Demand for bicycle trailers continues to grow, enough that people are just building their own rather than trying to find an inexpensive solution at a store.
From speaking with numerous people and using several different types of trailers I have learned quite a bit about what makes a successful one. First of all I recommend using a two-wheeled design because of it's stability and safety. The frame should be strong enough to comfortably enclose the wheels without any part of the structure coming close enough to be in contact with the rotating wheels. A square frame with rounded corners provides the most stable platform and minimal chance of scratching you or anything else. That's the easy part. More complicated is the hitch system. The hitch must allow all three axis of rotation. Since the bike must be able to lean side to side, turn on corners, and bend over steep hills and driveways. The most ideal location for a bicycle hitch is to connect as close to the rear axle as possible. This puts the force of the load directly in line with the wheels. For light loads of 30-90blbs (12-40kgs) it's best to connect to the bike in this manner. There are several commercial trailers which use a simple and cheap pneumatic coupler. This is an easy universal joint which you can find at most hardware stores. Simply screw or bolt it to a couple of metal plates and attach those to the bike near the rear axle. Here's a nicely designed lightweight hitch. For slightly heavier loads of approximately human weight, the coupler may not be strong enough. A good solution is a rod end, which is a ball & socket joint (1). You can get several different sizes so pick one which will be plenty strong. The only disadvantages to attaching at the wheel are the complexity, and the difficulty of keeping it away from the rear tire on turns. That's why for heavier loads of several hundred pounds or 150 kg. I recommend attaching to the seatpost. It's a little less safe in quick stops, but if you're carrying 300 lbs on your bicycle, than you should definately be a careful and experienced rider.
The fastest way to do this is to take two metal tubes (large enough to fit over your seatpost) and weld them at a 90 degree angle Slide one end over the seat tube, and drill a large hole perpendicular to the tube sticking out. Through this you can put a bolt and run your trailer tube from it (3). A more complex but very durable alternative, is to buy a windsail joint (not that kind of joint). This is a neoprene connection between the board and the windsail. A used one is cheap and provides a weatherproof joint (4).
I don't have much authority on metal connections, but from those poeple that I've talked to, welded connections are more homogenious than bolted. In other words the whole frame acts as one unit, and joints don't loosen. However since I don't know how to weld, I built a full trailer using self tapping screws. If you do use regular bolts, be sure to put lock washers or neoprene bolts on every one to prevent the bolts from loosening.
For more information on trailer design and use, visit bikes-at-work.
