I’ve decided to build a bicycle trailer, and the first step that I usually make in such a project is to explore the work that has already been done in the field. A cursory glance is enough to indicate that bicycle trailers can be divided in to two categories: single wheel and multi-wheel.
Trailers in the second category are probably the most common, and usually have two wheels. They are configured much like a standard car trailer; the weight is distributed in front of and behind the axle so that there is only a small amount of weight bearing down on the hitching mechanism. These trailers can be built to carry a remarkable load.
While two-wheel trailers are obviously useful workhorses, I am more captivated by single wheel designs. BOB trailers are probably the most well known example, and their basic design has been copied many times over.
There are some other interesting design features that have been explored. While many trailers attach to the rear axle or the nearby rear frame, some attach to the bicycle seat-post. The Oxtail trailer attaches to the seat-post, and it is correctly argued that such an arrangement is less likely to fish-tail and jack-knife when rolling downhill. The Oxtail trailer also folds down for easy storage and transport.
The Extrawheel is an innovative design for a single-wheel trailer, and is probably my favourite. Its large wheel gives stability and a smoother ride, and its simplicity allows it to be very light (only 4.8 kg). The large wheel in the centre would, however, make it difficult to carry any load that could not be divided up and placed on either side.