A semi-trailer is a trailer without a front axle. The combination of a semi-trailer and a tractor truck is called a semi-trailer truck (also known simply as a "semi-trailer", "tractor trailer", or "semi" in the United States). [1]
A large proportion of a semi-trailer's weight is supported by a tractor unit, or a detachable front-axle assembly known as a dolly, or the tail of another trailer. The semi-trailer's weight is semi-supported (half-supported) by its own wheels, at the rear of the semi-trailer. [2] [3] [4] [5] A semi-trailer is normally equipped with landing gear (legs which can be lowered) to support it when it is uncoupled. Many semi-trailers have wheels that are capable of being totally dismounted and are also relocatable [6] (repositionable) to better distribute load to bearing wheel weight factors. [7] Semi-trailers are more popular for transport than full trailers, which have both front and rear axles. Ease of backing is cited as one of the semi's chief advantages. A road tractor coupled to a semi-trailer is often called a semi-trailer truck or "semi" in North America and Australia, and an articulated lorry or "artic" in the UK.
Semi-trailers with two trailer units are called B-doubles (Australian English) or tandem tractor-trailers, [8] tandem rigs, or doubles (American English). Other terms used are "B-train" or (when there are three or more trailers) "road train". A double-trailer combination is possible with the use of a dolly, or "converter dolly" (Australian and American English), essentially one to three additional axles placed under the front of a second semi-trailer. The first semi-trailer is connected to the power unit using the tractor's fifth wheel coupling while the converter dolly, already attached to the second semi-trailer, is connected to the first semi-trailer with a drawbar. In Australian English, the tractor unit is called a "prime-mover", and the combination of a prime-mover and trailer is known as a "semi-trailer", "semi" or single. Some popular manufacturers are Kenworth, Iveco, Freightliner, MAN, Scania, Mercedes-Benz, DAF, Renault Trucks, Volvo, Peterbilt, and Mack.
Semi-trailers were invented by August Fruehauf in 1914.
Road tractors and semi-trailers are responsible for carrying a large proportion of cargo. With 1,170,117 million tonne-kilometers transported this way in the European Union, including the UK, road tractors and semi-trailers are 77.6% of the total tonne-kilometers transported in 2015, according to Eurostat. [9]
In road haulage, semi-trailers predominate over full trailers because of their flexibility. The trailers can be coupled and uncoupled quickly, allowing them to be shunted for loading and to be trucked between depots. If a power unit fails, another tractor can replace it without disturbing the cargo.
Compared with a full trailer, a semi-trailer attached to a tractor unit is easier to reverse, since it has only one turning point (the coupling), whereas a full trailer has two turning points (the coupling and the drawbar attachment). Special tractors are known as shunt trucks or shuttle trucks can easily maneuver semi-trailers at a depot or loading and unloading ferries. These tractors may lift the coupling so the trailer legs clear the ground.
A rigid truck and full trailer are articulated inside the cargo area length, so a semi-trailer can have a longer continuous cargo area. Because of this, a semi-trailer can haul longer objects, (logs, pipe, beams, railway track). This depends on the legislation; in some European countries, a full trailer can be as long as a semi-trailer. However, since a rigid truck is longer than a semi-tractor, this increases the overall length of the combination, making it less maneuverable.
For heavy haulage or for increased manoeuvrability, some semi-trailers are fitted with rear-wheel steering, controlled electro-hydraulically. The wheels on all or some of the rear axles may be turned through different angles to enable tighter cornering, or through the same angle (so-called 'crab' steering) to move the rear of the trailer laterally.
The two types of couplings are fifth-wheel coupling and automatic. In some applications, no separable coupling is fitted, and the trailer is bolted to the tractor unit, using a bearing, and rocker feet as are used under a fifth wheel skid plate.
The towing vehicle has a wide coupling plate known as a fifth-wheel coupling bolted onto its chassis, on which the semi-trailer rests and pivots. As the tractor reverses under the trailer, a kingpin under the front of the trailer slides into a slot in the skid plate, and the jaws of the fifth wheel close onto it. The driver has to raise the trailer legs manually and couple the airbrake lines and electrical cables. Some low-set trailers such as lowboys/low-loaders and car transporters have electrically powered landing gear due to the necessarily low clearance prohibiting conventional landing gear.
Different types of semi-trailers are designed to haul different cargoes.
Common widths are 8 ft (2.44 m), [10] and 2.6 metres (102.36 in). [11] Generally speaking, most North American type trailers use two axles with dual-tire hubs totaling 8 wheels, while most European type trailers use three axles with single-tire hubs totaling 6 wheels, with one of the axles being able to be lifted for lighter loads and saving on tire, brake, and axle wear. Nearly all sufficiently tall modern trailers are equipped with a rear underride guard to prevent cars from passing beyond the rear edge of the trailer, and most also have side underride guards for the same reason. There are also other smaller differences with regards to kingpin depth, lighting, door locks, et cetera, though most purpose-built tractor trucks can carry most types of trailer regardless of which continent it was built on and the differences therein.
A tank trailer is a semi-trailer specifically designed to carry liquids, fluids and gases. [13]
A semi-trailer truck is the combination of a tractor unit and one or more semi-trailers to carry freight. A semi-trailer attaches to the tractor with a type of hitch called a fifth wheel.
A road train, also known as a land train or long combination vehicle (LCV) is a semi-truck used to move road freight more efficiently than single-trailer semi-trucks. It consists of one semi-trailer or more connected together with or without a tractor. It typically has to be at least three trailers and one tractor. Road trains are often used in areas where other forms of heavy transport are not feasible or practical.
A trailer is an unpowered vehicle towed by a powered vehicle. It is commonly used for the transport of goods and materials.
A dump truck, known also as a dumping truck, dump trailer, dumper trailer, dump lorry or dumper lorry or a dumper for short, is used for transporting materials for construction as well as coal. A typical dump truck is equipped with an open-box bed, which is hinged at the rear and equipped with hydraulic rams to lift the front, allowing the material in the bed to be deposited ("dumped") on the ground behind the truck at the site of delivery. In the UK, Australia, South Africa and India the term applies to off-road construction plants only and the road vehicle is known as a tip lorry, tipper lorry, tipper truck, tip truck, tip trailer or tipper trailer or simply a tipper.
A flatcar (US) is a piece of rolling stock that consists of an open, flat deck mounted on trucks (US) or bogies (UK) at each end. Occasionally, flat cars designed to carry extra heavy or extra large loads are mounted on a pair of bogies under each end. The deck of the car can be wood or steel, and the sides of the deck can include pockets for stakes or tie-down points to secure loads. Flatcars designed for carrying machinery have sliding chain assemblies recessed in the deck.
A dolly is an unpowered vehicle designed for connection to a tractor unit, truck or prime mover vehicle with strong traction power.
The M19 tank transporter was a heavy tank transporter system used in World War II and into the 1950s. It consisted of a 12-ton 6×4 M20 Diamond T model 980 truck and companion 12-wheel M9 trailer.
A tractor unit, also known as a truck unit, lorry unit, power unit, prime mover, ten-wheeler, semi-tractor, semi-truck, semi-lorry, tractor cab, truck cab, lorry cab, big rig tractor, big rig truck or big rig lorry or simply a tractor, truck, lorry, semi, big rig or rig, is a characteristically heavy-duty towing engine that provides motive power for hauling a towed or trailered load. These fall into two categories: heavy- and medium-duty military and commercial rear-wheel-drive semi-tractors used for hauling semi-trailers, and very heavy-duty typically off-road-capable, often 6×6, military and commercial tractor units, including ballast tractors.
The fifth-wheel coupling provides the link between a semi-trailer and the towing truck, tractor unit, leading trailer or dolly. The coupling consists of a kingpin, a 2-or-3+1⁄2-inch-diameter vertical steel pin protruding from the bottom of the front of the semi-trailer, and a horseshoe-shaped coupling device called a fifth wheel on the rear of the towing vehicle. As the connected truck turns, the downward-facing surface of the semi-trailer rotates against the upward-facing surface of the fixed fifth wheel, which does not rotate. To reduce friction, grease is applied to the surface of the fifth wheel. The configuration is sometimes called a turn-table in Australia and New Zealand, especially if it is a rotating ball-race-bearing type. The advantage of this type of coupling is towing stability.
Towing is coupling two or more objects together so that they may be pulled by a designated power source or sources. The towing source may be a motorized land vehicle, vessel, animal, or human, and the load being anything that can be pulled. These may be joined by a chain, rope, bar, hitch, three-point, fifth wheel, coupling, drawbar, integrated platform, or other means of keeping the objects together while in motion.
A flatbed truck is a type of truck the bodywork of which is just an entirely flat, level 'bed' with no sides or roof. This allows for quick and easy loading of goods, and consequently they are used to transport heavy loads that are not delicate or vulnerable to rain, and also for abnormal loads that require more space than is available on a closed body. Flatbed trucks can be either articulated or rigid.
An articulated hauler, articulated dump truck (ADT), or sometimes a dump hauler, is a very large heavy-duty type of dump truck used to transport loads over rough terrain, and occasionally on public roads. The vehicle usually has all-wheel drive and consists of two basic units: the front section, generally called the tractor, and the rear section that contains the dump body, called the hauler or trailer section. Steering is made by pivoting the front in relation to the back by hydraulic rams. This way, all wheels follow the same path, making it an excellent off-road vehicle.
A ballast tractor is a specially weighted tractor unit of a heavy hauler combination. It is designed to utilize a drawbar to pull or push heavy or exceptionally large trailer loads which are loaded in a hydraulic modular trailer. When feasible, lowboy-style semi-trailers are used to minimize the height of a load's center of mass. Typical drivetrains are 6×4 and 6×6, but 8×6 and 8×8 are also available. Typical ballast tractor loads include oil rig modules, bridge sections, buildings, ship sections, and industrial machinery such as generators and turbines.
The M39 series 5-ton 6×6 truck (G744) was a family of heavy tactical trucks built for the United States Armed Forces. The basic cargo version was designed to transport a 5-ton (4,500 kg), 14 ft (4.3 m) long load over all terrain in all weather. In on-road service the load weight was doubled.
A specialized set of jargon describe the tools, equipment, and employment sectors used in the trucking industry in the United States. Some terms may be used within other English-speaking countries, or within the freight industry in general. For example, shore power is a term borrowed from shipping terminology, in which electrical power is transferred from shore to ship, instead of the ship relying upon idling its engines. Drawing power from land lines is more efficient than engine idling and eliminates localized air pollution. Another borrowed term is "landing gear", which refers to the legs which support the front end of a semi-trailer when it is not connected to a semi-truck. Some nicknames are obvious wordplay, such as "portable parking lot", in reference to a truck that carries automobiles.
A tank transporter is a combination of a heavy tractor unit or a ballast tractor and a mating full trailer, hydraulic modular trailer or semi-trailer, used for transporting tanks and other armoured fighting vehicles. Some also function as tank recovery vehicles, the tractors of which may be armoured for protection in combat conditions.
A lowboy is a semi-trailer with two drops in deck height: one right after the gooseneck and one right before the wheels. This allows the deck to be extremely low compared with other trailers. It offers the ability to carry legal loads up to 12 ft (3.66 m) tall, which other trailers cannot. Lowboys are used to haul heavy equipment such as bulldozers and large industrial equipment.
A car carrier trailer, also known as a car-carrying trailer, car hauler, or auto transport trailer, is a type of trailer or semi-trailer designed to efficiently transport passenger vehicles via truck.
The Mack M123 (G792) was a 10-ton 6x6 semi-tractor introduced in 1955. The Mack M125 was a heavy cargo truck version of the M123. The M123 was used to tow tank transporter trailers while the M125 towed field artillery pieces.
The M809 Series 5-ton 6x6 truck (G908) was a family of heavy tactical trucks built for the United States Armed Forces. The basic cargo version was designed to transport a 5-ton (4,500 kg), 14 ft (4.3 m) long load over all terrain in all weather. In on-road service the load weight was doubled. Built by AM General, they evolved into the M939 Series.