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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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1157 news items

Boston Globe

Boston Globe
Mon, 31 Aug 2015 07:39:45 -0700

... hours earlier, Route 24 northbound was closed near the rest stop in Bridgewater as firefighters doused a 2015 Freightliner tractor trailer that caught fire, Fitch said. The driver of that truck was not injured, but the fire eventually consumed the ...


Wed, 26 Aug 2015 12:12:26 -0700

"We believe part of the drive train of the tractor unit broke loose, popped up into the trailer, which caused a spark that ignited the jet fuel that was in the tanker truck," Sgt. James Senecal, of the Colorado State Patrol, said. "The driver was ...

Victorville Daily Press

Victorville Daily Press
Fri, 14 Aug 2015 09:19:34 -0700

4 lane around 12:10 a.m. Authorities said that it appeared he did not notice traffic slowing to around 55 mph in front of him and he collided with the back of the semi-truck tractor unit, which was not towing a trailer at the time. The force of the ...

Hull Daily Mail

Hull Daily Mail
Fri, 28 Aug 2015 01:55:21 -0700

Mr Shipman, of Dunkeld Drive, north Hull, has come up with the concept of an ambulance "pod" attached to a tractor unit. The pod would contain all the medical equipment currently inside an ambulance but could be detached from the tractor unit and left ...

Daily American Online

Daily American Online
Wed, 12 Aug 2015 19:59:55 -0700

An 86-mile section of the Pennsylvania Turnpike was closed in both directions between New Stanton and Breezewood Wednesday morning as a result of a tractor-trailer accident at milepost 138 eastbound. State police said the tractor-trailer was traveling ...

Market Rasen Mail

Market Rasen Mail
Fri, 28 Aug 2015 23:33:45 -0700

Additional attractions will include a display of model trucks – complemented by several full-size vehicles, including a vintage AEC Mandator tractor unit together with three or four more modern outfits, all of which is likely to appeal particularly to ...


Thu, 20 Aug 2015 05:11:15 -0700

From the information that we have been sent it seems that the tractor unit skidded off the road shortly after passing a toll booth and was then stranded on the grass. The crane, a Zoomlion QY30V was called in to lift it back on to the road and then ...

Oxford Mail

Oxford Mail
Tue, 25 Aug 2015 02:10:08 -0700

“In order to steal his truck, diesel had been syphoned from a tractor unit that was in the yard.” The court heard that 29-year-old Noon had been caught after the pipe used to syphon the diesel had been forensically tested and his DNA found on it. Miss ...

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