"heavy trucks" are the heaviest trucks that are allowed on the road. in the u.k. they are known as "lorries". usually heavy trucks have three axles: one at front and two at rear, and the rear axles have coupled wheels to enable heavy loads. often heavy trucks pull trailers. they can be either "full trailers" which have both front and rear axles or boggies and their own brakes, and are connected to the towing truck with a towbar, or "semi-trailers", which are attached to a special kind of a truck called "tractor unit" with a turntable coupling ("fifth wheel"). a semi-trailer is a kind of trailer which has wheels only in the back and the front rides on the back of the tractor unit. the tractor unit has an engine and the semi-trailer does not. driving a heavy truck requires a professional driver's license. truck drivers are called "truckers".