Tractor unit

A tractor unit (prime mover or traction unit) is a characteristically heavy-duty towing engine that provides motive power for hauling a towed or trailered load. These fall into two categories: very heavy-duty typically off-road capable, often 6×6, military and commercial tractor units, including ballast tractors, and heavy and medium duty military and commercial rear-wheel drive "semi tractors" used for hauling semi-trailers.

Tractor units typically have large displacement diesel engines for power, durability, and economy; several axles; and a multi-ratio transmission (10, 13, or 18 gears) for maximum flexibility in gearing.

The tractor-trailer combination distributes a load across multiple axles while being more maneuverable than an equivalently sized rigid truck. The most common trailer attachment system is a fifth wheel coupling, allowing rapid shift between trailers performing different functions, such as a bulk tipper and box trailer. Trailers containing differing cargos can be rapidly swapped between tractors, eliminating downtime while a trailer is unloaded or loaded.

Drawbar couplings are also found, particularly in dedicated exceptionally heavy-duty ballast tractors.

There have been three common cab configurations used in tractors, two are still widely used. The conventional has an engine and hood over the front axle in front of the cab, as in most automobiles. This style is almost universal in North America. The cab over engine or cab forward has a flat nose cab with the driver sitting in front of the front axle. Widely used in the EU and Japan, this style has the advantages of good vision, maneuverability, and allows maximum trailer length relative to overall length. In the North America this type of cab can be useful in straight trucks, but now has little advantage in tractors and is rarely used. A North American style cab over engine, now largely obsolete, had a flat nose cab located higher over the engine, with the driver sitting above the front axle. This allowed a sleeper compartment in a short tractor, and maximum wheelbase relative overall length, important for bridge formula weight restrictions. With the loosening of length restrictions in 1982 this style had limited applications, and is no longer manufactured for the U.S. market. This style is still however popular in Australia where length restrictions still apply on many roads and it is used to maximise the capacity of both single trailer and B double configurations, and American companies Freightliner and Kenworth still manufacture trucks in this style for this market. In Australia both styles of cab over engine truck as well as conventionals are in common use.

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Wikipedia