Yet Another Bicycle Trailer

Step 3 Internal frame

This structure needs to connect the wheel brackets to the plastic tub, the wheel brackets to each other (to prevent flexing of the box), and will provide somewhere to connect the hitch mount underneath the tub. Everything was connected to a piece of scrap plywood, resting at the bottom of the tub.

My original solution was to use a 2x4 to connect the wheels, with the wheel brackets lag bolted into the end grain of the wood. This might result in the brackets loosening, and the wheels rubbing against the top of the tub and/or falling off the trailer. Ask me how I know. FYI, putting screws into the end grain of wood is a very weak connection. I tried improving it by gluing in dowels at right angles to the screws, to give a stronger connection, but that didn't work either.

So the working solution was to use EMT (electrical metallic thinwall) electrical conduit and threaded rods, to form a "sandwich" with the wheel brackets at the ends. I used 1" nominal conduit. To match the angled sides of the plastic tub, I needed to cut angled ends to the tubing. I used a pipe cutter and angle grinder, but there are other options for sure (metal cutting bandsaw, hacksaw if you want a workout, etc.) In the future, I would change the angles slightly, so that the wheels are canted inwards at the top (like racing wheelchairs) slightly, to increase stability.

The lower EMT conduit is bolted to the plywood, through the plastic floor of the tub. The upper EMT is connected to the lower one using hose clamps. This results in a strong structure--probably not the lightest solution, but it seems to work so far.