What is a B-Double? Configuration, Mass Limits, and A-Trailer/B-Trailer Roles
A B-double is a prime mover towing two semi-trailers coupled directly to each other by a fifth wheel and kingpin, the same coupling type used between the prime mover and a standard semi-trailer. That fifth-wheel-to-fifth-wheel connection is what separates a B-double from a road train: a road train links its extra trailers with a drawbar and turntable, which lets each trailer steer somewhat independently. A B-double's rigid fifth-wheel coupling holds the combination in a tighter, more stable line through corners and lane changes.
That stability is why B-doubles carry more freight per trip than a single semi-trailer without the handling penalties of a longer drawbar combination, and why they're one of the most common linehaul configurations on Australian highways.
This guide covers how the lead (A-trailer) and rear (B-trailer) roles differ, the mass limits that apply at each length and axle count, and how to work out whether a B-double suits your freight task.
A B-double is a prime mover plus two semi-trailers joined by fifth wheel couplings, not drawbars. The lead trailer (A-trailer) carries a second fifth wheel at its rear to couple the rear trailer (B-trailer) directly. Under general access, a full 26 metre B-double is rated at 62.5 tonnes GML (63.0 tonnes with a complying steer axle), the highest tier in a graduated scale that starts at 50.0 tonnes for a 19 metre combination.
What Is a B-Double?
A B-double combination has three parts: the prime mover, the A-trailer (lead trailer), and the B-trailer (rear trailer). The prime mover couples to the A-trailer the same way it would to any semi-trailer, via a kingpin dropping onto the prime mover's fifth wheel. What makes the A-trailer different from a standalone semi is a second fifth wheel mounted at its own rear, which the B-trailer's kingpin couples onto in exactly the same way. The B-trailer has no independent front axle group of its own; it rests on the A-trailer's rear fifth wheel just as the A-trailer rests on the prime mover's.
That double fifth-wheel arrangement is the entire distinction from a road train. A road train (a prime mover towing one or more dog trailers via drawbar and turntable) allows each trailer to steer somewhat independently of the one in front, which increases the combination's swept path through corners. A B-double's rigid fifth-wheel-to-fifth-wheel coupling keeps both trailers tracking in a much tighter line behind the prime mover, which is why B-doubles are approved for a wider network of general access roads than road trains are.
Both the A-trailer and B-trailer are typically built as standard semi-trailer types (flat top, drop deck, skeletal, and others) fitted with the rear fifth wheel where required on the A-trailer. There's no separate "B-double trailer" product as such; a B-double is a combination of two purpose-built semi-trailers running together.
Australian Mass Limits for B-Doubles
General access dimension limits step up with combination length, and mass limits step up alongside them. B-doubles run under a graduated General Mass Limit (GML) scale set by axle count and overall combination length, distinct from the mass limits that apply to a single semi-trailer.
19 m (general access): 50.0 t GML
Up to approx. 21 to 23 m (restricted access): 55.5 t GML (56.0 t with complying steer axle)
Full 26 m B-double: 62.5 t GML (63.0 t with complying steer axle)
Source: NHVR, National Class 2 B-double Authorisation Notice 2026 (No.1): Operator's Guide. Concessional Mass Limits (CML) can add a further 1 tonne on combinations up to 55 tonnes, or 2 tonnes on combinations above 55 tonnes, where the operator holds the relevant accreditation. Higher Mass Limits (HML) and Performance Based Standards (PBS) approval can lift these figures further again, assessed on the specific combination and route.
Length limits also vary by state. General access sits at 19m in most jurisdictions, while a full 26m B-double requires the additional articulation and equipment conditions set out in the Notice. Tasmania runs its own general access figures (21.3m for a B-double fitted with a rear load restraining guard, 21m without).
B-Double Axle Configurations
4-axle combinations
A single drive axle prime mover paired with single-axle A-trailer and B-trailer gives the lightest common B-double configuration. It suits lower payload freight where the combination's main advantage is running two shorter trailers rather than maximum tonnage.
7-axle combinations
A tandem (dual) drive prime mover paired with tandem-axle A-trailer and B-trailer is the mid-tier configuration, lifting GML meaningfully over the 4-axle setup while keeping the combination inside the 19m general access length in many configurations.
10-axle combinations
A tandem drive prime mover paired with tri-axle A-trailer and tri-axle B-trailer is the configuration that reaches the top of the GML scale, up to 62.5 tonnes (63.0 tonnes with a complying steer axle) at the full 26m combination length. South Australia permits additional tri-drive prime mover variants (4 to 5 axles on the prime mover itself) for further axle count and mass options.
What Do People Use B-Doubles For?
General linehaul freight where a single semi-trailer's payload isn't enough for the route economics
Palletised and general freight, run as two flat top or drop deck trailers coupled together
Container and skeletal transport, including dangerous goods ISO container movements
Agricultural and bulk product haulage on approved B-double routes
Any freight task where the extra payload of a second trailer outweighs the reduced route access compared to a single semi-trailer
B-double route access is more restricted than general access single semi-trailers, since the extra length and mass require a road network approved for the configuration. Confirm your regular routes are on the approved B-double network before committing to the combination.
Midland's B-Double Trailers
Midland Pty Ltd manufactures both A-trailer and B-trailer configurations at its Parkes NSW and Kyneton VIC facilities, across flat top, drop deck, and skeletal builds. Because A-trailers need the rear fifth wheel fitted correctly to the specific B-trailer they'll run with, and B-trailers need to match the A-trailer's coupling height and axle configuration, most B-double builds are specced as a matched pair around the operator's actual freight and route.
Move More With Certainty means the combination is built to run together properly from day one, not fitted together after the fact.
How to Choose the Right B-Double Configuration
Is your route approved for B-double access? Confirm the specific roads you'll run on are on the approved B-double network before speccing the combination. B-double access is narrower than general access for a single semi-trailer.
What's your payload target? This drives axle count. A 4-axle combination suits lighter freight, while a 10-axle combination at the full 26m length and 62.5 tonne GML suits the heaviest general access payloads.
What trailer types do you need for the A- and B-trailer? Flat top, drop deck, and skeletal are all common choices depending on freight. See our flat top and drop deck guides for how those decisions work for a standalone semi-trailer; the same considerations apply to each trailer in a B-double pair.
Do you need HML, CML, or PBS approval? If your payload requirements exceed standard GML at your combination length, confirm what additional accreditation your operation holds or needs before finalising the spec.
Frequently Asked Questions
What's the difference between a B-double and a road train?
A B-double joins its trailers with a fifth wheel and kingpin, the same coupling used between a prime mover and a semi-trailer, which holds both trailers in a tight, stable line. A road train joins its extra trailers with a drawbar and turntable, letting each trailer steer somewhat independently and increasing the combination's swept path through corners. B-doubles are approved for a wider general access road network than road trains because of this tighter tracking.
What's the difference between an A-trailer and a B-trailer?
The A-trailer is the lead trailer, coupled to the prime mover's fifth wheel via a kingpin exactly like a standalone semi-trailer, but fitted with a second fifth wheel at its own rear. The B-trailer is the rear trailer, coupled to that second fifth wheel via its own kingpin. The B-trailer has no independent front axle group; it rests on the A-trailer's rear fifth wheel the same way the A-trailer rests on the prime mover.
How much can a B-double carry?
It depends on combination length and axle count. Under general access GML: a 19 metre B-double is rated at 50.0 tonnes, restricted access combinations up to roughly 21 to 23 metres reach 55.5 tonnes (56.0 tonnes with a complying steer axle), and a full 26 metre B-double reaches 62.5 tonnes (63.0 tonnes with a complying steer axle). These figures come from the NHVR's National Class 2 B-double Authorisation Notice Operator's Guide. Concessional Mass Limits can add a further 1 to 2 tonnes for accredited operators, and HML or PBS approval can lift the figures further again.
What axle configurations do B-doubles run?
Common configurations are 4-axle (single drive prime mover with single-axle A- and B-trailers), 7-axle (tandem drive with tandem-axle trailers), and 10-axle (tandem drive with tri-axle A- and B-trailers), the last of which reaches the top of the GML scale at the full 26 metre length. South Australia permits additional tri-drive prime mover variants for further axle and mass options.
Do I need a permit to run a B-double?
B-doubles require access to a road network specifically approved for the configuration, which is narrower than general access for a single semi-trailer. Confirm your intended routes are on the approved B-double network before committing to the combination. Combinations at the full 26 metre length or requiring HML, CML, or PBS mass also carry their own approval conditions on top of standard route access.
Can any semi-trailer be used as a B-double A-trailer?
Not without modification. An A-trailer needs a second fifth wheel fitted at its rear, positioned and rated to match the specific B-trailer it will run with. A standard semi-trailer without that rear fifth wheel can only run as a standalone semi or as the rear (B-trailer) position, not the lead position, in a B-double combination.
What trailer types are used in B-double combinations?
Flat top, drop deck, and skeletal trailers are all common choices for both the A-trailer and B-trailer positions, matched to the freight being carried. The trailer type decision for each position in a B-double follows the same logic as choosing a standalone semi-trailer, covered in our flat top and drop deck guides.
Talk to Midland
Midland Pty Ltd manufactures matched A-trailer and B-trailer combinations at Parkes NSW and Kyneton VIC. If you're planning a B-double for a specific route and freight task, tell us what you're carrying and we'll work through the configuration with you.
Related guides:
Flat top trailers | Drop deck trailers | Semi trailer types explained