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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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1012 news items

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Mon, 23 Mar 2015 05:56:15 -0700

Raymond Brown Waste Solutions has taken delivery of a new TGS 26.440 6x4 tractor unit, following a 10-year absence from the German giant. Steve Clasby, managing director of Raymond Brown, says on-site manoeuvrability was among the key deciding ...

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Thu, 26 Mar 2015 18:07:30 -0700

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Thu, 19 Mar 2015 12:49:34 -0700

A truck and trailer unit containing highly flammable materials burst into flames near Wellington this morning, blocking off State Highway 1. The fire broke out just after 6am near the Paremata roundabout. It has since been brought under control.


Thu, 19 Mar 2015 15:26:51 -0700

So I detached the tractor unit from the front trailer and drove the truck up the highway." The two trailers, containing paint thinner and aerosol cans, were burnt out. McGregor picked the rear trailer - which was the first to catch fire - at a Taupo ...
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