Bicycle trailer
A bicycle trailer is a motorless wheeled frame with a hitch system for transporting cargo by bicycle. It can greatly increase a bike's cargo capacity, allowing point-to-point haulage of objects up to 4 cubic yards (3 cubic metres) in volume that weigh as much as half a ton. Types. Different types of trailer are designed for various purposes, cargo requirements and riding conditions: Components. Axle. Most trailers have a separate axle for each wheel, like those used on a bicycle. These separate axles usually mount directly on the frame using either threaded nuts, a quick -release mechanism, or some press fit arrangement. The use of separate axles for two-wheel trailers allows the load carrying area to be between the wheels with its base below the axles, so as to keep the centre of gravity relatively low. Some trailers support a normal axle on two sides, others mount the wheel off one side with a stub axle (a one sided axle). Fender/mud guard. If included, the fender (or just a mud flap) helps to protect the cargo and the towing bicycle from road spray and dirt. On heavy-duty trailers, the fender may be  designed to be capable of bearing heavy loads. Hitch. There are various types, from home made to those supplied by the trailer manufacturers. Hitch positions: 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. Standards. A voluntary European standard (EN 15918) for two-wheel bicycle trailers was published in September 2011. This standard applies to trailers carrying cargo and/or up to 2 passengers, and enforces a maximum gross weight of 60 kg. There are many safety-related requirements, including restraints, structural integrity, stability, small parts, sharp edges, etc.