Semitrailer
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A semi-trailer is a trailer without a
front axle An axle or axletree is a central shaft for a rotating wheel or gear. On wheeled vehicles, the axle may be fixed to the wheels, rotating with them, or fixed to the vehicle, with the wheels rotating around the axle. In the former case, bearin ...
. The combination of a semi-trailer and a
tractor truck 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, ...
is called a ''
semi-trailer truck A semi-trailer truck (also known by a wide variety of other terms – see below) 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 ...
'' (also known simply as a "semi-trailer", "tractor trailer", or "semi" in the
United States The United States of America (USA), also known as the United States (U.S.) or America, is a country primarily located in North America. It is a federal republic of 50 U.S. state, states and a federal capital district, Washington, D.C. The 48 ...
). A large proportion of a semi-trailer's weight is supported by a
tractor unit 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, ...
, 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. 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 (repositionable) to better distribute load to bearing wheel weight factors. 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 A semi-trailer truck (also known by a wide variety of other terms – see below) 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 ...
or "semi" in North America and Australia, and an
articulated lorry A semi-trailer truck (also known by a wide variety of other terms – see below) 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 ...
or "artic" in the UK. Semi-trailers with two trailer units are called B-doubles (Australian English) or tandem tractor-trailers, tandem rigs, or doubles (American English). Other terms used are "
B-train A road train, also known as a land train or long combination vehicle (LCV) is a semi-trailer truck, semi-trailer used to move road freight more efficiently than single-trailer semi-trailers. It consists of one semi-trailer or more connected ...
" or (when there are three or more trailers) "
road train A road train, also known as a land train or long combination vehicle (LCV) is a semi-trailer used to move road freight more efficiently than single-trailer semi-trailers. It consists of one semi-trailer or more connected together with or wit ...
". 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 Kenworth Truck Company is an List of American truck manufacturers, American truck manufacturer. Founded in 1923 as the successor to Gersix Motor Company, Kenworth specializes in production of heavy-duty (Truck classification#Class 8, Class 8) ...
,
Iveco Iveco S.p.A., an acronym for Industrial Vehicles Corporation, is an Italian multinational transport vehicle manufacturing company with headquarters in Turin, Italy. It designs and builds light, medium, and heavy Commercial vehicle, commercial veh ...
, Freightliner,
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,
Scania Scania ( ), also known by its native name of Skåne (), is the southernmost of the historical provinces of Sweden, provinces () of Sweden. Located in the south tip of the geographical region of Götaland, the province is roughly conterminous w ...
,
Mercedes-Benz Mercedes-Benz (), commonly referred to simply as Mercedes and occasionally as Benz, is a German automotive brand that was founded in 1926. Mercedes-Benz AG (a subsidiary of the Mercedes-Benz Group, established in 2019) is based in Stuttgart, ...
, DAF,
Renault Trucks Renault Trucks was a French commercial truck manufacturer with corporate headquarters at Saint-Priest near Lyon. Originally part of Renault, it was a subsidiary of Volvo since 2001. In July 2024, John Cockerill (company) completed the takeover ...
,
Volvo The Volvo Group (; legally Aktiebolaget Volvo, shortened to AB Volvo, stylized as VOLVO) is a Swedish multinational manufacturing corporation headquartered in Gothenburg. While its core activity is the production, distribution and sale of truck ...
,
Peterbilt Peterbilt Motors Company is an List of American truck manufacturers, American truck manufacturer specializing in the production of heavy-duty (Truck classification#Class 8, Class 8) and medium-duty (Classes 5–7) commercial vehicles. The names ...
, and Mack.


Properties

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. 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 truck A terminal tractor, known in the United States as a shag truck, shunt truck, spotter truck, spotting tractor, yard truck, yard shifter, yard dog, yard goat, yard horse, yard mule, yard jockey, yard spotter, hostler, or mule, is a kind of semi-t ...
s 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 Railway track ( and UIC terminology) or railroad track (), also known as permanent way () or "P way" ( and Indian English), is the structure on a railway or railroad consisting of the rails, fasteners, sleepers ( railroad ties in American ...
). 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.


Couplings

The two types of couplings are
fifth-wheel coupling 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 vertical steel pin protruding from the bottom of the front of the semi- ...
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.


Fifth-wheel coupling

The towing vehicle has a wide coupling plate known as a
fifth-wheel coupling 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 vertical steel pin protruding from the bottom of the front of the semi- ...
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.


Types

Different types of semi-trailers are designed to haul different cargoes. Common widths are , and . 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. *Box or van trailers are the most common type. They consist of a metal box on wheels with some doors on the back, though some offer additional access doors on the sides. Standard lengths in North America are , , , , , , , and . Due to maximum length regulations and the need to maximize cargo within said regulations, almost all European semi-trailers are in length. * Bus-bodied trailers are hitched to a tractor unit to form a trailer bus, a simple alternative to building a rigid bus. *
Car carrier trailer A car carrier trailer, also known as a car-carrying trailer, car hauler, or auto transport trailer, is a type of trailer (vehicle), trailer or semi-trailer designed to efficiently transport passenger vehicles via truck. Modern car carrier trailer ...
s carry multiple cars, usually new cars from the manufacturer. In the US and elsewhere, car carriers often carry used vehicles as well. Similar variants can carry forklift trucks, light commercial vehicles, and agricultural tractors. Single-deck versions are used for larger vehicles. *Conestoga trailers are a special form of a flatbed trailer with a flexible retractable roof and siding used to carry large pieces of equipment that can only be lifted by an overhead crane, but which also need protection from the weather during transit. * Curtain siders or tautliners are similar to box trailers except the sides are movable curtains made of reinforced fabric coated with a waterproof coating. To put it another way, it is basically a flatbed trailer with additional aerodynamic and weather protection as well as greater load security. They generally have a restraining system of straps and buckles every foot (30 cm) or so to keep the curtain tight and adverse to the elements. Also, some have removable gates mounted into the trailer, to help reinforce the load and prevent bulges. The purpose of a curtain sider is to combine the security and weather resistance of a box trailer with the ease of loading of a flatbed. Curtain siders are one of the most common trailer types in Europe. *Drop-deck trailers (also called step deck, or step-frame trailers) have a floor that drops down a level once clear of the tractor unit; the most common types of drop-deck trailers are flatbeds and curtain siders. *Double deckers or deckers have either a fixed, hinged, or moveable second floor to enable them to carry more palletized goods. In general, a double-decker can carry 40 pallets, as opposed to 26 for a standard trailer. Double-deck trailers are generally a step-frame construction with the majority being either box or curtain siders, with box trailers having either a fixed or movable (floating) deck, and curtain sides having either a fixed or hinged second deck; this hinged second deck generally swings into a position down the length of the trailer and can be divided into two or three sections to allow greater load flexibility. In Australia and New Zealand, they are known as mezzanine trailers or mezz-decks for short. *Dry bulk ("British" powder tankers) trailers resemble big tankers but are used for cement, sand,
barite Baryte, barite or barytes ( or ) is a mineral consisting of barium sulfate (Ba S O4). Baryte is generally white or colorless, and is the main source of the element barium. The ''baryte group'' consists of baryte, celestine (strontium sulfate), ...
, flour, and other dry powder materials. *Dump trailers ("British" tippers) are trailers in which one end can be raised to allow the cargo (often building materials or agricultural produce) to slide out the other end. Commonly, they are hinged at the rear and raised at the front, but side-unloading dump trailers also are used. *Flatbeds, or flat decks, consist of just a load floor and removable side rails and a bulkhead in front to protect the tractor in the event of a load shift. They can haul almost anything that can be stacked on and strapped down. *Hopper bottoms or grain hopper trailers are usually used to haul grain, but can be used to haul other materials. *Inloaders are used to transport glass panes on stillages. They feature an open bottom to maximize loading height and the entire trailer can be lifted and lowered during loading, to allow safe loading of fragile glass panels. *Intermodal trailers are similar in frame design to box trailers, but without any sort of integral cargo-carrying ability as they are designed to carry a variety of standard
intermodal container An intermodal container, often called a shipping container, or a freight container, (or simply "container") is a large metal crate designed and built for intermodal freight transport, meaning these containers can be used across different Mode ...
s * Live bottom trailers have a conveyor belt on the bottom of the trailer tub that pushes the load material out of the back of the trailer. The tub does not have to be raised to deposit the materials. * Livestock trailers are used to haul livestock such as cattle, horses, pigs, sheep, etc. Commonly, they have two levels (or three for hogs) to maximize capacity. * Lowboy ("British/Australian" low-loader) trailers are a type of flatbed in which the load floor is as close to the ground as possible, most commonly used to haul heavy equipment, cranes, bulldozers, etc. * Refrigerator trailers are box trailers with a heating/cooling unit (''reefer'') attached and insulated walls, used for hauling produce, frozen foods, meat, flowers, etc. Not quite as common but they are also produced in tautliner models with thicker curtains to sustain temperature. * Refrigerator tank trailers are well insulated or refrigerated to haul bulk liquid foods, such as liquid sugar, water, wine, milk or juices. *Semi-trailers with
sidelifter A sidelifter is a specialised vehicle or semi-trailer used to hoist and transport International Organization for Standardization, ISO standard intermodal containers over longer distances. Overview The sidelifter loads and unloads containers via a ...
s have a hydraulic crane mounted at each end of the chassis allowing for the loading and unloading of
shipping container A shipping container is a container with strength suitable to withstand shipment, storage, and handling. Shipping containers range from large reusable steel boxes used for intermodal shipments to the ubiquitous corrugated box design, corrugated b ...
s without the need of a forklift or other container-handling equipment. Also known as a sideloader. *
Side bay trailer Side or Sides may refer to: Geometry * Edge (geometry) of a polygon (two-dimensional shape) * Face (geometry) of a polyhedron (three-dimensional shape) Places * Side, Turkey, a city in Turkey * Side (Ainis), a town of Ainis, ancient Thessaly, G ...
s have a series of roll-up or panel doors down each side. Each door opens into individual compartments. These trailers are most commonly used in the beverage distribution industry. * Tank chassis or
tank trailer 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). A large pr ...
s are used for hauling liquids such as
gasoline Gasoline ( North American English) or petrol ( Commonwealth English) is a petrochemical product characterized as a transparent, yellowish, and flammable liquid normally used as a fuel for spark-ignited internal combustion engines. When for ...
and
alcohol Alcohol may refer to: Common uses * Alcohol (chemistry), a class of compounds * Ethanol, one of several alcohols, commonly known as alcohol in everyday life ** Alcohol (drug), intoxicant found in alcoholic beverages ** Alcoholic beverage, an alco ...
, or various types of gases. They are similar in principle to intermodal trailers but with a very different frame intended to be attached to a liquid or gas tank, hence the name. Some are designed with a lowered centre of gravity to ensure greater stability. Generally bottom loaded with the ability to recover any waste vapor for safety reasons. * A "frac" tank trailer has a single and fixed axle, and is typically used during
hydraulic fracturing Fracking (also known as hydraulic fracturing, fracing, hydrofracturing, or hydrofracking) is a well stimulation technique involving the fracturing of Formation (geology), formations in bedrock by a pressurized liquid. The process involves the ...
at
oil well An oil well is a drillhole boring in Earth that is designed to bring petroleum oil hydrocarbons to the surface. Usually some natural gas is released as associated petroleum gas along with the oil. A well that is designed to produce only gas m ...
s or for petrochemical industries. It is shaped like a wedge, and when it is unhitched, its bottom side lies flat on the ground. * "Wing" trailers are mostly found in Japan and are something of a hybrid between a curtain sider and a box trailer, with rigid, motorized
gull-wing door In the automotive industry, a gull-wing door, also known as a falcon-wing door, McLaren anhedral door, or an up-door, is a car door that is hinged at the roof rather than the side, as pioneered by Mercedes-Benz 300 SL, first as a race car in ...
s in place of the fabric curtains. Also found as part of a rigid box truck.


Tank trailer

A tank trailer is a semi-trailer specifically designed to carry
liquid Liquid is a state of matter with a definite volume but no fixed shape. Liquids adapt to the shape of their container and are nearly incompressible, maintaining their volume even under pressure. The density of a liquid is usually close to th ...
s,
fluid In physics, a fluid is a liquid, gas, or other material that may continuously motion, move and Deformation (physics), deform (''flow'') under an applied shear stress, or external force. They have zero shear modulus, or, in simpler terms, are M ...
s and
gas Gas is a state of matter that has neither a fixed volume nor a fixed shape and is a compressible fluid. A ''pure gas'' is made up of individual atoms (e.g. a noble gas like neon) or molecules of either a single type of atom ( elements such as ...
es.Tank trailer
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Gallery

File:2008-08-02 UPS long trailer 1.jpg, Box trailer (US) File:KühneNagelAuflieger.jpg, Curtain sider trailer (EU) File:WieltonMuldenkipper.jpg, Semi dump trailer (EU) File:SinoTruk HOWO Iran.jpg, Flatbed trailer (Iran) File:Live bottom pic 1.jpg, Live bottom trailer (US) File:Glas-Innenlader für Flachglas.jpg, Glass inloader trailer (Germany) File:Globe Trailers Lowboy with Volvo A300.jpg, Lowboy (US) File:Sainsburys lorry refrigerated trailer.jpg,
Refrigerated trailer A refrigerator truck or chiller lorry (also called a reefer), is a van or truck designed to carry perishable freight at low temperatures. Most long-distance refrigerated transport by truck is done in articulated trucks pulling refrigerated ha ...
(UK) File:Sidelifter in forest.jpg,
Sidelifter A sidelifter is a specialised vehicle or semi-trailer used to hoist and transport International Organization for Standardization, ISO standard intermodal containers over longer distances. Overview The sidelifter loads and unloads containers via a ...
(New Zealand) File:Tank truckタンクローリー9274341.jpg, Tank trailer (Japan) File:Gullwing Trailer.jpg, Gullwing trailer File:Road train (27).jpg, 53 foot container turnpike doubles File:Loves Freightliner tanker truck, southern Oklahoma.jpg, Tank trailer File:Arrivée d'un grumier dans le clos de l'entreprise des Frères Allot.jpg, French
logging truck A logging truck or timber lorry is a large truck used to carry logs. Some have integrated flatbeds, some are discrete tractor units, and some are configured to spread a load between the tractor unit and a dollied trailer pulled behind it. Oft ...
File:2015 Peterbilt 388 car hauler in Black, Front Left, 09-26-2022.jpg,
Car carrier trailer A car carrier trailer, also known as a car-carrying trailer, car hauler, or auto transport trailer, is a type of trailer (vehicle), trailer or semi-trailer designed to efficiently transport passenger vehicles via truck. Modern car carrier trailer ...


See also

*
Air brake (road vehicle) An air brake or, more formally, a compressed-air-brake system, is a type of friction brake for vehicles in which compressed air pressing on a piston is used to both release the parking/emergency brakes in order to move the vehicle, and also to ...
* Articulated lorries *
Articulated vehicle An articulated vehicle is a vehicle which has a permanent or semi-permanent coupling in its construction. This coupling works as a large pivot joint, allowing it to bend and turn more sharply. There are many kinds, from heavy equipment to Arti ...
*
Brake A brake is a machine, mechanical device that inhibits motion by absorbing energy from a moving system. It is used for Acceleration, slowing or stopping a moving vehicle, wheel, axle, or to prevent its motion, most often accomplished by means of ...
*
Cargo In transportation, cargo refers to goods transported by land, water or air, while freight refers to its conveyance. In economics, freight refers to goods transported at a freight rate for commercial gain. The term cargo is also used in cas ...
*
Fifth-wheel coupling 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 vertical steel pin protruding from the bottom of the front of the semi- ...
*
Fruehauf Corporation Fruehauf Trailer Corporation, previously Fruehauf Trailer Company (1918–1963) and Fruehauf Corporation (1963–1989), was an American company engaged in the manufacture and sale of truck trailers, and other machinery and equipment, with headqu ...
*
Gladhand connector A gladhand connector or gladhand coupler is an interlocking hose coupling fitted to hoses supplying pressurized air from a tractor unit to air brakes on a semi-trailer, or from a locomotive to railway air brakes on railroad cars. Gladhand connec ...
*
Globe Trailers Globe Trailers is a privately held corporation headquartered in Bradenton, Florida, midway between Tampa and Naples. It is a semi-trailer manufacturer. History This heavy equipment-manufacturing company was founded in 1982 and was bought by the ...
* Trailer jack *
Jackknifing Jackknifing is the folding of an articulated vehicle so that it resembles the acute angle of a folding pocket knife. If a vehicle towing a trailer skids, the trailer can push the towing vehicle from behind until it spins the vehicle around and ...
* Live bottom trailer *
Long combination vehicle A road train, also known as a land train or long combination vehicle (LCV) is a semi-trailer used to move road freight more efficiently than single-trailer semi-trailers. It consists of one semi-trailer or more connected together with or wit ...
*
Refrigerated container A refrigerated container or reefer is an intermodal container (shipping container) used in intermodal freight transport that is capable of refrigeration for the transportation of temperature-sensitive, perishable cargo such as fruits, vegetab ...
*
Refrigerator truck A refrigerator truck or chiller lorry (also called a reefer), is a van or truck designed to carry perishable freight at low temperatures. Most long-distance refrigerated transport by truck is done in articulated trucks pulling refrigerated hard ...
*
Roll trailer A roll trailer or "Mafi" is a trailer platform that requires towing by a powered vehicle. It is commonly used for the transport of heavy static goods and materials in the maritime shipping industry. Roll trailers are similar to shipping flat ra ...
*
Self unloading trailer A belt trailer or self unloading belt trailer is a semi-trailer that uses either a chain and flap assembly or a continuous belt that runs lengthwise on the bottom of the trailer. The belt is bolted to bars that in turn bolt to a chain that runs the ...
*
Semi-trailer truck A semi-trailer truck (also known by a wide variety of other terms – see below) 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 ...
*
Shunt truck A terminal tractor, known in the United States as a shag truck, shunt truck, spotter truck, spotting tractor, yard truck, yard shifter, yard dog, yard goat, yard horse, yard mule, yard jockey, yard spotter, hostler, or mule, is a kind of semi-t ...
*
Side stake A stanchion () is a sturdy upright fixture that provides support for some other object. It can be a permanent fixture. Types In architecture, stanchions are the upright iron bars in windows that pass through the eyes of the saddle bars or horiz ...
* Swap body * Tank chassis *
Teardrop trailer (truck) A teardrop trailer is an aerodynamically shaped semi-trailer with a curved-roof that differs in shape from the traditional rectangularly shaped trailer. The trailer is meant to be paired with a compatibly designed tractor unit leading to greater f ...
* Trailer * Trailer skirt * Trailer tail *
Vehicle category A vehicle category classifies a land vehicle or trailer for regulatory purposes. UNECE categories United Nations Economic Commission for Europe Information from Consolidated Resolution on the Construction of Vehicles (R.E.3), Revision 6. Some ...


References


External links

* {{Commons category-inline, Semi-trailers
ISO Standards
Specific aspects for light and heavy commercial vehicles, busses and trailers Articulated vehicles Trailers