Semi-trailer truck

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FAW semi-trailer truck in China
Tractor with a dump trailer.
A tractor with an auto-transport semi-trailer.
Semi-trailer truck with sleeper behind the cab.

A semi-trailer truck is a large vehicle that consists of a towing engine, known as a tractor in the United States and truck in many other places, attached to one or more semi-trailers to carry freight. It is also known as a transport in Canada; prime mover in Australia; semi, tractor-trailer, big rig, or 18-wheeler in the United States; and articulated lorry, abbreviated artic, in Britain and Ireland.

A semi-trailer does not trail completely behind the towing vehicle, but is attached at a point that is just forward of the rear-most axle of the towing unit. This is done so that a large portion of the weight of the trailer is carried by the prime mover. This arrangement means that both tractor and semi-trailer will have a distinctly different design than a rigid truck and trailer.

Regional configurations[edit]

North America[edit]

Tractor unit hauling tractor units in Idaho.

In North America, the combination vehicles made up of a powered truck and one or more detachable trailers, are known as "semi-tractor-trailers", "tractor-trailers", "semis", "big rigs" or "eighteen-wheelers".[a]


View of a truck's interior dashboard

The "tractors", or powered trucks, have two or three axles; those built for hauling heavy-duty commercial-construction machinery may have as many as four or five axles, some often being lift axles.

The most common tractor-cab layout has a forward engine, one steering axle, and two drive axles. The "fifth-wheel" trailer coupling on most tractor trucks is movable fore and aft, to allow adjustment in the weight distribution over its rear axle(s).

Ubiquitous in Europe, but less common in the USA since the 1990s, is the cabover configuration, where the driver sits next to, or over the engine. With changes in the US to the maximum length of the combined vehicle, the cabover was mostly phased out of North-American "over-the-road" or "long-haul" service by 2007. Cabovers were notorious for being difficult to service, as the cab could not be lifted on its hinges to a full 90-degree forward tilt, and this severely limited access to the front part of the engine.

Trucks average between 4 and 8 miles per gallon, with fuel economy standards requiring more than 7 miles per gallon efficiency by 2014.[1]


The cargo trailer usually has a "tandem" axle pair at the rear, each of which has dual wheels, or eight wheels on the trailer, four per axle.[b] Many trailers are equipped with a movable tandem to allow adjusting the weight distribution. The combination of eight tires on the trailer and ten tires on the tractor is what led to the moniker "eighteen wheeler".

Rocky Mountain Double

The United States also allows two-axle tractors to pull two single-axle 28.5 ft (8.7 m) semi-trailers, known officially as STAA doubles, and colloquially as doubles, a set, or a set of joints, on all highways that are part of the national network. To connect the second of a set of doubles to the first trailer, and to support the front half of the second trailer, a converter gear, also known as a con-gear or dolly is used. This apparatus has one or two axles, a fifth-wheel coupling for the rear trailer, and a tongue with a ring-hitch coupling for the forward trailer. Individual states may further allow longer vehicles, known as "longer combination vehicles" or LCVs), and may allow them to operate on roads other than those that are part of the national network.

LCV types include:

Future LCV's under consideration and study for the U.S. MAP-21 transportation bill are container doubles. These combinations are under study for potential recommendation in November 2014:

40 foot container turnpike double

Regulations on LCVs vary widely from one state or province to another. None allows more than three trailers without a special permit. Reasons for limiting the legal trailer configurations include both safety concerns and the impracticality of designing and constructing roads that can accommodate the larger wheelbase of these vehicles and the larger minimum turning radii associated with them.

Most states restrict operation of larger tandem trailer setups such as triple units, "turnpike doubles" and "Rocky-Mountain doubles". In general, these configurations are restricted to turnpikes. Except for these units, tandem setups are not restricted to certain roads any more than a single setup. They are also not restricted by weather conditions or "difficulty of operation". The Canadian province of Ontario, however, does have weather-related operating restrictions for larger tandem trailer setups.

In the United States, 80,000 lb (36,000 kg) is the maximum allowable legal gross vehicle weight without a permit.

The axle-weight breakdown is:[2]

Over-length and overweight permits are issued by each individual state whose roads will be traveled. The permits are usually issued in advance, for a specific period of time, over a specific route, with a specific load. Most over-length loads require escorts.[3] An escort is an accompanying automobile and its driver, who communicates with the driver of the payload vehicle regarding the position of the load in relation to the road and shoulder, and about other situational considerations.

A trailer's dimensions can vary greatly, depending on the amount and type of cargo it is designed to haul. In the United States, they are normally limited to 8.5 feet (2.6 m) in width.[4] (See types of trailers under Construction, below.)

Although dual wheels are the most common, use of two single, wider tires, known as "super singles", on each axle is becoming popular among bulk cargo carriers and other weight-sensitive operators. With increased efforts to reduce greenhouse gas emissions, the use of the super-single tire is gaining popularity. There are several advantages to this configuration. The first of these is that super singles reduce fuel consumption. In 1999, tests on an oval track showed a 10% fuel savings when super singles were used. These savings are realized because less energy is wasted flexing fewer tire sidewalls. Second, the lighter overall tire weight allows a truck to be loaded with more freight. The third advantage is that the single wheel covers less of the brake unit, which allows faster cooling and reduces brake fade. One of the major disadvantages of the super singles is that they are currently not as widely available as a standard tire. In addition, if a tire should become deflated or be destroyed, there isn't another tire attached to the same hub to maintain the dynamic stability of the vehicle, as would be the case with dual wheels. With dual wheels, the remaining tire may be overloaded, but it will typically allow the vehicle to be safely stopped or driven to a repair facility.

Another innovation rapidly growing in popularity is the skirted trailer. The area between the road and the bottom of the trailer frame was left open until it was discovered that the air moving under the trailer is a source of aerodynamic drag. Three Split Skirt (SS) concepts were EPA-verified to provide fuel savings greater than 5%, and four Split Skirts concepts had EPA-verified fuel savings between 4 and 5%.[5] The drawback to skirts is that they make the trailers more vulnerable to high crosswind loading and therefore more susceptible to "dog-legging," which is the misalignment of truck and trailer, or rolling over in a crosswind situation. Another drawback is that skirted trailers require more steps to remove wheels after a blowout.


An articulated lorry (Renault Magnum) in London, England.

The noticeable difference between tractor units in the North American and Europe is that almost all European models are "cab over engine" (COE or forward control), while the majority of North American trucks are conventional (or normal control). For repairs, the entire cab hinges forward to allow maintenance access. European trucks, whether small rigid or fully articulated, have a sheer face on the front. This allows for shorter trucks with longer trailers (with larger freight capacity) within the legal maximum total length. Furthermore, it offers greater manoeuvrability and better overview for the driver. Conversely, "conventional" cab tractors offer the driver a more comfortable driving environment and better protection in a collision as well as eliminating the need to empty the driver's personal effects from the tractor whenever the engine requires service.

In Europe usually only the rear tractor axle has twin wheels, while larger size single wheels are used for the cargo trailer. The most common combination used in Europe is a semi tractor with two axles and a cargo trailer with three axles, giving five axles and 12 wheels in total. Lesser used (common in Scandinavia) are tractors with three axles, which feature twin wheels either on one or both rear axles. In addition to the most common three axles variant, cargo trailers with only two or only one axle are in use, again usually with larger single wheels.

In Sweden lumber trucks and long distance freight is run on 7 or 8 axle combinations up to 60,000 kg in weight and 25,25 metres long. Semi-trailers are used for short distance freight. In Finland since Oct 1 2013 new law N 407/2013 allows 76 tons max if the truck has 9 axles and 68 tons max if the truck has 8 axles; truck can be 25,25 metres long and 4,4 metres high in both cases (full law text in Finnish language, explanation by Finnish Forest Association in English, 76 tons in the newspaper). The forest sector plans to submit to the Finnish Transport Safety Agency an application for testing lorries weighing 90 tons and 30 meters long on Finnish roads.

United Kingdom[edit]

In the UK the maximum permitted gross weight of a semi-trailer truck, without the use of a Special Type General Order (STGO), is 44,000 kg (97,000 lb), which is the second heaviest permitted legal weight for a single semi-trailer truck in the world (50,000 kg is allowed in the Netherlands). In order for a 44 tonne semi-trailer truck to be permitted on UK roads the tractor and semi-trailer must have 3 or more axles each. Lower weight semi-trailer trucks can mean some tractors and trailer having fewer axles. In practice, like double decker buses and coaches in the UK, there is no legal height limit for semi-trailer trucks; however, bridges over 16.5 ft (5.03 m) do not have the height marked on them. Semi-trailer trucks on Continental Europe have a height limit of 4.0 metres.

Vehicles heavier than 44,000 kg (97,000 lb) are permitted on UK roads but are indivisible loads, which would be classed as abnormal (or oversize). Such vehicles are required to display an STGO (Special Types General Order) plate on the front of the tractor unit and, under certain circumstances, are required to travel by an authorised route and have an escort.

Articulated lorries with extended tents, representing Renault at Silverstone.

Most UK trailers are 45 feet (13.7 metres) long and, dependent on the position of the fifth wheel and kingpin, a coupled tractor unit and trailer will have a combined length of between 50 and 55 feet (15.25 and 16.75 metres). Although the Construction and Use Regulations allow a maximum rigid length of 60 feet (18.2 metres), this, combined with a shallow kingpin and fifth wheel set close to the rear of the tractor unit, can give an overall length of around 75 feet (22.75 metres), although combinations of this length are usually used only to carry steel or concrete beams. Providing certain requirements are fulfilled, a Special Types General Order (STGO) allows for vehicles of any size or weight to travel on UK roads. However, in practice any such vehicle has to travel by a route authorised by the Department of Transport and move under escort. The escort of abnormal loads in the UK is now predominantly carried out by private companies, but extremely large or heavy loads that require road closures must still be escorted by the police.

In the UK, some articulated trucks have 8 tyres on 3 axles on the tractor; these are known as 6-wheelers or "6 leggers", with either the centre or rear axle having single wheels which normally steer as well as the front axle and can be raised when not needed (i.e. when unloaded or only a light load is being carried; an arrangement known as a TAG axle when it is the rear axel, or mid-lift when it is the centre axel). Some trailers have 2 axles which have twin wheels on each axle; other trailers have 3 axles, of which 1 axle can be a lift axle which has super-single wheels. In the UK, two wheels bolted to the same hub are classed as a single wheel, therefore a standard six-axle articulated truck is considered to have twelve wheels, even though it has twenty tyres. The UK also allows articulated truck tractors which have 6 tyres on 2 axles; these are known as 4-wheelers.

Denby Eco-Link B-Train[edit]

In 2009 the operator Denby Transport designed and built a 25.25 metre long B-Train (or B-Double) semi-trailer truck called the Denby Eco-Link to show the benefits of such a vehicle, which were a reduction in road accidents and result in less road deaths, a reduction in emissions due to the one tractor unit still being used and no further highway investment being required. Furthermore Denby Transport asserted that two Eco-Links would replace three standard articulated lorries while if limited to the current UK weight limit of 44 tonnes, it was claimed the Eco-Link would reduce carbon emissions by 16 per cent, and could still halve the number of trips needed for the same amount of cargo carried in conventional lorries. This is based on the fact that for light but bulky goods such as toilet paper, plastic bottles, cereals and aluminium cans, conventional lorries run out of cargo space before they reach the weight limit.[2][4] At 44 tonnes, as opposed to 60 tonnes usually associated with B-Trains, the Eco-Link also exerts less weight per axle on the road compared to the standard 6 axle 44 tonne articulated combination.

The vehicle was built after Denby Transport believed they had found a legal-loophole in the present UK law to allow the Eco-Link to be used on the public roads. The relevant legislation concerned the 1986 Road Vehicles Construction and Use Regulations. The 1986 regulations state that "certain vehicles" may be permitted to draw more than one trailer and can be up to 25.9 m (85 ft) in length.[2] The point of law reportedly hinged on the definition of a "towing implement", with Denby prepared to argue that the second trailer on the Eco-Link was one. The Department for Transport were of the opinion that this refers to recovering a vehicle after an accident or breakdown, but the regulation does not explicitly state this.

During BTAC performance testing the Eco-Link was given an "excellent" rating for its performance in manoeuvrability, productivity, safety and emissions tests, superseding ordinary lorries in many respects.[23] Private trials had also reportedly shown the Denby vehicle had a 20 per cent shorter stopping distance than conventional lorries of the same weight, due to having extra axles. The active steer system meant that the Eco-Link had a turning circle of just 41 feet, the same as a conventional articulated lorry.

Although the DfT advised that the Eco-Link was not permissible on public roads, Denby Transport gave the Police prior warning of the timing and route of the test drive on the public highway, as well as outlining their position in writing to the Eastern Traffic Area Office. On 1 December 2009 Denby Transport were preparing to drive the Eco-Link on public roads, but this was cut short because the Police pulled the lorry over as it left the gates in order to test it for its legality "to investigate any... offences which may be found". The Police said the vehicle was unlawful due to its length and Denby Transport was served with a notice by the Vehicle and Operator Services Agency (VOSA) inspector to remove the vehicle from the road for inspection. Having returned to the yard, Denby Transport was formally notified by Police and VOSA that the lorry could not be used while the Eco-Link, or any other B-Train, have since not been permnitted on UK roads. However, this prompted The Department for Transport to undertake a desk study in to semi-trailer trucks, which has resulted in the longer semi-trailer trial which commenced in 2012.

Longer semi-trailers[edit]

Starting in January 2012 the Department for Transport is conducting a trial of longer semi-trailers. The trial involves 900 semi-trailers of 14.6m in length (i.e. 1 metre longer than the current maximum), and a further 900 semi-trailers of 15.65m in length (i.e. 2.05 metres longer). This will result in the total maximum length of the semi-trailer truck being 17.5 metres (for trailers of 14.6 metre in length) and 18.55 metres (for trailers of 15.65 metres in length). The increase in length will not result in the 44,000 kg (97,000 lb) weight limit being exceeded and will allow some operators to approach the weight limit which may not have been previously possible due to the previous length of trailers. The trial will run for a maximum of 10 years.

Continental Europe[edit]

The maximum overall length applying in the EU and EEA member states is 18.75 metres with a maximum weight of 40 tonnes, or 44 tonnes if carrying an ISO container.[6] However, rules limiting the semi-trailers to 16.5 metres and 18.75 are met with trucks carrying a standardized 7.82 metre body with one additional 7.82 metre body on tow as a trailer.[7] Since 1996, when Sweden and Finland formally won a final exemption from the European Economic Area rules with 60- tonne and 25.25- metre combinations, all other member states gained the ability to adopt the same rules. In Italy the maximum permitted weight (unless exceptional transport is authorized) is 44000 kg for any kind of combination with 5 axel or more.

Effort to increase the maximum overall length[edit]
All EuroCombi variants being considered for Europe wide adoption

The 25.25 metres truck combinations were developed under the branding of EcoCombi which influenced the name of EuroCombi for an ongoing standardization effort where such truck combinations shall be legal to operate in all jurisdictions of the European Economic Area. With the 50% increase in cargo weight, the fuel efficiency increases with an average of 20% with a corresponding relative decrease in carbon emissions and with the added benefit of one third fewer trucks on the road.[6] The 1996 EU regulation defines a Europe Module System (EMS) as it was implemented in Sweden. The wording of EMS combinations and EuroCombi are now used interchangeably to point to truck combinations as specified in the EU document; however apart from Sweden and Finland the EuroCombi is only allowed to operate on specific tracks in other EU member states.

From 2006, 25.25 m truck trailer combinations are to be allowed on restricted routes within Germany, following a similar (on-going) trial in The Netherlands. Similarly, Denmark have allowed 25.25 metre combinations on select routes. Like in Sweden and Finland, these vehicles in continental Europe will run a 60 ton weight limit. Two types are to be used: 1) a 26 ton truck pulling a dolly and semi-trailer, or 2) an articulated tractor unit pulling a b-double. The UK government has so far decided not to have its own trial of these 60 ton vehicles, but to keep an eye on the other countries' trials.

A truck with a swap body pulling a trailer using a dolly; the overall length is 25.25 m.

When using a dolly, which generally has to be equipped with lights and a license plate, rigid trucks can be used to pull semitrailers. The dolly is equipped with a fifth wheel to which the trailer is coupled. Because the dolly attaches to a pintle hitch on the truck, maneuvering a trailer hooked to a dolly is different from maneuvering a fifth wheel trailer. Backing the vehicle requires same technique as backing an ordinary truck/full trailer combination, though the dolly/semi setup is probably longer, thus requiring more space for maneuvering. The tractor-semitrailer configuration is rarely used on timber trucks, since these will use the two big advantages of having the weight of the load on the drive wheels, and the loader crane used to lift the logs from the ground can be mounted on the rear of the truck behind the load, allowing a short (lightweight) crane to reach both ends of the vehicle without uncoupling. Also construction trucks are more often seen in a rigid + midaxle trailer configuration instead of the tractor + semitrailer setup.

Denmark, the Netherlands, Norway, Sweden and Finland[edit]

Denmark, the Netherlands and Norway all allow 25.25 m trucks (the Netherlands from 2000, Denmark from 2008, and Norway from 2008 on selected routes).

In Sweden the allowed length is 24 metres (78.7 ft) since 1967. Before that, the maximum length was unlimited - the only limitations were on axle load. What stopped Sweden from adopting the same rules as the rest of Europe, when securing road safety was the national importance of a competitive forestry industry.[6] Finland with the same road safety issues and equally important forestry industry followed suit. The change made trucks able to carry three stacks of cut-to-length logs instead of two, as it would be in a short combination. They have one on stack together with a crane on the 6x4 truck, and two additional stacks on a four axle trailer. The allowed gross weight in both countries is up to 60 tonnes (130,000 lb) depending on the distance between the first and last axle.

In the negotiations starting in the late 80s preceding the two countries' entries to the European Economic Area and later the European Union, they insisted on exemptions from the EU rules citing environmental concerns and the transportation needs of the logging industry. In 1995, after Sweden and Finland's entry to the union, the rules changed again, this time to allow trucks carrying a standard CEN unit of 7.82 metres to draw a 13.6 metres standard semi-trailer on a dolly, a total overall length of 25.25 metres (82.8 ft). Later B-double combinations came into use, often with one 20 ft container on the b-link and a 40 ft container (or two 20 ft containers) on a semi-trailer bed. In allowing the longer truck combinations, what would take two 16.5 metre semi-trailer trucks and one 18.75 metre truck and trailer to haul on the continent now could be handled by just two 25.25 metre trucks - greatly reducing overall costs and emissions. Prepared since late 2012 and effective on January 2013, Finland has changed its regulations to allow total maximum legal weight of a combination to be 76 tonnes (168,000 lb). At the same time the maximum allowed height would be increased by 20 cm; from current maximum of 4.2 metres to 4.4 metres. The effect this major maximum weight increase would cause to the roads and bridges in Finland on the long run, is heated debate.

However, longer and heavier combinations are regularly seen on public roads - special permits are issued for special cargo - not a too uncommon occurrence. Others are: The mining company Boliden AB have a standing special permit for 80 ton combinations on select routes between mines in the inland and the processing plant in Boliden, taking a load of 50 ton ore. Volvo has a special permit for a 32 metres (105 ft), steering B-trailer-trailer combination carrying two 40 ft containers to and from Gothenburgs harbour and Volvo Trucks factory, all on the island of Hisingen.[8] Another example is the ongoing project En Trave Till (lit. One more pile/stack) started in December 2008. It will allow even longer vehicles to further rationalize the logging transports. As the name of the project points out, it will be able to carry four stacks of timber, instead of the usual three. The test is limited to Norrbotten county and the European route E4 between the timber terminal in Överkalix and the sawmill in Munksund (outside Piteå). The vehicle is a 30 metre long truck trailer combination with a gross weight exceeding 90 tonnes (200,000 lb). It is estimated that this will give a 20% lower cost and 20-25% CO2 emissions reduction compared to if the timber instead would have been transported with regular 60 ton truck combinations. As the combinations spreads its weight over more axles, braking distance, road wear and traffic safety is believed to be either the same or improved with the 90 ton truck-trailer. In the same program two types of 74 tonnes (163,000 lb) combinations will be tested in Dalsland and Bohuslän counties in western Sweden. An enhanced truck and trailer combination for use in the forest and a b-double for plain highway transportation to the mill in Skoghall. The Northland Mining has in 2012 got permission for 90 ton combinations with normal axle load (an extra dolly) 150 km Kaunisvaara-Svappavaara, carrying iron ore.[9]


Australian road transport has a reputation for using very large trucks and road trains. This is reflected in the most popular configurations of trucks generally having dual drive axles and three axles on the trailers, with 4 tires on each axle. This means that Australian single semi-trailer trucks will usually have 22 wheels which is generally more than their counterparts in other countries. Long haul transport usually operates as B-doubles with two trailers (each with three axles), for a total of nine axles (including steering). In some lighter duty applications only one of the rear axles of the truck is driven, and the trailer may have only two axles.

From July 2007 the Australian Federal and State Governments allowed the introduction of B-triple trucks on a specified network of roads.[10] B-Triples are set up differently to conventional road trains. The front of their first trailer is supported by the turntable on the prime mover. The second and third trailers are supported by turntables on the trailers in front of them. As a result, B-Triples are much more stable than road trains and handle exceptionally well. True road trains only operate in remote areas, regulated by each state or territory government.

In total, the maximum length that any articulated vehicle may be (without a special permit and escort) is 53.5 metres (175.5 ft), its maximum load may be up to 164 tons (361,558 lb) gross and may have up to 4 trailers. However, heavy restrictions apply to the areas where such a vehicle may travel in most states. In remote areas such as the Northern Territory great care must be taken when sharing the road with longer articulated vehicles that often travel during the day time, especially 4 trailer road trains.

Articulated trucks towing a single trailer or two trailers (commonly known as "short doubles") with maximum overall length of 19 metres (62 ft) are referred to as "General access heavy vehicles" and are permitted in all areas, including metropolitan. B-doubles are limited to a maximum total weight of 62.5 ton (137,788 lb) and overall length of 25 metres (82 ft) or 26 metres (85 ft) if they are fitted with approved FUPS (Front Underrun Protection System) devices. B-doubles may only operate on designated roads, which includes most highways and some major metropolitan roads. B-doubles are very common in all parts of Australia including state capitals and on major routes they outnumber single trailer configurations.

Maximum width of any vehicle is 2.5 metres (8 ft 2 in) and height of 4.3 metres (14 ft 1 in). In the past few years, allowance has been made by several states to allow certain designs of heavy vehicles up to 4.6 metres (15 ft 1 in) high but they are also restricted to designated routes. In effect, a 4.6 metres high B-double will have to follow two sets of rules: they may access only those roads that are permitted for B-doubles AND for 4.6 metres high vehicles.

In Australia, both conventional tractor units and cabovers are common, however cabovers are most often seen on B-Doubles on the eastern seaboard where the reduction in total length allows the vehicle to pull longer trailers and thus more cargo than it would otherwise.

Super single tires are sometimes used on tri-axle trailers. The suspension is designed with travel limiting, which will hold the rim off the road for one blown or deflated tire for each side of the trailer, so a trailer can be driven at reduced speed to a safe place for repair. Super singles are also often used on the steer axle in Australia to allow greater loading over the steer axle. The increase in loading of steer tires requires a permit.

Semi-truck manufacturers[edit]

These are for tractor units, not straight, rigid, box or other heavy vehicles.

Currently in the United States and Canada
Currently in Europe
Currently in Japan and some other Asia Pacific regions
Currently in Other countries


Side view and underside view of a conventional 18-wheeler semi-trailer truck with an enclosed cargo space. The underside view shows the arrangement of the 18 tires (wheels). Shown in blue in the underside view are the axles, drive shaft, and differentials. The legend for labeled parts of the truck is as follows:
1. tractor unit
2. semi-trailer (detachable)
3. engine compartment
4. cabin
5. sleeper (not present in all trucks)
6. air dam
7. fuel tanks
8. fifth wheel coupling
9. enclosed cargo space
10. landing gear - legs for when semi-trailer is detached
11. tandem axles

Types of trailers[edit]

There are many types of semi-trailers in use, designed to haul a wide range of products. See semi-trailer for more detail.

Coupling and uncoupling[edit]

The cargo trailer is, by means of a king pin, hooked to a horseshoe-shaped quick-release coupling device called a fifth wheel or a turntable hitch at the rear of the towing engine that allows easy hook up and release. The truck trailer cannot move by itself because it only has wheels at the rear end: it requires a forward axle, provided by the towing engine, to carry half the load weight. The vehicle has a tendency to fold at the pivot point between the towing vehicle and the trailer when braking hard at high speeds. Such a truck accident is called a 'trailer swing', although it is commonly described as a 'jackknife'. 'Jackknifing' is a condition where the tractive unit swings round against the trailer: not vice-versa. See: jackknifing.


Semi trucks use air pressure, rather than hydraulic fluid, to actuate the brakes mainly due to the much larger braking forces required. This also allows for ease of coupling and uncoupling of trailers from the tractor unit, as well as reducing the potential for problems common to hydraulic systems, such as leakage or brake failure caused when overheated brake fluid vaporizes in the hydraulic lines. The most common failure is "brake fade," usually caused when the drums or discs and the linings of the brakes overheat from excessive use.

The parking brake of the tractor unit and the emergency brakes of the trailer are spring brakes that require air pressure in order to be released. They are applied when air pressure is released from the system, and disengaged when air pressure is supplied. This is an emergency feature which ensures that if air pressure to either unit is lost, the trailer will stop to a grinding halt instead of not stopping and becoming uncontrollable.

A view of semi-trailer "Suzies" at the back of an Australian prime mover. The red line is emergency/supply and the blue is control line.

The trailer controls are coupled to the tractor through two "gladhand connectors", which provide air pressure, and an electrical cable, which provides power to the lights and any specialized features of the trailer.

"Glad-hand" connectors (also known as "palm couplings,") are air hose connectors, each of which has a flat engaging face and retaining tabs. The faces are placed together, and the units are rotated so that the tabs engage each other to hold the connectors together. This arrangement provides a secure connection, but allows the couplers to break away without damaging the equipment if they are pulled, as may happen when the tractor and trailer are separated without first uncoupling the air lines. These connectors are similar in design to the ones used for a similar purpose between railroad cars. Two air lines control the trailer unit. An emergency or main air supply line pressurizes the trailer's air tank and disengages the emergency brake, and a second service line controls the brake application.

In the UK male/female quick release connectors "red line" or emergency, have a female on the truck and male on the trailer and a "yellow line" or service has a male on the truck and female on the trailer. This avoids coupling errors (causing no brakes) plus the connections will not come apart if pulled by accident. The three electrical lines will fit one way round a primary black a secondary green and an ABS lead, all of these lines are collectively known as "suzies" or "suzie coils".

Another braking feature of semi-trucks is the engine braking, which could be either compression brake (usually shortened to "Jake brake") or exhaust brake or combination of both. The use of compression brake alone however produces a loud and distinctive noise, and owing to noise pollution, some local municipalities have prohibited or restricted the use of engine brake systems inside their jurisdictions, particularly in residential areas. The advantage to using this instead of conventional brakes is that a truck can travel down a long grade without overheating its wheel brakes. Some vehicles can also be equipped with hydraulic or electric retarders which have an advantage of near silent operation.


Because of the wide variety of loads the "semi" may carry, they usually have a manual transmission to allow the driver to have as much control as possible. However, all truck manufacturers now offer semi-automatic transmissions (manual gearboxes with automated gear change), as well as automatic transmissions.

Older trucks often had twin gearshifts, usually with 4 or 5 ratios on main shift and 3 or 4 on the auxiliary shift. This example has 5 main ratios and 3 auxiliaries, thus a "5x3."

"Semi" truck transmissions can have as few as 9 forward speeds or as many as 18 forward speeds (plus 2 reverse speeds). A large number of transmission ratios means the driver can operate the engine more efficiently. Modern on-highway diesel engines are designed to provide maximum torque in a narrow RPM range (usually 1200-1500 RPM); having more gear ratios means the driver can hold the engine in its optimum range regardless of road speed (drive axle ratio is also a critical factor).

A ten speed manual transmission, for example is controlled via a six-slot H-box pattern, similar to that in five-speed cars — five forward and one reverse gear. Gears six to ten (and high speed reverse) are accessed by a Lo/High range splitter; gears 1-5 are Lo range; gears 6-10 are High range using the same shift pattern. A Super-10 transmission, by contrast, has no range splitter; it uses alternating "stick and button" shifting (stick shifts 1-3-5-7-9, button shifts 2-4-6-8-10). 13,15 and 18 speed transmission have the same basic shift pattern, but include a splitter button to access to additional ratios found in each range. Some may have 12.

Another difference between semi-trucks and cars is the way the clutch is set up. On an automobile, the clutch pedal is depressed full stroke to the floor for every gear shift to ensure the gearbox is disengaged from the engine. On a semi-truck with constant mesh transmission (non synchronized), such as by the Eaton Roadranger series, not only is double clutching required, but a clutch brake is required as well. The clutch brake stops the rotation of the gears, and allows the truck to be put into gear without grinding when stationary. The clutch is pressed to the floor only to allow smooth engagement of low gears when starting from a full stop; when moving, the clutch pedal is pressed only far enough to break torque for gear changes.


An electrical connection is made between the tractor and the trailer through a cable often referred to as a "pigtail." This cable is a bundle of wires in a single casing. Each wire controls one of the electrical circuits on the trailer, such as running lights, brake lights, turn signals, etc. A standard cable would break when the rig went around corners so it is coiled and retains these coils when not under tension. It is these coils that cause the cable to look like a pigtail.

In most countries a trailer or semi-trailer must have minimum

Underride guard[edit]

This is an assembly hanging down from the bottom of the rear of the trailer. It is intended to provide some protection for cars which start to run into the rear of the trailer. This came into use in the aftermath of the accident that killed Jayne Mansfield on June 29, 1967; the car she was in hit the rear of a tractor-trailer. The bottom of the rear of the trailer is near head level for an adult in a car, and without the underride guard, the only protection for such an adult's head in such an accident would be the car's windshield.

Driver's license[edit]

A special driver's license is required to operate various commercial vehicles.


Regulations vary by province. A license to operate a vehicle with air brakes is required (i.e., normally a Class I, II, or III commercial license with an "A" or "S" endorsement in provinces other than Ontario). In Ontario, a "Z" endorsement[12] is required to drive any vehicle using air brakes; in provinces other than Ontario, the "A" endorsement is for air brake operation only, and an "S" endorsement is for both operation and adjustment of air brakes. Anyone holding a valid Ontario driver's license (i.e., excluding a motorcycle license) with a "Z" endorsement can legally drive any air-brake-equipped truck-trailer combination with a registered- or actual-gross-vehicle-weight (i.e., including towing- and towed-vehicle) up to 11 metric tonnes, that includes one trailer weighing no more than 4.6 tonnes if the license falls under the following three classes: Class E (school bus—maximum 24-passenger capacity or ambulance), F (regular bus—maximum 24-passenger capacity or ambulance) or G (car, van, or small-truck). A Class B (any school bus), C (any urban-transit-vehicle or highway-coach), or D (heavy trucks other than tractor-trailers) license enables its holder to drive any truck-trailer combination with a registered- or actual-gross-vehicle-weight (i.e., including towing- and towed-vehicle) greater than 11 tonnes, that includes one trailer weighing no more than 4.6 tonnes.[13] Anyone holding an Ontario Class A license (or its equivalent) can drive any truck-trailer combination with a registered- or actual-gross-vehicle-weight (i.e., including towing- and towed-vehicles) greater than 11 tonnes, that includes one or more trailers weighing more than 4.6 metric tonnes.

United States[edit]

Drivers of semi-trailer trucks generally require a Class A commercial driver's license to operate any combination vehicles with a combined Gross Vehicle Weight Rating (or CGVWR) in excess of 26,000 pounds (11.8 t) if the gross vehicle weight rating (GVWR) of the towed vehicle(s) is in excess of 10,000 lbs. Some states (such as North Dakota) provide exemptions for farmers, allowing non-commercial license holders to operate semis within a certain air-mile radius of their reporting location. State exemptions, however, are only applicable in intrastate commerce, only the stipulations of the Code of Federal Regulations (CFR) may be applied in interstate commerce. Also a person under the age of 21 cannot operate a commercial vehicle outside the state where the commercial license was issued. This restriction may also be mirrored by certain states in their intrastate regulations. A person must be at least 18 in order to be issued a commercial license.

In addition, Endorsements are necessary for certain cargo and vehicle arrangements and types;


Taiwanese sign prohibiting heavy trailers

The Road Traffic Security Rules (zh:道路交通安全規則) require a combination vehicle driver license (Chinese: 聯結車駕駛執照) to drive a combination vehicle (Chinese: 聯結車). These rules define a combination vehicle as a motor vehicle towing a heavy trailer, i.e., a trailer with a gross weight of more than 750 kilograms (1,653 lb).


A category CE driving licence is required to drive a Tractor-trailer.


Truck drivers in Australia require an endorsed license. These endorsements are gained through training and experience. The minimum age to hold an endorsed license is 18 years, and/or must have held open (full) driver's license for minimum 12 months. The following are the heavy vehicle license classes in Australia:

In order to obtain a HC License the driver must have held an MR or HR license for at least 12 months. To upgrade to an MC License the driver must have held a HR or HC license for at least 12 months.

From licenses MR and upward there is also a "B" Condition which may apply to your license if you do your testing in a Synchromesh or Automatic Transmission Vehicle. To remove the "B"Condition you must prove to the Motor Registry (In any jurisdiction) that you have the ability to drive a constant mesh Transmission using the clutch.

Constant Mesh transmission refers to "Crash Box" transmissions, predominantly Road Ranger 18 Speed Transmission in Australia.

New Zealand[edit]

In New Zealand drivers of heavy vehicles require specific licences, termed as 'classes'. A Class 1 Drivers Licence (aka a Car License) will allow the driving of any vehicle with Gross Laden Weight (GLW) or Gross Combination Weight (GCW) of 4500 kg or less. For other types of vehicles the Classes are separately licensed as follows:

Further information on the New Zealand Licensing system for Heavy Vehicles can be found at Land Transport New Zealand.

Role in industry[edit]

Modern day semi-trailer trucks often operate as a part of a domestic or international transport infrastructure to support containerized cargo shipment. Various types of rail flat bed train cars are modified to hold the cargo trailer or container with wheels or without. This is called "Intermodal" or "piggy-back" or "piggyback". The system allows the cargo to switch from the highway to railway or vice versa with relative ease by using gantry cranes.

The large trailers pulled by a tractor unit come in many styles, lengths, and shapes. Some common types are: vans, reefers, flatbeds, sidelifts and tankers. These trailers may be refrigerated, heated, ventilated, or pressurized, depending on climate and cargo. Some trailers have movable wheel axles that can be adjusted by moving them on a track underneath the trailer body and securing them in place with large pins. The purpose of this is to help adjust weight distribution over the various axles, to comply with local laws.





Video games[edit]

See also[edit]


  1. ^ "Eighteen-wheeler" is a cultural nickname from counting the number of tires on a five axle over the road combination, number of tires are not often counted elsewhere.
  2. ^ In the UK and US it is more common to refer to the number of wheel hubs, rather than the number of tires. In the US an axle can have either single or dual tires with no legal difference.


External links[edit]