What a B-double is
A B-double is a prime mover towing two semi-trailers. The lead (A) trailer has a fifth wheel at its rear, and the second (B) trailer couples to it. A common Australian set-up has a shorter A-trailer and a longer B-trailer, for example a 7 m A-trailer and a 13 m B-trailer.
A standard 9-axle B-double (steer axle, tandem drive, and a tri-axle group under each trailer) is limited to 62.5 tonnes gross combination mass at General Mass Limits, and up to 26 m long on approved B-double routes. Operators accredited for Higher Mass Limits can run more on approved roads.
Axle group limits to plan around
| Axle group | General Mass Limit |
|---|---|
| Steer axle (single, single tyres) | 6.0 t |
| Drive axles (tandem, dual tyres) | 16.5 t |
| A-trailer tri-axle group | 20.0 t |
| B-trailer tri-axle group | 20.0 t |
| Gross combination (9-axle B-double) | 62.5 t |
The total can be under 62.5 t and the truck still be overloaded, if too much weight sits over one group. Payload per trailer depends on the tare of your prime mover and trailers, so use your own vehicle figures. Limits here are the national General Mass Limits; check the rules for your state, route and accreditation.
Step by step
- Write down each trailer's usable deck length, width, height and payload, plus any site rules (for example a maximum pack length on the A-trailer).
- List the freight with dimensions, weights, quantities and stacking rules.
- Put long and heavy items on the B-trailer first, since it is usually the longer deck and its tri-axle carries the rear of the load.
- Fill the A-trailer with shorter items, keeping its weight within its own limit.
- Keep the heaviest items low and spread along each deck, not bunched at one end.
- Check each axle group: steer, drive, A-trailer group and B-trailer group.
- Plan the restraint for each trailer: tie-down or direct restraint, lashing count and dunnage.
- Print a plan per trailer so the loaders build each deck the same way it was planned.
Common B-double mistakes
- Planning the combination as one 20 m deck, which ignores the gap between trailers and each trailer's own limits
- Loading the A-trailer first with the heaviest freight, overloading the drive group
- Leaving the rear of the B-trailer light, which moves weight forward onto the A-trailer group
- Exceeding the height limit on one trailer while the other has room
- Splitting a list by count instead of by weight and length
Planning a B-double in TRUXE
TRUXE plans the A and B trailers as separate decks with their own dimensions and weight limits, splits a list across both, and shows the load on each axle group. If a group is over its limit it suggests a rebalance. Each trailer gets its own 3D view, restraint plan and page in the PDF report.
Frequently asked questions
How much weight can a B-double carry in Australia?
A 9-axle B-double is limited to 62.5 t gross combination mass at General Mass Limits, or more under Higher Mass Limits with accreditation on approved routes. Payload is that figure minus the tare of the prime mover and both trailers, typically well under 40 t.
Which trailer should the heavy freight go on?
Usually the B-trailer, which is normally the longer deck. Spread heavy items along its length and keep them low, then check that the B-trailer tri-axle group and the A-trailer group are both within limits.
How long is a B-double?
Up to 26 m overall on approved B-double routes. Deck lengths vary by combination, for example a 7 m A-trailer with a 13 m B-trailer.
Can software split a load across the A and B trailers?
Yes. TRUXE splits a list across both trailers using each trailer's own length, height and weight limits, then checks the axle groups for the combination.