A semi-trailer truck, also known as tractor-trailer or (in the United Kingdom and Ireland) articulated lorry, is an articulated truck or lorry consisting of a towing engine (tractor in the United States, prime mover in Australia, and truck in the UK, Canada and New Zealand), and a semi-trailer (plus possible additional trailers) that carries the freight.
Colloquial terms for semi-trailer truck include truck and trailer, transfer truck, 18-wheeler, semi, Diesel, Mack truck (named for a prominent brand), big rig (US), transport (Canada), artic (UK and Ireland), and juggernaut (UK).
In North America, semi tractors usually have 3 axles, the front, or "steer", axle having two wheels, and each of the two rear, "drive", axles having a pair of "dualies" (double) wheels on each side. Thus, the most common configuration of tractor has 10 wheels, however in some cases dual wheels are replaced by tires known as "super singles" to reduce the weight of the tractor. In this case the tractor will only have six wheels. "Super singles" are substantially wider than normal tires. A smaller tractor having a single drive axle (six wheeler) is often used to pull shorter trailers in tight urban environments, such as downtown areas where a 60-foot rig would be too difficult to maneuver.
The cargo trailer usually has two "tandem" axles at the rear, each of which has dual wheels, or 8 wheels on the trailer. Many trailers are equipped with moveable tandems that can be set to balance the weight of the trailer to stay within legal limits.
Although the cargo's weight added to the semi's weight can equal a certain amount of gross some roads are marked with a different gross restriction so the roads are not damaged. Cargos that exceed allowed weights are usually marked with overweight load and must obtain a permit to use certain roads.
Rules governing the maximum size and weight of vehicles differ among states in the US. However, since the majority of hauling is done on the interstate system, the vast majority of trucks and trailer made in the US are built to the specifications of the Department of Transportation which governs the use of the interstate system. The D.O.T. has established vehicle limits of: 102 inches wide, 13.5 feet in height, and 80,000 lbs gross weight. These limits can be exceeded as individual states have the right to issue temporary oversize and/or overweight permits.
Trailer dimensions vary greatly depending on amount and type of cargo it was designed to haul. See types of trailers under Construction below.
Although dual wheels are most common, use of a single, wider tire (known as "super singles") on each axle is becoming popular, particularly among bulk cargo carriers and other weight-sensitive operators. The advantages of this configuration are dual: the lighter tire weight allows a truck to be loaded with more freight, and the single wheel covers less of the brake unit, which allows faster cooling. The biggest disadvantage is that when a tire becomes deflated or destroyed, it is not possible to drive the vehicle to a service location without risking damage to the rim, as it is with dual wheels.
The United States also allows 2-axle tractors to tow two 1-axle 28.5-foot (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. The second trailer in a set of doubles uses a converter gear, also known as a con-gear. This apparatus supports the front half of the second 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 part of the National Network.
LCV types include:
Regulations on LCVs vary widely from state to state. No state 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, the "Turnpike Double" (twin 48-53 ft units) or the "Rocky Mountain Double." (A full 48-53 ft unit and a shorter 28 ft unit) In general, these types of setups are restricted to tolled turnpikes such as I-80 through Ohio and Indiana, and select Western states. Tandem setups are not restricted to certain roads anymore than a single setup. The exception are the units listed above. They are also not restricted because of weather or "difficulty" of operation.
Used in the US:
See also: List of dump truck manufacturers.
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. Like in Scandinavia, these vehicles in The Netherlands will run a 60 tonne weight limit. Two types are to be used: 1) a 26 tonne truck pulling a dolly and semi-trailer, or 2) an articulated tractor unit pulling two semi-trailers linked by a dolly. The UK government has so far decided not to have its own trial of these 60 tonne vehicles, but to keep an eye on the other countries' trials.
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.
The largest trailer manufacturer in Europe is Schmitz Cargobull.
In continental Europe, most semi tractors have two axles, again with the front, steer, having two wheels, and rear, drive, having twin wheels on each side. Thus, the most common configuration has six wheels. The cargo trailer usually has three axles at the rear, with single wheels, or six wheels in total. The entire vehicle thus usually has five axles and 12 wheels in total, although the trailers can vary in number of wheels.
In Sweden the allowed length is 24 metres (78.7 ft) for all vehicles and 25.25 metres (82.8 ft) for trucks with two trailers. In 1997 the rules were changed, following the European Economic Area rules, allowing trucks to pull two trailers with a total length of 25.25 metres (82.8 ft), assuming certain conditions were met, like ABS on all vehicles. In Finland most trucks can tow any trailer as long as total length stays within 25.25 metres (82.8 ft). The exception to this is a tractor unit pulling a semi-trailer, which can be only 16.5 metres (54.1 ft) long. The allowed gross weight in both countries is up to 60 tonnes (132,277 lb) depending on the distance between the first and last axle. In Sweden the old style tractor-trailer is still the most common, but in some areas, especially container haulage, 25.25 metre (82.8 ft) vehicles are available. In Finland most new trucks and trailers are built with 25.25 metre (82.8 ft) in mind.
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. The 1996 EU regulation defines a Europe Module System (EMS) which was implemented by Swedish manufacturer Volvo into its product line. 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.
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), while some trailers have 2 axles which have twin wheels on each axle, and other trailers with 3 axles of which 1 axle can be a TAG axle which have 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 Arctic truck tractors which have 6 tyres on 2 axles; these are known as 4 wheelers.
Most [UK] trailers are 45 feet (13.5 meters) 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 meters). Although the Construction and Use Regulations allow a maximum rigid length of 60 feet (18.2 meters), 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 meters), 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.
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).
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 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 tonnes (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.
In most areas a truck is generally limited to two trailers and a total length of 26 metres (85 ft) and in urban areas this length limit is further reduced to 19 metres (62 ft). 25 metre (82 ft), 62.5 tonne (137,788 lb) B-doubles are very common in all parts of Australia including state capitals and on major routes may outnumber single trailer configurations.
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 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, that unit will not lose all braking capacity and become uncontrollable.
The trailer controls are coupled to the tractor through two "glad-hand" 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 polution, 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 equiped 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.
"Semi" truck transmissions usually provide at least nine or ten gear ratios, but possibly as many as eighteen (e.g. Australian Road Trains). A large number of transmission ratios means the engine itself can operate within a narrow range of speeds. The range of speeds over which an engine is expected to perform well has implications for the design - the narrower the range, the more the engine can be optimised for that. Also having so many gears allows fine-grained control of engine braking for better control on downhills and in curves.
A ten speed manual transmission 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 toggling a selector control - so that first gear becomes sixth, second becomes seventh, etc.
Another difference between semi-trucks and cars is the way the clutch is set up. On a regular car 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 e.g. Eaton Roadranger series constant mesh transmission (non synchronized) not only double clutching is required, there is an additon of clutch brake 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 bottom of the clutch pedal stroke is where the clutch brake activates and as a result only partial or "half" clutch pedal stroke is used when a vehicle is in motion.
In most countries a trailer or semi-trailer must have minimum
In addition, Endorsements are necessary for certain cargo and vehicle arrangements and types;
The Road Traffic Security Rules (zh:道路交通安全規則) require a combination vehicle driver license to drive a combination vehicle (). 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).
Further information on the New Zealand Licensing system for Heavy Vehicles can be found at Land Transport New Zealand
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.