PBS A doubles combinations
An A Double combination refers to a category of combination that consists of a Prime Mover truck towing two trailers which is where the "double" part of the name comes from. The A in the title refers to the drawbar coupling between the two trailers. This is different from the conventional B Double, which also has two trailers but is connected through a B coupling, which involves the lead trailer having a fifth wheel on the rear. The advantages of an A Double are that the additional articulation point increases manoeuvrability, allowing for better on-road performance and therefore safety at longer overall lengths, while the lead and rear trailers can be the same standard semi-trailers as opposed to a B double which requires a specific lead trailer. The additional axle group also allows for significantly higher overall combination masses, resulting in higher productivity, which can be a game changer for operators.
What are the different types of PBS A double combinations?
There are two ways that the trailers of an A double combination may be configured. The first is where a truck towing a semi-trailer is coupled to a dog trailer (generally called a full trailer in Europe). Alternatively, a semi-trailer may be coupled to a converter dolly (also called road train dolly or simply dolly) with its own fifth wheel which is then coupled to another semi-trailer. This configuration of two semi-trailers and a dolly is the more common type in Australia as it allows the greatest flexibility to mix and match trailers and make use of existing semi-trailer assets.
The most common axle configuration of semi-trailers used in A Doubles is a standard tri-axle group. This is typically paired with a tandem axle dolly as the kingpin weight capacity of the dolly is similar to that of a conventional 6x4 prime mover. This means that operators can use the same lead and rear trailers and load them in the same way as they have the same payload capacity. Often this type of configuration is referred to as a 3-2-3 A Double, with these numbers corresponding to the axle groups on each trailing unit in the combination. 3-3-3 A Doubles that use a tri-axle dolly are becoming increasingly popular as they can have better road access and carry slightly more weight. Additional considerations when running a tri-axle dolly are:
Is the Aggregate Trailer Mass (ATM) of my rear trailer high enough to allow for maximum payload? (The standard 38.0t for a tri-axle semi is typically not sufficient).
Does my rear trailer and dolly have enough landing leg clearance to prevent contact while in use?
Can I load the rear trailer with enough weight over the kingpin to make use of the additional dolly axle?
This last consideration is why typically 3-3-3 A Doubles are used for bulk materials such as gravel, grain & fertiliser where the distribution of weight on the trailer can be easily altered. Another less common type of A Double is a 2-2-2 configuration, sometimes referred to as a pocket road train due to its shorter overall length of up to 26m.
What freight tasks are PBS A double combinations suitable for?
A Doubles are well suited to both cubic and mass freight and therefore can be used for a wide variety of transport tasks within the transport industry.
Cubic freight is typically carried with two longer trailers and a shorter dolly drawbar. For Performance Based Standards Level 2B combinations up to 30m in length this would typically be 2x20 pallet trailers for 40 pallets of overall space. With a PBS Level 3A vehicle up to 36.5m in length the trailers can be extended to 24 pallet variants for a total carrying capacity of 48 pallets. This represents a significant productivity increase over the prescriptive B double combination which typically consists of a 12 pallet lead trailer and 22 pallet rear trailer for 34 pallets of overall capacity. This makes PBS A Doubles an attractive solution for line haul, general freight and refrigerated freight.
The weight limits for a 3-2-3 and 3-3-3 A double can be up to 86.1t & 91.6t when operating at higher mass limits (HML). This can result in a large productivity jump over conventional mass carrying freight vehicles such as truck and 4A dogs and B Doubles. Time savings can also be present where ordinarily a B double may need to be uncoupled and the axle group slid forwards to be able to tip. This makes A Doubles an attractive choice to transport grain, livestock feed, fertiliser, sand, gravel and other construction aggregates.
PBS A Doubles are also well suited to container carrying. A typical 30m A Double can carry two 40ft containers or four 20ft containers. The additional weight that these vehicles can carry provides additional flexibility and the ability to carry two heavy containers at once. B doubles, while having space to carry more, can typically only carry one heavy container at a time without exceeding mass limits.
How can BTT Engineering help you get the most out of your PBS A Double combination?
BTT Engineering has extensive industry experience with PBS vehicles, including A doubles, and has the team to help with understanding the complexities of the entire certification process, including
Helping identify maximum mass limits that your combination dimensions allow and compliance with relevant reference vehicles
Assistance with compliance to additional network requirements for example the High Productivity Freight Vehicle (HPFV) scheme in Victoria
Inspecting PBS combinations as a Performance Based Standards certifier to get your vehicles approved
Arranging for a PBS assessor to perform assessment for a design approval
Being able to provide guidance on route and road access including permits, notices and gazetted
Working with partners to supply additional required systems such as TMA and smart on board mass (Smart OBM)
Contact us today to discuss your specific equipment and requirements and find out how we can help with the PBS process for you.
Related PBS services & other combinations we can assist with
A Triples
AB Triples
B Triples
B Quad
Quad axle semis
PBS A Doubles combination FAQs
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According to the National Heavy Vehicle Regulator there are three different definitions for a road train which are briefly explained below:
Road Train Labelling
Any vehicle that exceeds 30m in overall length must display 'ROAD TRAIN' signage at the front and rear of the vehicle
Road Train Certification
The National Heavy Vehicle Law has the following definition "Road trains are multi-vehicle combinations consisting of a Prime mover towing at least two trailers (and is not a B-double)". Therefore for certification, an A Double regardless of overall length must be considered a road train combination. As a result, each prime mover and trailer used in a road train combination must be road train certified. If Prime Movers or trailers are not Road Train certified from the factory they will require an upgrade and engineering certificate (Modification Plate). More details on road train trailer requirements can be found at https://www.nhvr.gov.au/road-access/access-management/applications/road-train-permit/road-train-trailer-requirements
Road Train Access
Certain jurisdictions have class 2 road networks for road trains, the same way that they also have B Double networks. This is outside of the Performance Based Standards scheme and typically involves some prescriptive limits such as minimum and maximum dimensions, minimum horsepower etc. If a combination meets the conditions it may operate on the network without the need for a PBS vehicle approval or access permit. These networks are typically found in the western states, North West Victoria, New South Wales, west of the Newell and Queensland. A PBS A Double may operate on the road train network without permit provided it meets the specific requirements. Access to any route outside this network will require a permit.
To summarise, all vehicles within an A Double must be road train certified, road train signs displayed if the combination length is greater than 30m (Under 30m 'LONG VEHICLE' signs are displayed) and PBS A Doubles can operate on road train networks provided they meet the specific requirements of that network.
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This depends on the configuration of axles and weight limit they are operating at. Maximum masses at Higher Mass Limits (HML) are:
74.5t for a 2-2-2 A Double with 6x4 Prime Mover
86.1t for a 3-2-3 A Double with 6x4 Prime Mover
91.6t for a 3-3-3 A Double with 6x4 Prime Mover
Certain bridges may have lower mass limits that can prevent the above limits being utilised.
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While A Doubles can be a range of different lengths, they are typically classified into different levels to assist with road access:
PBS Level 2A Less than 26m overall length
PBS Level 2B Between 26 and 30m overall length
PBS Level 3A between 30 and 36.5m overall length