A tractor unit, semi tractor (U.S.), prime mover (Australian English), road tractor, or traction unit is typically a heavy-duty military tractor or commercial freight vehicle within the large goods vehicle category, though medium duty models are made. Tractor units typically have large displacement diesel engines for power, durability, and economy, and several axles for maximum flexibility in gearing.
A tractor unit's function it to pull trailers (most often semi-trailers). Different trailers performing different functions, such as a bulk tipper and box trailer, or containing differing cargos can be rapidly swapped between tractors, eliminating downtime while a trailer is unloaded or loaded.
The tractor-trailer combination also 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.
In the USA and Australia in particular, owner drivers, and even fleet operators, go to great lengths to customise their tractors. This often involves liberal chrome plating, fitting larger exhaust systems and extra lights, and custom paint. Customized tractors are sometimes mated with matching trailers.
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 the North America. The cab over engine has a flat nose cab with the driver sitting in front of the front axle. Widely used in the EU, 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 strait 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 in the U.S.
A tractor unit can have many axles depending on axle load legislation. The most common varieties are those of 4x2, 6x2 and 6x4 types[a]. However, some manufacturers offer 6x6, 8x6, 8x8, 10x8, and 10x10 axle configurations. A 6x4 has three axles, with two of the axles driven. 6x4 units are more common in long distance haulage in larger countries such as the USA and Australia. In Europe, the 4x2 and 6x2 variants are more commonplace.
Tractors with three axles or more can have more than one steering axle, which can also be driven. Most 6x2 units allow the undriven rear axle to be raised when lightly loaded, or running without a trailer, to save tyre wear, and increase traction. The 6x6 units have three axles, all driven, and 8x6 units have four axles, with the rear three usually driven and the front axle for steering. The 8x8 units also have four axles, but with all of them driven, and 10x8 units have five axles with the rear four usually driven and the front axle for steering. All five axles of 10x10 units are driven. The front two axles are usually both steer axles. The axle configurations are usually based on axle load legislation, and maximum gross vehicle weight ratings (BDM).
Heavier versions of tractor units, such as those used in heavy haulage and road trains, tend to have four or more axles, with more than two axles driven. In certain countries (such as Switzerland), a certain amount of weight must be spread over driven axles, which lead to heavier varieties having six-wheel drive, otherwise another tractor unit would have to be used. Heavy haulage variants of tractor-units are often turned into a ballast tractor by fitting temporary ballast, which may require special permitting.
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- Total wheels X driven wheels, with 2 wheels per axle regardless of single or dual tires.