Australia

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 metres, its maximum load may be up to 115.5 tonnes 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 metres long and in urban areas this length limit is further reduced to 19 metres. 25 metre, 62 tonne 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.

Construction

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.

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, containerlifts 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.

Etymology

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.