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A Leyland T45 tractor unit

A tractor unit, semi tractor (U.S.) [a], 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.

Cab configurations[edit]

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.[1][2][3][4][5][6]

Axle configuration[edit]

A tractor unit can have many axles depending on axle load legislation. The most common varieties are those of 4x2, 6x2 and 6x4 types[b]. 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 permits.[1][2][7]


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 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[clarification needed] - 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.

Tanker haulage[edit]

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 although all customizations are done within legal limits. 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.

See also[edit]


  1. ^ a b "Freight Management and Operations: Bridge Formula Weights". U.S. Department of Transportation. Retrieved 25 June 2013. 
  2. ^ a b "Freight Management and Operations: Size Regulations". U.S. Department of Transportation. Retrieved 25 Jun 2013. 
  3. ^ "International Trucks (U.S.)". Navistar, Inc. 2013. Retrieved 25 June 2013. 
  4. ^ "Mack Trucks (U.S.)". Mack Trucks, Inc. 2013. Retrieved 25 June 2013. 
  5. ^ "Mercedes-Benz Truck (Germany)". Daimler AG. 2013. Retrieved 25 June 2013. 
  6. ^ "Volvo Trucks (Worldwide)". AB Volvo. Retrieved 25 June 2013. 
  7. ^ "Guidelines on Maximum Weights…Criteria (EU)". Road Safety Authority. 2013. Retrieved 25 June 2013. 


  1. ^ In the U.S. full tractors are not used in on-highway applications.
  2. ^ Total wheels X driven wheels, with 2 wheels per axle regardless of single or dual tires.

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48 news items


Sun, 20 Jul 2014 23:00:00 -0700

John, 77, was on his way home from covering a car launch event in Spain when the accident occurred on 16 January. His Nissan Qashqai was crushed between Rourke's lorry and a Mercedes-Benz Tractor Unit in front. He was pronounced dead at the scene.
New Zealand Police (press release)
Thu, 17 Jul 2014 01:15:00 -0700

Sergeant Graeme Crosson Officer in Charge Hurunui Police advises that the operation to locate the missing driver from the serious crash on State Highway One, Hundalee's yesterday morning at about 5.30am is still yet to locate him. Today the tractor ...

The Courier

The Courier
Thu, 17 Jul 2014 02:35:12 -0700

He admitted causing the trailer to topple over, detach from the tractor unit and fall on to the off side lane of the dual carriageway where it collided with a safety barrier. The court heard from defence solicitor Mike Ferrie that while working for ...
Thu, 17 Jul 2014 11:56:15 -0700

The police operation to find a missing driver of the truck which crashed on State Highway 1, near Hundalee, Wednesday about 5.30am is on-going. Sergeant Graeme Crosson says the drivcer has not been located yet. Yesterday, the tractor unit and trailer ...
Wed, 16 Jul 2014 13:03:33 -0700

A crane has managed to lift the back of unit onto the road today, while salvagers are still working to get to the tractor unit of the truck. The driver of the truck, a man believed to be aged in his 50s, is presumed dead. "I think there's always a ...

Newstalk ZB

Newstalk ZB
Thu, 17 Jul 2014 15:38:17 -0700

A crane has lifted the trailer and tractor unit out and police will now sort through the scattered freight the truck was carrying in an attempt to find the missing driver. Sergeant Graeme Crosson is also hopeful a second go with a sniffer dog will have ...
Motor Transport (blog)
Mon, 14 Jul 2014 09:52:30 -0700

Carrier Transicold UK has modified Arla Foods' tri-axle combination trailer so it can power the motors used when loading and discharging raw milk, instead of using a tractor unit with power take-off (PTO). The MT Award-winning trailer, which is made up ...

International Railway Journal

International Railway Journal
Fri, 11 Jul 2014 01:48:45 -0700

The crane will be hauled to the worksite by a purpose-built tractor unit, and Kirow says it will be possible to control the crane remotely from the powered vehicle. Like the crane, the underframe and superstructure of the single-cab tractor unit will ...

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