Semi-Trailer Definition for the Freight Industry presented by Apparel Search 

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A semi-trailer truck or tractor-trailer (colloquially known as an 18-wheeler, semi, or big-rig in the US, as a semi in Australia, US, and Canada, and as an articulated lorry, artic, or truck and trailer in the UK, Ireland, and New Zealand) is an articulated truck or lorry consisting of a towing engine (tractor in the US, prime mover in Australia, "truck" in the UK and New Zealand), and a trailer that carries the freight.

Regional Configurations

United States

In United States, 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 "dual" (double) wheels on each side. Thus, the most common configuration of tractor has 10 wheels. The cargo trailer usually has two "tandem" axles at the rear, each of which has dual wheels, or 8 wheels on the trailer.

Although dual wheels are most common, use of a single, wider tire on each axle is becoming increasingly popular, particularly among bulk cargo carriers and other weight-sensitive operators. The advantages of this configuration are two: The lighter weight allows a truck to be loaded with more weight of product, 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.

However, the United States also allows 2-axle tractors to tow two 1-axle 28-foot (8.5 m) semi-trailers known colloquially as doubles, a set, or a set of joints. Some states also allow towing up to three 28-foot trailers known colloquially as triples or road trains. A 2-axle full-sized semi-trailer pulling a 28-foot "pup" trailer known as a Rocky Mountain Double is also permitted in some regions. Very few states allow two full-sized semi trailers which are similar to the Australian road trains. 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.

Overall lengths often range from 50 to 70 ft. (15 to 25 m) in the US, and most US states limit the overall weight to 80,000 lb. (36 ton) The long-haul towing engines used in interstate travel are often equipped with a "sleeper" behind the driver's cab, which can be anything from a small bunk to a rather elaborate miniature apartment.

Europe in general

In Europe, most semi tractors have 2 axles, again with the front, steer, having two wheels, and rear, drive, having a pair of double wheels on each side. Thus, the most common configuration has 6 wheels. Conversely, the cargo trailer usually has three axles at the rear, with single wheels, or 6 wheels in total. One way or the other, the entire vehicle thus usually has 5 axles and 12 wheels in total, although the trailers can vary in number of wheels.

The noticeable difference between trucks in the US and lorries in Europe is the lack of a nose on European models. While some US trucks are built without a nose, they are not as common. In European design, the driver's cab is positioned above the engine. For repairs, the entire cab hinges forward to allow maintenance access. European lorries, whether small or fully articulated, have a sheer face on the front. This allows greater maneuverability when steering, as the driver need only gauge distances behind his seating point, and this allows for shorter trucks with longer trailers (with larger freight capacity) within the legal maximum total length. In Europe the entire length of the vehicle is measured as total length, while in US the cabin of the truck is normally not part of the measurement.

Sweden and Finland

In Sweden the allowed length is 24 meters for all vehicles and 25.25 meters for trucks with two trailers. In 1997 the rules were changed, under pressure from the EU, allowing trucks to pull two trailers with a total length of 25.25 meters, assuming certain conditions were met, like ABS on all vehicles. In Finland most trucks can tow trailer as long as total length stays within 25.25 meters. The exception to this is a tractor unit pulling semi-trailer, which can be only 16.5 meters long. The allowed gross weight in both countries is up to 60 metric tons depending on the distance between the first and last axle. In Sweden the old style tractor-trailer is still the most common overall, but in some areas, especially container haulage, 25.25 meter vehicles are coming strong. In Finland most new trucks and trailers are built with 25.25 meter in mind.

Using a dolly, which has to be equipped with lights and a license plate, rigid trucks can be used to pull semitrailers. The dolly is equipped with fifth wheel to which the trailer is coupled. The dolly and trailer together act like a regular trailer, so driving it and backing up is usually no different.
The old truck-trailer configuration is almost the only style used on timber trucks. There are at least two big advantages with this, one is the weight of the load on the drive wheels, and two, that the crane used to lift the logs from the ground can be mounted on the rear of the truck behind the load, instead of behind the cab which would make it difficult to reach to the end of the trailer.


Australia has a reputation of having very large trucks. This is reflected in that the most popular configurations of trucks generally have axles in groupings of 3 rather than 2, with either 4 or 6 tires on each axle. This means that Australian semi-trailers will often have 26 or even 32 wheels which is generally more than their counterparts in other countries. In total, the maximum length that any articulated vehicle may be is 53.5 meters, its maximum load may be up to 115.5 tons gross and may have up to 4 trailers. However heavy restrictions apply to the areas where such a vehicle may travel in many of the more densely populated states. In less remote areas a truck is generally limited to two trailers to 25 meters long and in urban areas this length limit is further reduced to 19 meters. 25 meter, 62 ton B-doubles are very common in all parts of Australia including state capitals and on certain roads actually outnumber single trailer configurations, however these vehicles typically travel at night and by law stay on main roads so are not encountered as often by passenger vehicle drivers. 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 Australia, both semi-trailers with and without "noses" are common, however those without noses are most often seen on B-Doubles on the south east coast where the reduction in total length allows the vehicle to pull longer trailers and thus more cargo than it would otherwise.


The cargo trailer is hooked to a horseshoe-shaped coupling device called a fifth wheel at the rear of the towing engine that allows easy hook up and release. The trailer cannot move by itself because it only has wheels at the rear end, hence the name semi-trailer: it only carries half its own weight. The vehicle has a tendency to fold at the pivot point between the semi and the trailer when braking hard at high speeds. Such a truck accident is appropriately called a jack-knife, or jack-knifing.

Semi trucks use air pressure, rather than hydraulic fluid to actuate the brakes. This 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-fade" caused when overheated brake fluid vaporizes in the hydraulic lines. (Brake fade may also occur when the lining of the braking unit becomes severely overheated. This has no connection to the fluid lines.)

The "parking brake" of the tractor unit and the "emergency brake" of the trailer are applied when air pressure is released, 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 are air couplers, 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 when they are pulled, as may happen when the tractor and trailer are separated without first uncoupling the air lines.

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.

Another braking feature of semi-trucks is the "engine brake", colloquially known as the "Jake brake". This feature uses the engine to slow the vehicle, which allows trucks to travel down long grades without overheating their wheel brakes. Due to noise concerns, some locales have prohibited or restricted the use of engine brake systems inside their jurisdictions

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. A special driver's license is required to operate a semi-trailer in most countries.

Road Trains

In Australia, semi-trailers with more than one trailer are known as road trains. In certain areas "B-doubles" are permitted. These include a modified trailer with a turntable at the rear to allow a second trailer to be tightly coupled to the rig without the extra cost and handling problems of a dolly.

On some interstate highways in the US, long-haul semi-trailer trucks can tow another full trailer at the end, which makes the vehicle look like a two-car small train. Some of the second cars are full trailers with wheels on both ends, while others are just regular semi-trailer cars hooked to the standard coupling device on another set of wheels in tow (sometimes referred to as a "dolly"). Some states further allow a third trailer to be added to the vehicle, against the objections of some car drivers who must share the highways with these longer trucks.

Role in Industry

Modern day semi-trailer trucks often operate as a part of an international transport infrastructure to support containerized cargo shipment. Some flat bed train cars are modified to hold the cargo trailer with wheels and all. This is called "piggy-back" in North America. The system allows the cargo to switch from the highway to railway or vice versa with ease.

The large trailers pulled by a semi come in many styles, lengths, and shapes. Some common types are: vans, reefers, flatbeds, container lifts 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.


Semi-trailer is the name of the kind of trailer that a prime mover pulls, but in general use refers to the whole assembly. "Semi" refers to the fact that when attached to the tractor unit the cargo is half supported by the rear wheels of the tractor.

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Definition modified by Apparel Search 12/27/05.
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