trailer attached, the mechanism that joins the trailer to the motorcycle must allow for an appreciable lean in order for a trailer to stay upright. Without the ability to freely rotate on a vertical axis, the coupler will bind and the trailer will jack-knife or flip. As explained above in "Ball and socket", the movement in the coupler and ball combination is limited to mostly horizontal movement and tilting slightly. Therefore, this type of coupling is best suited for three-wheeled motorcycles and four-wheeled vehicles. In contrast, as pivot ball hitches and swivel adapters do allow the appreciable lean necessary to keep all the trailer wheels on the ground while turning, these devices are the better option for towing trailers behind a two-wheeled motorcycle. However, the standard car type "ball and socket" couplings used in the United Kingdom are perfectly adequate for two wheeled motorcycles providing plenty of lean angle. Towing restrictions in the United Kingdom In the United Kingdom, there are some legal restrictions on towing trailers. Only motorcycles with engine displacement over may tow. The motorcycle must be marked with its kerbside weight. The trailer must not be wider than . The distance between the rear axle of the motorcycle and the rear-most part of the trailer must not exceed . The laden weight of the trailer must not exceed or two-thirds of the kerbside weight of the motorcycle, whichever is lighter. The trailer must be clearly and indelibly marked with its unladen weight. The UK speed limit is 60 mph on motorways and dual carriageways and 50 mph on all other roads unless a lesser limit is in force. The trailer must display the same registration plate as the towing vehicle. Trailers must have two red sidelights, two red stop lights, a number plate light, two triangular red reflectors and amber indicators (which flash between 60 and 120 times per minute) at the rear. All trailers built after September 30, 1990 also require white front reflectors. Safety Tips for towing include: Motorcycle manufacturers do not recommend that trailers be towed by their motorcycles because of the increased safety hazards. All towing is done at the operator's own risk. A ball hitch, where used, should be well greased at all times to facilitate smooth cornering. While towing a trailer, one must remember to ride closer to the center of the lane, taking the width of trailer into account. Be