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For more details on "prime mover", see ballast tractor.
Oshkosh M1070 8×8 military tractor unit
Peterbilt 386 cab style commercial 6×4 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 hauleing semi-trailers.

Tractor units typically have large displacement diesel engines for power, durability, and economy, and several axles 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.



A Leyland T45 cab-over 4×2 tractor unit

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]


An 8×8 Oshkosh M1070 Heavy Equipment Transporter tractor unit pulling a lowboy style trailer

A tractor unit can have many axles depending on axle load legislation. The most common varieties are those of 4×2, 6×2 and 6×4 types[a]. However, some manufacturers offer 6×6, 8×6, 8×8, 10×8, and 10×10 axle configurations. A 6×4 has three axles, with two of the axles driven. 6×4 units are more common in long distance haulage in larger countries such as the USA and Australia. In Europe, the 4×2 and 6×2 variants are more commonplace.

Tractors with three axles or more can have more than one steering axle, which can also be driven. Most 6×2 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 6×6 units have three axles, all driven, and 8×6 units have four axles, with either the rear three driven and the front axle not or the front and rear-most two axles powered and an unpowered lifting bogey axle to spread the load when needed. The 8×8 units also have four axles, but with all of them driven, and 10×8 units have five axles with the rear four usually driven and the front axle for steering. All five axles of 10×10 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.[1][2][7]

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. ^ Total wheels X driven wheels, with 2 wheels per axle regardless of single or dual tires.

Original courtesy of Wikipedia: http://en.wikipedia.org/wiki/Tractor_unit — Please support Wikipedia.
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1065 news items

Materials Handling World Magazine

Materials Handling World Magazine
Wed, 08 Apr 2015 02:22:30 -0700

Gasrec, Europe's leading supplier of liquefied gas for road transport, has confirmed that the UK's first dedicated Euro 6 gas-powered heavy goods vehicle, a Scania tractor unit which recently entered service with Argos and has already been trialled by ...
Rosemount Town Pages
Mon, 13 Apr 2015 13:23:36 -0700

It is expected to be in use in September of 1965.... The cost with the tractor unit was $22,589.08 with one half to be paid by the State Library Division. This will be the third bookmobile type of unit to be put in service by the Dakota-Scott Regional ...

International Railway Journal

International Railway Journal
Thu, 16 Apr 2015 05:11:15 -0700

The tractor unit can then leave the terminal, even if the train has not arrived. When the train reaches the terminal, freight can be loaded and unloaded simultaneously, and according to CargoBeamer a 36-wagon consist can be handled in just 15 minutes.

Guinness World Records

Guinness World Records
Wed, 25 Mar 2015 10:37:30 -0700

Click the player below to watch wheelman Zhang Hua slide a CGC4222 Dayun tractor unit into a gap just 90 cm (2 ft 11.43 in) wide to set a new record for Tightest truck parallel parking. Zhang, who is part of the Zhengzhi Car Stunts Performance Co .Ltd ...

Belfast Telegraph

Belfast Telegraph
Fri, 10 Apr 2015 02:08:52 -0700

The coffin of 19-year-old Johnny Black was placed on a specially built base on the back of the tractor unit of a lorry as his funeral was held yesterday. Its driver blessed himself as he passed the spot marked by flowers where an accident occurred on ...

Irish Independent

Irish Independent
Sun, 12 Apr 2015 23:37:25 -0700

The engineers at the Blue Oval Badge have excelled this time with a little dynamo that could power a big-rig tractor unit. While it's not peel-your-face-off quick and lacks the massive torque of the diesel, it's punching well above its weight and never ...

Ballymoney and Moyle Times

Ballymoney and Moyle Times
Fri, 10 Apr 2015 23:11:15 -0700

On Thursday the remains of Mr Black were taken from his home at Glenshesk near Ballycastle in a coffin placed on a specially constructed base behind the truck cab of a lorry tractor unit which is normally more associated with pulling 40ft long trailers.
Commercial Motor
Tue, 14 Apr 2015 21:52:30 -0700

The Scania tractor unit recently entered service with Argos and Gasrec said it has already been trialled by a number of other leading retail and logistics companies. Plated at 40 tonnes GTW, the unit is equipped with Scania's nine-litre, Euro-6 engine.

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