Into the Details: All About Road Trains

Road trains, as they are known in Australia, are large commercial freight trucks (aka prime movers) that haul two or more trailers or semi-trailers. They are a critical component of the trucking industry in Australia. Vintage Road Haulage reports that Australia has road-legal road trains weighing as much as 200 tonnes—the longest and heaviest in the world.

In Australia, road trains are divided into three categories, depending on how the trailers are connected to one another (coupled). The first category is the B-double, or B-train. The B-train differs from a standard road train in that a fifth wheel is attached to the back of the lead trailer, enhancing the train’s stability and safety. B-trains can measure up to 26 metres in length.

Thanks to a turntable mounted on the forward trailer, a B-train does not require a converter dolly like other road trains do¹. It has a large turning radius and is easier to reverse than other configurations. The B-train is resistant to yaw (twisting or oscillation of a moving vehicle about a vertical axis) in all but the most extreme circumstances.

Unlike A-type (converter dolly) couplings, B-type (fifth wheel) couplings will transfer roll and load between vehicle elements.

B-trains are especially popular with many food service companies around the world, as “a set of two [trailers] can be pulled, usually overnight, to a city within 500 miles of the distribution center,” according to Big Truck Guide. The B-train is also frequently used to haul flatbed, bulk and liquid goods.

The second category of road train includes the B-triple road train (two lead trailers), double road train (a semi-trailer and five-axle dog trailer), AB-triple road train (a semi-trailer, converter dolly and B-train) and BAB-quad road train (two B-trains linked by a converter dolly).

The third category, the triple road train, is made up of a prime mover and three semi-trailers hooked together using two converter dollies.

With certain road train configurations, a unit’s Gross Combination Mass (GCM) is limited by law to less than the sum of its axle groups. For example, a 7-axle B-train can have a maximum GCM of 50.5 tonnes for general access, or 56.0 tonnes for restricted access. All configurations must satisfy minimum axle spacing requirements for the load in question.

 

¹ The Australian Trucking Association, TAPsDescription of Truck Configurations. https://www.truck.net.au/sites/default/files/TAPs%20-%20description%20of%20truck%20configuration%20September%202016.pdf P.3-6, September 2016, Retrieved 30 March 2019.

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