow. The problem with option 2 is the lack of sidewall support and creating reliefs for side rail bolt heads.
Therefore the author opted for making the bed 47.25 inches wide. This allows 0.5 inches of sidewall support and is wide enough to finesse bolts through the bed into the existing holes in the trailer frame.
Constructing the Bed
After the decision is made, the task is fairly simple. Cut a 4x8 5/8" thick sheet of plywood to the correct width and build a 2x2 pine frame to support it. The 2x2s that go across for support should be placed to align with the trailer cross members. You can place extra cross member near the front half where your weight will be when inside the trailer. Use "Polyurethane Glue" to glue it all together.
Next you'll need to buy a gallon of Black Roof Coating (roofing tar) and coat the entire underside to protect the wood from the elements. Do not coat the sides as you will be gluing them. After it's dry you can flip it over onto the trailer, line and center it up and drill holes down into the existing trailer frame holes. And finally bolt it down. Have washers on hand to space out any gaps between the cross members and the bed.
You should feel pretty good at this point. You've completed the boring work on the trailer. Next comes the fun part - putting up the side walls.