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).
Regional Configurations
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" or Wide-base 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. These tractors are referred to as day cabs and do not have sleepers.
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 movable 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 (D.O.T.) 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 or dolly. 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:
Triples: Three 28.5-foot (8.7 m) trailers; maximum weight up to 129,000 pounds (58.5 t).
Turnpike Doubles: Two 48-foot (14.6 m) trailers; maximum weight up to 147,000 pounds (66.7 t)
Rocky Mountain Doubles: One 40 (12.2 m) to 53 (16.2 m) foot trailer (though usually no more than 48 feet) and one 28.5-foot (8.7 m) trailer (known as a "pup"); maximum weight up to 129,000 pounds (58.5 t)
In Canada, a Turnpike Double is two 53-foot trailers and a Rocky Mountain Double is a 50-foot trailer with 24-foot "pup"
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.
The long-haul tractors 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.
