heavy loads. Frame Metal: usually steel or aluminum alloy tubing, assembled by brazing, welding, or nuts and bolts. Wood (or bamboo): seldom seen but sometimes used in makeshift and home-built trailers, fastened with glue, nails, screws, bolts, or a combination thereof. Hitch There are various types, from home made to those supplied by the trailer manufacturers. Hitch positions: Seatpost: Temporary or permanent clamp assembly attaching the trailer hitch to the seat post. Rear axle: Special attachment points, integral to the rear quick release skewer or bolted onto solid axles, hold dropouts cut into the trailer hitch. Chainstay: Two-piece sandwiching clamp screws tight over left rear triangle, with protruding socket-and-pin receiver. Rear cargo or pannier rack: Some improvised hitches attach to the rear cargo rack or pannier frame. Since rear racks are not structural parts of the bicycle they cannot handle much weight or torque loading. Improvised: Ropes, bungee cord, chain, cable, etc. Usually not dependable, often dangerous to rider and cargo. Single-wheel trailers generally use a special frame hitch which attaches to both sides of the rear axle, and which incorporates a vertical hinge to allow cornering. Attaching the trailer at the seat-post can (dependent on the design) place the load at a lower point relative to the longitudinal pivot axis of the attached trailer. This can make it possible to wheel a loaded single-wheel trailer around while disconnected from the bike, as the seat-post hitch is a natural height for grasping while walking upright. Hitches for two-wheel trailers must incorporate some form of universal joint or flexible coupling. If the joint relies on a certain amount of play to give the required movement, then there is the possibility of resonance at certain pedaling speeds and trailer loads. The effect is that the trailer feels as if it is 'bumping' the bicycle. However, there are other methods such as spring with a nylon tube inserted through the middle to provide rigidity and flexibility, allowing the bicycle to lean while the trailer remains upright and isolates the bicycle from any resonance. Two-wheel trailers which attach to the rear axle or chainstay generally have an angled towbar to help keep the trailer central behind the bicycle. Wheel Traditional spoked bicycle wheel in various sizes. Has the advantage of being light, strong, readily available. Pneumatic tires provide some suspension for the load, larger diameters ride smoothly and have much less drag than many