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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 hauling 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 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)" (PDF). 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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The Globe and Mail

The Globe and Mail
Tue, 06 Oct 2015 12:18:45 -0700

Police say a semi-tractor unit livestock trailer carrying a full load of two-week old pigs swerved to avoid a moose, lost control and rolled onto its side in the ditch. The impact caused the trailer to break open, allowing the piglets to escape onto ...

Wakefield Express

Wakefield Express
Mon, 05 Oct 2015 00:07:30 -0700

Police say the collision involved a tractor unit - known as a shunter vehicle - and a male pedestrian, and that the incident took place in the loading area of the Wincanton distribution centre. The pedestrian, a man aged in his fifties from the ...
Chandigarh Tribune
Thu, 01 Oct 2015 13:26:15 -0700

HMT Workers Union president Diwan Chand said in 2013, the Centre announced a revival package of Rs 1,083 crore for the tractor unit. The package could not realise the desired objectives as it was “short-sighted” and did not take into confidence all ...

Belfast Newsletter

Belfast Newsletter
Mon, 05 Oct 2015 22:26:15 -0700

He explained: “The Montgomery Transport Group started out with just one tractor unit back in 1970 and has continued to invest and develop throughout the past four and a half decades. We now operate a large fleet of modern distribution and articulated ...

Knutsford Guardian

Knutsford Guardian
Tue, 29 Sep 2015 01:57:02 -0700

Patrick put an appeal for help on Facebook, and four hours after his plea on social media the missing Scania tractor unit was spotted near Liverpool. The Facebook user who saw it contacted the police, and later that day the vehicle was recovered and ...

SHD Logistics

SHD Logistics
Mon, 05 Oct 2015 05:33:45 -0700

British Touring Car Championship contender and RHA-sponsored Tom Ingram drove a 44 tonne HGV tractor unit around the track following his own rigorous HGV training process. “Haulage and logistics is an exciting and dynamic industry in which to forge a ...

Essex Chronicle

Essex Chronicle
Thu, 17 Sep 2015 09:57:22 -0700

A TRACTOR DRIVER who careered into a nearside bank off the A12 whilst drunk was banned from driving yesterday. Kevin Rayner, of Walters Close, Leigh-on-Sea, was driving a Volvo heavy tractor unit at around midnight on August 31. Police were called to ...

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Fri, 02 Oct 2015 16:03:45 -0700

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