A truck pulling a semi-trailer using a trailer dolly
A semi-trailer is a trailer without a front axle. A large proportion of its weight is supported by a road tractor, a detachable front axle assembly known as a dolly, or the tail of another trailer. A semi-trailer is normally equipped with landing gear (legs which can be lowered) to support it when it is uncoupled.
A road tractor coupled to a semi-trailer is often called a semi-trailer truck or semi, or in the UK an articulated lorry. The fifth wheel on a truck connects to a semi trailer Kingpin. Kingpins come in many guises, however the most common within the UK market is the 2.0" (50.8mm) EEC approved type. This Kingpin is fully interchangeable and, given a strict maintenance schedule, it should last the life of a trailer.
In Australian English, the tractor unit is usually referred to as a prime-mover; and the combination of a prime-mover and trailer is known as a semi-trailer or semi. Semi-trailers with two trailer units are B-Doubles or road trains. A B-double consists of a prime mover towing two semi-trailers, where the first semi-trailer is connected to the prime mover by a fifth wheel coupling and the second semi-trailer is connected to the first semi-trailer by a fifth wheel coupling. A road train means a combination, other than a B-Double, consisting of a motor vehicle towing at least two trailers (counting as a single trailer a converter dolly supporting a semi-trailer).
Advantages and disadvantages
A 1920 advertisement for semi-trailers
In road haulage, semi-trailers predominate over full-trailers because of their flexibility.
- The trailers can be coupled and uncoupled quickly, allowing them to be shunted for loading and to be trunked between depots.
- In the event of a breakdown, a tractor unit can be exchanged quickly and the load delivered to its destination without undue delay and without having to trans-ship.
- It is also possible to use a dolly to tow a semi-trailer behind a rigid truck, or behind another semi-trailer.
- Special tractors known as Shunt trucks (also as tugs, yard dogs, switchers, jockey horses, yard trucks or just shunts) are often used for internal transport, for example, maneuvering semi-trailers at a depot or loading and unloading ferries. These tractors may lift the coupling so that the trailer legs clear the ground.
- Compared with a full scale trailer, a semi-trailer attached to a tractor unit is easier to reverse, since it has only one turning point (the coupling), whereas a full trailer has two turning points (the coupling and the drawbar attachment).
- Compared with a rigid vehicle, a semi-trailer truck has a turning circle smaller than its overall length making it more maneuverable.
- Because of the longer overall length of the cargo bed, a semi-trailer can haul longer objects (logs, pipe, beams, railway track) than a full trailer.
- Given equal lengths of the composition, a semi-trailer has greater load capacity, since the drawbar adds to the overall length of the composition.
- A semi-trailer has a better ratio between tare and cargo weights.
- A semi-trailer leads to more weight on the driven axles of the tractor than a truck plus trailer combination, an advantage on snow or mud.
- Since a semi-trailer rests on top of a tractor it has a higher centre of gravity which makes it more prone to tipping than a rigid vehicle.
- Articulated vehicles can be more difficult to drive in snow and ice since the trailer loses traction more easily than a rigid truck. Using the retarder and/or engine brake only worsens the situation, making the composition more prone to jackknifing.
- A tractor-trailer composition is more likely to jackknife on ice because the semi-trailer weighs significantly more than the tractor.
- A rigid truck can be used without a trailer, whereas a semi-tractor has no use on its own.
There are two types of couplings: fifth wheel coupling and automatic. In some applications, no separable coupling is fitted, and the trailer is bolted to the tractor unit, using a bearing and rocker feet as are used under a fifth wheel skid plate.
Fifth wheel coupling
The towing vehicle has a wide coupling plate known as a fifth wheel coupling bolted onto its chassis on which the semi-trailer rests and pivots. As the tractor reverses under the trailer, a king-pin under the front of the trailer slides into a slot in the skidplate, and the jaws of the fifth wheel close on to it. The driver has to raise the trailer legs manually, and couple the airbrake lines and electrical cables.
Many years ago, automatic couplings predominated but are now quite rare. Automatic couplings were generally used for payloads of 12 tons or less, e.g. on the Scammell Mechanical Horse.
There is no coupling plate on the tractor. There is a turntable permanently fixed to the underside of the trailer. This locks to the chassis of the tractor. When the tractor reverses under the trailer, its legs rise and the brake and electrical connections are made automatically. Almost the entire coupling and uncoupling procedure is operated by the driver from inside the cab, except that he or she has to descend to release (or apply) the trailer parking brake.
A curtain sider semi-trailer without truck
Different types of semi-trailers are designed to haul different cargoes.
Common widths are 8 feet (2.44 m), and 2.6 metres (8 ft 6.4 in).
- The most common type of trailer. Also called a van trailer.
- Standard lengths in North America are 28 ft 0 in (8.53 m), 32 ft 0 in (9.75 m), 34 ft 0 in (10.36 m), 36 ft 0 in (10.97 m), 40 ft 0 in (12.19 m), 45 ft 0 in (13.72 m), 48 ft 0 in (14.63 m) and 53 ft 0 in (16.15 m).
- A bus bodied trailer hitched to a tractor unit to form a trailer bus, a simple alternative to building a rigid bus.
- Car-carrying trailer
- Carries multiple cars; usually new cars from the manufacturer. In the U.S., car carriers often carry used vehicles as well.
- Curtain sider
- A curtain sider is similar to a box trailer except that the sides are movable curtains made of reinforced fabric coated with a waterproof coating. The purpose of a curtain sider is to allow the security and weather resistance of a box trailer with the ease of loading of a flatbed.
- Drop-deck trailer
- A drop-deck trailer is a trailer on which the floor drops down a level once clear of the tractor unit; the most common types of drop-deck trailer are flatbeds and curtain siders.
- Double decker
- Double deckers or deckers are trailers with either a fixed, hinged or moveable second floor to enable them to carry more palletised goods. In general a double decker can carry 40 pallets, as opposed to 26 for a standard trailer. Double deck trailers are generally a stepped box or curtain siders, with box trailers having either a fixed or movable (floating) deck, and curtain sides having either a fixed or hinged second deck; this hinged second deck generally swings into a position down the length of the trailer, and can be divided into 2 or 3 sections to allow greater load flexibility.
- Dry Bulk
- Resembles a big tanker, but is used for sugar, flour, and other dry powder materials.
- Consists of just a load floor and removable side rails and a bulkhead in front to protect the tractor in the event of a load shift. Can haul almost anything that can be stacked on and strapped down.
- Hopper Bottom
- Usually used to haul grain, but can be used to haul other materials.
- Live bottom
- Has a conveyor belt on the bottom of the trailer tub that pushes the material out of the back of the trailer. The tub does not have to be raised to deposit the materials.
- Livestock Trailer
- Used to haul livestock such as cows, pigs, sheep, etc. Commonly have two levels to maximize capasity.
- Type of flatbed in which the load floor is as close to the ground as possible. Most commonly used to haul heavy equipment, cranes, bulldozers, etc.
- Reefer - see Refrigerator truck
- Box trailer with a heating/cooling unit (reefer) attached. Used for hauling produce, ice cream, meat, flowers, etc.
- Semi-trailer with hydraulic cranes mounted at both ends of the chassis allowing for the loading and unloading of shipping containers without the need of a forklift or other container handling equipment.
- Tanker - see Tank truck
- Used for hauling liquids such as gasoline, milk, orange juice, and alcohol.
- A type of tank trailer with a single and fixed axle, typically used during hydraulic fracturing at oil wells. It is shaped like a wedge, and when it is unhitched its bottom side lies flat on the ground.