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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 to carry freight. A semi-trailer attaches to the tractor with a type of Tow hitch called a fifth wheel.
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 (midlift) with a total of 14 wheels. The lift axles used on both tractors and trailers allow the trucks to remain legal when fully loaded (as weight per axle remains within the legal limits); on the other hand, these axle set(s) 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 (safe) 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, 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 on the truck, maneuvering a trailer hooked to a dolly is different from maneuvering a fifth wheel trailer. Backing up 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 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 (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/semi-trailer setup.
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 Swap body 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 (or two containers) 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 Group has a special permit for a , steering B-trailer-trailer combination carrying two containers to and from Gothenburg 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 (outside Piteå). 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 (an extra dolly) for use on the Kaunisvaara-Svappavaara route, carrying iron ore. full law text in Finnish language, explanation by Finnish Forest Association in English, 76 tons in the newspaper
, 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. Maximum length of a vehicle combination 34.5 metres Maximum length of a vehicle combination 34.5 metres
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 (i.e. longer than the current maximum), and a further 900 semi-trailers of in length (i.e. longer). 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 (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 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 (i.e. when unloaded or only a light load is being carried; an arrangement known as a TAG axle when it is the rear axle, or mid-lift when it is the center axle). 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 (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 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 (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 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.
The tractor unit typically has two or three ; those built for hauling heavy-duty commercial-construction machinery may have as many as five, some often being lift axles.
The most common tractor-cab layout has a forward engine, one steering axle, and two . 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 North America since the 1990s, is the Cab over 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 largely phased out of North American over-the-road (long-haul) service by 2007. Cabovers were difficult to service; for a long time, the cab could not be lifted on its hinges to a full 90-degree forward tilt, severely limiting access to the front of the engine.
, a truck could cost , while the diesel fuel cost could be $70,000 per year. Trucks average from , with fuel economy standards requiring better than efficiency by 2014. Power requirements in standard conditions are at or at , and somewhat different power usage in other conditions.
The semi-trailer usually has tandem axles at the rear, each of which has dual wheels, or eight tires on the trailer, four per axle. In the US it is common to refer to the number of wheel hubs, rather than the number of tires; an axle can have either single or dual tires with no legal difference. The combination of eight tires on the trailer and ten tires on the tractor is what led to the moniker eighteen wheeler, although this term is considered by some truckers to be a misnomer (the term "eighteen-wheeler" is a nickname for a five-axle over-the-road combination). Many trailers are equipped with movable tandem axles to allow adjusting the weight distribution.
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 known as a "dolly" is used. This 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" (road train), and may allow them to operate on roads other than Interstates.
Long combination vehicle types include:
The US federal government, which only regulates the Interstate Highway System, does not set maximum length requirements (except on auto and boat transporters), only minimums. Tractors can pull two or three trailers if the combination is legal in that state. Weight maximums are on a single axle, on a tandem, and total for any vehicle or combination. There is a maximum width of and no maximum height.
Roads other than Interstates are regulated by individual states, and laws vary widely. Maximum weight varies between to , depending on the combination. Most states restrict operation of larger tandem trailer setups such as triple units, turnpike doubles, and Rocky Mountain doubles. Reasons for limiting the legal trailer configurations include 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. In general, these configurations are restricted to the Interstates. Except for these units, double 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.
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. B-Triples are set up differently from 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 , its maximum load may be up to 164 tonnes gross, and may have up to four 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 daytime, especially four-trailer road trains.
Articulated trucks towing a single trailer or two trailers (commonly known as "short doubles") with a maximum overall length of 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 tonnes and overall length of , or 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 and a height of . In the past few years, allowance has been made by several states to allow certain designs of heavy vehicles up to high but they are also restricted to designated routes. In effect, a 4.6 meter 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 meter high vehicles.
In Australia, both conventional prime movers and 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.
Common combinations in New Zealand are a standard semi-trailer, a B-train, or a rigid towing vehicle pulling a trailer with a drawbar, with a maximum of nine axles. Standard maximum vehicle lengths for trailers with one axle set are:
High-productivity motor vehicle (HPMV) permits are issued for vehicles exceeding 44 tonnes, or the above dimensions. Trucks up to 62 tonnes were allowed, with an initial bridge strengthening program costing $12.5m.
The parking brake of the tractor unit and the emergency brake 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 a fail-safe design feature which ensures that if air pressure to either unit is lost, the vehicle will stop to a grinding halt, instead of continuing without brakes and becoming uncontrollable. 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 . Two air lines typically connect to 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 during normal operation.
In the UK, male/female quick release connectors ( red line or emergency), have a female on the truck and male on the trailer, but 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 around a primary black, a secondary green, and an ABS lead, all of which are collectively known as suzies or suzie coils.
In New Zealand all trucks and trailers use a DUOMATIC air coupler which has female receivers mounted on the truck/tractor and on the trailer, and male on both ends of the suzie lines (they can be completely removed and stored in the cab to prevent theft). Connecting the red and blue lines is one operation at each end. The red and blue lines are always on the same side of every fitting so they can never hook up in reverse or the wrong way around. The same system is used in Europe.
Another braking feature of semi-trucks is engine braking, which could be either a compression brake (usually shortened to Jake brake) or exhaust brake or combination of both. However, the use of compression brake alone produces a loud and distinctive noise, and to control 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 engine braking instead of conventional brakes is that a truck can descend 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.
Semi-truck transmissions can have as few as three 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 (Gear ratio ratio must also be considered).
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 one to five are Lo range; gears six to ten 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). The 13-, 15-, and 18-speed transmissions have the same basic shift pattern but include a splitter button to enable additional ratios found in each range. Some transmissions may have 12 speeds.
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 a smooth engagement of low gears when starting from a full stop; when the truck is moving, the clutch pedal is pressed only far enough to break torque for gear changes.
Theoretically, semi-trucks could have diesel-electric transmission, as electric motors have better torque at 0 RPM than diesel engines, but this would significantly increase the weight of the truck itself, above the maximum legal weight for road vehicles.
In most countries, a trailer or semi-trailer must have minimum
In Europe, super singles became popular when the allowed weight of semitrailer rigs was increased from 38 to 40 tonnes. In this reform the trailer industry replaced two axles with dual wheels, with three axles on wide-base single wheels. The significantly lower axle weight on super singles must be considered when comparing road wear from single versus dual wheels. The majority of super singles sold in Europe have a width of . The standard 385 tires have a legal load limit of . (Note that expensive, specially reinforced 385 tires approved for do exist. Their market share is tiny, except for mounting on the steer axle.)
Skirted trailers are often combined with Underrun Protection Systems ( underride guards), greatly improving safety for passenger vehicles sharing the road.
The bottom rear of the trailer is near head level for an adult seated in a car, and without the underride guard, the only protection for such an adult's head in a rear-end collision would be the car's windshield and A pillars. The front of the car goes under the platform of the trailer rather than making contact via the passenger car bumper, so the car's protective crush zone becomes irrelevant and are ineffective in protecting the passengers. If installed, the underride guard can provide a rigid area for the car to contact that is lower than the lip of the bonnet/hood, thus preventing the vehicle from squatting and running under the truck, instead ensuring that the vehicle's crush zones and engine block can absorb the force of the collision.
In addition to rear underride guards, truck tractor cabs may be equipped with a Front Underrun Protection System (FUPS) at the front bumper of the truck, if the front end is not low enough for the bumper to provide the adequate protection on its own. The safest tractor-trailers are also equipped with side underride guards, also called Side Underrun Protection System (SUPS). These additional barriers prevent passenger cars from skidding underneath the trailer from the side, such as in an oblique or side collision, or if the trailer jackknifing across the road, and this also helps protect cyclists, pedestrians, and other vulnerable road users. In the 1969 proposal for rear underride guards, the Federal Highway Administration indicated that, "It is anticipated that the proposed rear standard will be amended, after technical studies have been completed, to extend the requirement for underride protection to the sides of large vehicles". However, to date, a side underride guard mandate has yet to ever be proposed by the USDOT or NHTSA. In fact, for side underride guards, NHTSA has disregarded successful crash tests that stop a passenger vehicle from underriding a semitrailer, ignored recommendations, disregarded administrative petitions, and denied petitions. In fact, for decades NHTSA has ignored credible scientific research on side underride guards and failed to take simple steps to stop these crashes.
In Europe, side and rear underrun protection are mandated on all lorries and trailers with a gross weight of or more. Several US states and cities have adopted or are in the process of adopting truck side guards, including New York City, Philadelphia, and Washington DC. The NTSB has recommended that the National Highway Traffic Safety Administration (NHTSA) develop standards for side underride protection systems for trucks, and for newly manufactured trucks to be equipped with technology meeting the standards.
In addition to safety benefits, these underride guards may improve fuel mileage by reducing air turbulence under the trailer at highway speeds. Another benefit of having a sturdy rear underride guard is that it may be secured to a loading dock with a hook to prevent "trailer creep", a movement of the trailer away from the dock, which opens up a dangerous gap during loading or unloading operations. Underride Guard on Everything2; Retrieved: 29 November 2007.
Former semi-truck manufacturers include:
The following are the heavy vehicle license classes in Australia:
In order to obtain an 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 the license if testing in a synchromesh or automatic transmission vehicle. The B Condition may be removed upon the driver proving the ability to drive a constant mesh transmission using the clutch. Constant mesh transmission refers to crash box transmissions, predominantly Road Ranger eighteen-speed transmissions in Australia.
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. 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 tonnes.
Further information on the New Zealand licensing system for heavy vehicles can be found at the New Zealand Transport Agency.
In addition, endorsements are necessary for certain cargo and vehicle arrangements and types;
Various types of rail Flatcar are modified to hold the cargo trailer or container with wheels or without. This is called Intermodal or piggyback. The system allows the cargo to switch from 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, 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.
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