Semi-trailer truck


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.

Other terms

There are a wide variety of English-language terms for a semi-trailer truck, including:
American English:
  • Semi-trailer
  • Semi-truck
  • Truck & trailer
  • Semi
  • Big rig
  • Tractor-trailer
  • Eighteen-wheeler
British English:
  • Articulated lorry
  • Artic
  • Juggernaut
  • Heavy Goods Vehicle/HGV
Canadian English:
  • Transport truck
  • Transfer truck

    Regional configurations

Europe

The main difference between tractor units in Europe and North America is that European models are cab over engine, while the majority of North American trucks are "conventional". European trucks, whether straight trucks or fully articulated, have a sheer face on the front. This allows shorter trucks with longer trailers within the legal maximum total length. Furthermore, it offers greater maneuverability in confined areas, a more balanced weight-distribution, and better overall view for the driver. The major disadvantage is that for repairs on COE trucks, the entire cab has to hinge forward to allow maintenance access.
In Europe, usually only the driven tractor axle has dual wheels, while single wheels are used for every other axle on the tractor and the trailer. The most common combination used in Europe is a semi tractor with two axles and a cargo trailer with three axles, one of which is sometimes a lift axle, giving 5 axles and 12 wheels in total. This format is now common across Europe as it is able to meet the EU maximum weight limit of without overloading any axle. Individual countries have raised their own weight limit. The U.K., for example, has a limit, an increase achieved by adding an extra axle to the tractor, usually in the form of a middle unpowered lifting axle with a total of 14 wheels. The lift axles used on both tractors and trailers allow the trucks to remain legal when fully loaded ; on the other hand, these axle set can be raised off the roadway for increased maneuverability or for reduced fuel consumption and tire wear when carrying lighter loads. Although lift axles usually operate automatically, they can be lowered manually even while carrying light loads, in order to remain within legal limits when, for example, navigating back-road bridges with severely restricted axle loads. For greater detail, see the United Kingdom section, [|below].
When using a dolly, which generally has to be equipped with lights and a license plate, rigid trucks can be used to pull semi-trailers. 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 the 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/semi-trailer configuration is rarely used on timber trucks since they use the two major 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 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/semi-trailer setup.

Continental Europe

The maximum overall length in the EU and EEA member states was with a maximum weight of if carrying an ISO container. However, rules limiting the semi-trailers to and 18.75 m are met with trucks carrying a standardized body with one additional 7.82 m body on tow as a trailer. 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 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. The 1996 EU regulation defines a Europe Module System 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 roads in other EU member states. Since 1996 Sweden and Finland formally won a final exemption from the European Economic Area rules with 60 tonne and combinations. From 2006, 25.25 m truck trailer combinations are to be allowed on restricted routes within Germany, following a similar a 26-tonne truck pulling a dolly and semi-trailer, or 2) an articulated tractor unit pulling a B-double, member states gained the ability to adopt the same rules. In Italy the maximum permitted weight is 44 tonnes for any kind of combination with five axles or more. Czech Republic has allowed 25.25 m combinations with a permission for a selected route.

Nordic countries

Denmark and Norway allow trucks. In Sweden, the allowed length has been 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. 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 stack together with a crane on the 6×4 truck, and two additional stacks on a four axle trailer. The allowed gross weight in both countries is up to depending on the distance between the first and last axle.
In the negotiations starting in the late 1980s preceding Sweden and Finland's 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 their entry to the union, the rules changed again, this time to allow trucks carrying a standard CEN unit of to draw a standard semi-trailer on a dolly, a total overall length of 25.25 m. Later, B-double combinations came into use, often with one container on the B-link and a container on a semi-trailer bed. In allowing the longer truck combinations, what would take two semi-trailer trucks and one truck and trailer to haul on the continent now could be handled by just two 25.25 m trucks – greatly reducing overall costs and emissions. Prepared since late 2012 and effective in January 2013, Finland has changed its regulations to allow total maximum legal weight of a combination to be. At the same time the maximum allowed height would be increased by ; from current maximum of to. The effect this major maximum weight increase would cause to the roads and bridges in Finland over time is strongly debated.
However, longer and heavier combinations are regularly seen on public roads; special permits are issued for special cargo. The mining company Boliden AB have a standing special permit for combinations on select routes between mines in the inland and the processing plant in Boliden, taking a load of ore. Volvo has a special permit for a, steering B-trailer-trailer combination carrying two containers to and from Gothenburg harbour and the Volvo Trucks factory, all on the island of Hisingen. Another example is the ongoing project En Trave Till 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. The vehicle is a long truck trailer combination with a gross weight exceeding . It is estimated that this will give a 20% lower cost and 20–25% emissions reduction compared to the regular truck combinations. As the combination spreads its weight over more axles, braking distance, road wear and traffic safety is believed to be either the same or improved with the truck-trailer. In the same program two types of 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. In 2012, the Northland Mining company received permission for combinations with normal axle load for use on the Kaunisvaara-Svappavaara route, carrying iron ore.
, the longest and heaviest truck in everyday use in Finland is operated by transport company Ketosen Kuljetus as part of a pilot project studying transport efficiency in the timber industry. The combined vehicle is long, has 13 axles, and weighs a total of.
Starting from 21 January 2019 the Government of Finland changed the maximum allowed length of truck from. New types of vehicle combinations that differ from the current standards may also be used on the road. The requirements for combinations also include camera systems for side visibility, an advanced emergency braking and lane detector system, electronic driving stability system and electronically controlled brakes.

United Kingdom

In the United Kingdom, a semi-trailer truck is known as an 'articulated lorry'. The maximum permitted gross weight of a semi-trailer truck without the use of a Special Type General Order is. In order for a 44,000 kg semi-trailer truck to be permitted on UK roads the tractor and semi-trailer must have three or more axles each. Lower weight semi-trailer trucks can mean some tractors and trailer having fewer axles. In practice, as with double decker buses and coaches in the UK, there is no legal height limit for semi-trailer trucks; however, bridges over do not have the height marked on them. Semi-trailer trucks in continental Europe have a height limit of. Vehicles heavier than 44,000 kg are permitted on UK roads but are indivisible loads, which would be classed as abnormal. Such vehicles are required to display an STGO plate on the front of the tractor unit and, under certain circumstances, are required to travel by an authorized route and have an escort.
Most UK trailers are 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. Although the Construction and Use Regulations allow a maximum rigid length of, 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.
In January 2012, the Department for Transport began conducting a trial of longer semi-trailers. The trial involves 900 semi-trailers of in length, and a further 900 semi-trailers of in length. This will result in the total maximum length of the semi-trailer truck being for trailers in length, and for trailers long. The increase in length will not result in the 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. Providing certain requirements are fulfilled, a Special Types General Order 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 authorized 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 semi-trailer trucks have eight tyres on three axles on the tractor; these are known as six-wheelers or "six 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. Some trailers have two axles which have twin tyres on each axle; other trailers have three axles, of which one 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 semi-trailer truck which have six tyres on two axles; these are known as four-wheelers.
In 2009, the operator Denby Transport designed and built a B-Train 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 fewer 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 semi-trailer trucks while, if limited to the current UK weight limit of, it was claimed the Eco-Link would reduce carbon emissions by 16% and could still halve the number of trips needed for the same amount of cargo carried in conventional semi-trailer trucks. This is based on the fact that for light but bulky goods such as toilet paper, plastic bottles, cereals and aluminum cans, conventional semi-trailer trucks run out of cargo space before they reach the weight limit. At, as opposed to usually associated with B-Trains, the Eco-Link also exerts less weight per axle on the road compared to the standard six-axle semi-trailer truck.
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. 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 maneuverability, productivity, safety and emissions tests, exceeding ordinary semi-trailer trucks in many respects. Reportedly, private trials had also shown the Denby vehicle had a 20% shorter stopping distance than conventional semi-trailer trucks of the same weight, due to having extra axles. The active steer system meant that the Eco-Link had a turning circle of, the same as a conventional semi-trailer truck.
Although the Department for Transport 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 semi-trailer truck over as it left the gates in order to test it for its legality "to investigate any... offenses 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 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 semi-trailer truck could not be used. Neither the Eco-Link, nor any other B-Train, have since been permitted on UK roads. However, this prompted the Department for Transport to undertake a desk study into semi-trailer trucks, which has resulted in the longer semi-trailer trial which commenced in 2012.