A tractor unit, semi tractor (U.S.), prime mover (Australian English), road tractor, or traction unit is a heavy-duty commercial vehicle within the large goods vehicle category, usually with a large displacement diesel engine, and several axles. The tractor unit serves as a method of moving trailers (most often semi-trailers). Different trailers can be swapped between tractor units quickly, so the tractor unit does not stand idle whilst the load is unloaded or loaded, unlike a rigid, and is not restricted to one type of goods as trailer types can be swapped e.g. bulk tipper to a box van. The tractor-trailer combination also means a load can be shared across many axles, yet be more maneuverable than an equivalently sized rigid truck (see trucks). The tractor unit couples to the trailer using some sort of mechanical lock system, usually a fifth wheel coupling. A tractor unit is primarily designated a freight vehicle, and is most common in the heavy goods vehicle (HGV) class of vehicles, although smaller, van-based tractor units do exist.
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. 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. Those 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. The heavy haulage variants of tractor-units are often turned into a ballast tractor by fitting temporary ballast, because their chassis allows a high BDM which may go beyond the legal limit, hence requiring special permit.
A tractor unit can sometimes have a mounted crane located behind the cab. The tractor unit can still pull a trailer using its fifth wheel, although it will carry less payload, due to the extra bulk of the crane. A crane truck is able to lift its load on site, and usually comes with stabilisers like those on mobile cranes. These cranes can come in sizes from a few tonnes - up to special heavy-duty ones that can lift in excess of 80 ton/mtr - for lifting intermodal containers and office units. Unlike a rigid truck with a crane, the tractor and crane have the advantage that they can be separated and repositioned relative to the load if necessary.
In the European Union and non-EU countries that have signed the ADR agreement, tractor units which haul tankers carrying flammable liquids must be specially modified to reduce the fire risk. This includes using a fully insulated electrical system (neither side earthed to the frame), relocation of exhaust components, and enclosing all wiring in conduit. Additional requirements include mandatory ABS on both the tractor and the trailer, additional 6-kilogram (13 lb) fire extinguishers for putting out a fire in the tanker, leak containment equipment, etc.
In countries that have signed the ADR agreement, a driver is required to have special training for hauling dangerous goods (including flammable liquids) in tanks. This is a supplement to the main ADR training certificate.
In the USA and Australia in particular, owner drivers, and even fleet operators, go to great lengths to customise their vehicle in a similar fashion to a custom car. This involves fitting larger exhaust systems (often highly polished stainless steel or chrome plating), extra lights and custom paint, among other things. Such tractor units attract lots of attention, and can be beneficial for promoting their businesses; such tractors can be matched with customised trailers. Some companies have one custom truck, or at least one tractor unit that is different, or more powerful, than the rest, to act as their flagship.
