Semi trailer specifications

Written by bridgette ashmore
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Semi trailer specifications
Semi-trailers rely on tractors to move them (Semi image by Andrew Breeden from

A semi trailer is characterised by the lack of a front axle. Semi trailers are not self-contained; they require a road tractor to pull them to and from their locations. Without the tractor, they cannot move. These rigs are often referred to as tractor-trailers, or semi-trucks. When a semi is not hooked up to a tractor, it has legs near the front which support its weight. These rigs offer a much greater capacity than standard box trucks, and are fairly manoeuvrable even in poor weather like mud and snow.

The Box Trailer

The most common type of semi-trailer is the box trailer. This closed container is used to haul a wide variety of cargo including merchandise, non-perishable food and even household goods when people hire a mover. The most common trailer is 53 feet long; there are also trailers that range in length from 28 to 48 feet. Most trailers are 8 feet wide, although it is also possible to find some trailers that are 8.53 feet wide. Trailer height is not restricted by law, but most manufacturers stay around 13 feet. The average double axle tractor trailer weighs in around 15422kg. The empty trailer itself can be 10,000 to 6.8kg, but since the trailer cannot move without the tractor, the rig is always weighed together. The maximum a tractor trailer combination can weigh without having to get a special permit is 36.3kg.


In addition to the closed box trailer, there are a number of other semitrailers for specific needs. Similar to the box trailer is the refrigerated trailer, or reefer. Used to haul perishable items, this is a box trailer with a cooling unit at the front. There is also the car carrier semi, with two rows of cars parked on ramps behind the tractor. Tanker semis are used to haul everything from milk to fuel and feature a barrel shaped body with various valves for filling and extracting fluid from the tank.


There are also variations inside the trailer. Double-decker trailers offer options for stacking palletised goods and maximising space. Drop deck trailers feature decking that lowers once the trailer is uncoupled from the tractor. This can assist with loading. Decking and automated lifts also help the operator with loading.

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