Semi-trailer

A tractor unit pulling a semi-trailer
A truck pulling a semitrailer using a trailer dolly

A semi-trailer is a trailer without a front axle. The combination of a semi-trailer and a tractor truck is called a semi-trailer truck (also known simply as a "semi-trailer", "tractor trailer", or "semi" in the United States).[1]

A large proportion of a semi-trailer's weight is supported by a tractor unit, or a detachable front-axle assembly known as a dolly, or the tail of another trailer. The semi-trailer's weight is semi-supported (half-supported) by its own wheels, at the rear of the semi-trailer.[2][3][4][5] A semi-trailer is normally equipped with landing gear (legs which can be lowered) to support it when it is uncoupled. Many semi-trailers have wheels that are capable of being totally dismounted and are also relocatable[6] (repositionable) to better distribute load to bearing wheel weight factors.[7] Semi-trailers are more popular for transport than full trailers, which have both front and rear axles. Ease of backing is cited as one of the semi's chief advantages. A road tractor coupled to a semi-trailer is often called a semi-trailer truck or "semi" in North America and Australia, and an articulated lorry or "artic" in the UK.

Semi-trailers with two trailer units are called B-doubles (Australian English) or tandem tractor-trailers,[8] tandem rigs, or doubles (American English). Other terms used are "B-train" or (when there are three or more trailers) "road train". A double-trailer combination is possible with the use of a dolly, or "converter dolly" (Australian and American English), essentially one to three additional axles placed under the front of a second semi-trailer. The first semi-trailer is connected to the power unit using the tractor's fifth wheel coupling while the converter dolly, already attached to the second semi-trailer, is connected to the first semi-trailer with a drawbar. In Australian English, the tractor unit is called a "prime-mover", and the combination of a prime-mover and trailer is known as a "semi-trailer", "semi" or single. Some popular manufacturers are Kenworth, Iveco, Freightliner, MAN, Scania, Mercedes-Benz, DAF, Renault Trucks, Volvo, Peterbilt, and Mack.

  1. ^ "Definition of SEMITRAILER". www.merriam-webster.com.
  2. ^ "Etymology - Why is it called a semi-truck? Doesn't "semi" mean "part"?".
  3. ^ "10 Things You Never Knew About Semitrucks". 6 November 2018.
  4. ^ "What is a Semi Truck?". 27 August 2019.
  5. ^ "Why are they called Semi-Trucks anyway?". 25 February 2015.
  6. ^ "Know how to slide your Tandems". www.bigtruckride.com.
  7. ^ "Positioning The Trailer Tandems - High Road Online CDL Training". www.truckingtruth.com.
  8. ^ "TRANSPORT & MACHINERY :: ROAD TRANSPORT :: TRUCKING :: TANDEM TRACTOR TRAILER image - Visual Dictionary Online". visual.merriam-webster.com.