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In the middle of the 20th century, some tractor units became widespread on our roads. A tractor unit is a type of tractor that works with semi-trailers that are attached to the vehicle using a special coupling mechanism, a fifth-wheel coupling. One of the most common modifications of medium and heavy trucks. It is usually created on the basis of existing conventional trucks by partially changing, or rather simplifying the design. The frame is made much shorter than that of a conventional truck, and the body superstructure is replaced with a fifth-wheel coupling.