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What is a semitrailer and why are they so common?

A semitrailer or ‘semi’ is a combination vehicle comprising a tractor unit towing a trailer via a fifth wheel coupling. The trailer only has wheels at the back (hence being called a semitrailer rather than a full trailer), and the front of it is supported on the fifth wheel.

The tractor unit can have up to four axles and the semitrailer (the name for the trailer as well as the combination of tractor unit and trailer) can also have up to 4 axles (called a ‘quad’).

The driver reverses the tractor unit under the front of the semitrailer. A kingpin under the semitrailer locks into the fifth wheel and the weight of the semitrailer is taken up by the fifth wheel when the trailer’s landing legs are raised.

Advantages of using a semitrailer over other types of trucks

Compared to a B-train, semitrailer are much easier to reverse and drive.

Compared to a rigid truck and trailer, a semitrailer can be dropped off to be unloaded, and can leave immediately with a new load, without having to wait to be unloaded or loaded.

Compared to a B-train, rigid truck, or rigid truck plus trailer, the cargo length is much longer because it is one trailer rather than two.

A tractor unit is very flexible. Assuming it has sufficient specifications, it can be pulling a linehaul curtainsider one day, a bottom dumper spreading aggregate the next day and a log trailer the day after, whereas it’s difficult to repurpose a rigid truck. If fitted with a bolt and pin coupling, a tractor unit can tow a full trailer.

They are common and therefore relatively cost-effective to purchase and resell.

The minimum number of axles on a semitrailer combination is three, whereas with a truck and trailer it is four.

A rigid truck can tow a semitrailer if using a dolly.

You can hire a semitrailer in many different configurations easily.

Dimensions

A regular tractor unit and semitrailer combination is a maximum of 19m long, 4.3m high and 2.55m wide. The maximum trailer length is 13.5m. The maximum gross mass is 39,000kg, although permits for larger vehicles are available for specific routes.

What licence do you need to drive a semitrailer?

If you’re driving it on a road (or what NZTA says is a road), then you need a class 5 licence. Here’s how to get a class 5 licence.

What sort of semitrailer combinations are available?

Curtainside

Read more about curtainsiders here

Hardside

A hardside refrigerated trailer

Transporters

Quad-axle transporter – transporters are generally used for moving machinery, long loads like steel bars, and building materials

Car transporter

Cars are driven onto the car transporter to be transported long distances by road

Tipulators or tippers

Tipper truck being loaded by a front-end loader at a quarry

Tipulators have a hydraulic ram that lifts the front of the truck so that the load can be ejected from the back.

Skeletal and sideloaders

Skeletal trailers are designed to have ISO containers placed on them by a crane, whereas sideloaders can automatically load the ISO container from the side.

Skeletal semitrailer
Sideloader semitrailer

Specialist transporters

Specialist semitrailers move things like houses, boats and specific machinery

What other names do semitrailers have

If you come from the UK you’ll know a semitrailer as an HGV (heavy goods vehicle), LGV (large goods vehicle) or ‘artic’, which is short for articulated lorry, and if you’re older, you might know them as juggernauts (coming from the definition, which means a massive inexorable force or an object that crushes whatever is in its path).

If you’re from Australia, you might call them a single. If it’s a skeletal trailer, they’re often called a semiskel.

If you’re from North America, you might know them as an 18-wheeler, a truck, a big rig, a tractor trailer or semi-tractor truck.

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Darren is a member of the Institute of Advanced Motorists and the NZ Motoring Writers' Guild

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