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Main article: truck
For the agricultural machine; see: Tractor

A tractor unit, traction unit, road tractor or prime mover (Australian English) is a heavy-duty vehicle, usually with a large diesel engine and several axles. The tractor unit serves as a method of moving trailers. Trailers can be swapped between tractor units quickly so the 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 that a load can be shared across many axles, yet be more manoeuvrable than a equivalently sized rigid truck (see trucks). The tractor unit couples to the trailer using some sort of mechanical lock system, usually a fifth wheel. A tractor unit is primarily designated a freight vehicle and is most common in the heavy goods vehicle class of vehicles although smaller van based tractor units do exist.

Axle configuration

A tractor unit can have many axles depending on axle load legislation. The most common varieties are those of 4x2, 6x2 and 6x4 types, where a 6x4 has 3 axles with 2 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 3 axles or more can have more than one steering axle, which can be also driven. Most 6x2 units allow the un-driven rear axle to be raised when lightly loaded or running without a trailer to save tyre wear and increase traction. The axle configurations are usually based upon axle load legislation and maximum GVWs. Heavier versions of tractor units, such as those used in heavy haulage and road trains, tend to have 4 or more axles with more than one axle driven. In certain countries (such as Switzerland), a certain amount of weight must be spread over driven axles which lead to heavier varieties having all 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 GVW which may go beyond the legal limit, hence requiring special permits.


See also: Grapple truck

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 stabilizers like those on mobile cranes. These cranes can come in sizes from a few tones up to special heavy duty ones that can lift in excess of 80 ton/mtr for lifting 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.

Tanker haulage

In the UK, 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) and enclosing all wiring in conduit.


In the USA and Australia in particular, owner drivers, and even fleet operators go to great lengths to customize their vehicle. This involves fitting larger exhausts, extra lights and custom paint among other things. Such tractor units attract lots of attention and can be beneficial for promoting their business, such tractors can be matched with a customized trailer. Some companies have one custom truck, or at least one tractor unit that is different or more powerful than the rest to act as a flag ship.

Regional variations

In the United Kingdom this type of "tractor" is often called an "artic",short for articulated lorry.

In the United States and Canada they are referred to as a Semi-trailer trucks .

See also

References / sources

External links

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