Rigid truck and dog trailer over a weighbridge
You drive a three-axle rigid truck towing a tandem-axle dog trailer on a general access route in the ACT. At a roadside weighbridge your combination shows: steer axle 6.2 t, rigid drive axle group 16.8 t, dog trailer axle group 15.4 t. The truck's GVM plate reads 24,000 kg and the dog trailer's ATM plate reads 16,000 kg. Your permit-free general access limits are 6.0 t on the steer axle and 16.5 t on a tandem group. The load is bagged cement, and the client wants it delivered within two hours. You have 40 km still to travel and no other weighbridge on the route.
1. Identify every mass limit that is breached and explain how you would determine which one is the binding constraint before you move.
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Three separate limits must be checked, not one:
- Axle and axle-group limits. The steer axle is 6.2 t against a 6.0 t general access limit - over by 0.2 t. The rigid drive group at 16.8 t is over the 16.5 t tandem limit by 0.3 t. The dog trailer group at 15.4 t is under 16.5 t, so it is compliant on group mass.
- Vehicle plate mass. The rigid combination mass must be compared with the truck GVM and the trailer ATM. The dog trailer at 15.4 t is under its 16,000 kg ATM, but you must also confirm the aggregate (GCM) is not exceeded - the plate figures govern even where the statutory axle limits are satisfied.
- Aggregate/total combination mass. Add all axles: 6.2 + 16.8 + 15.4 = 38.4 t. If the truck's rated GCM is below this, the combination is overloaded regardless of individual axles.
The binding constraint is the lowest of these ceilings. Here the steer axle is the tightest margin relative to its limit, but the drive group is the largest absolute excess. Because the load is bagged and movable, the correct action is to redistribute or remove load so that every axle, every group, the GVM/ATM and the GCM are simultaneously satisfied - fixing one axle by shifting weight usually overloads another, so re-weigh after adjustment rather than assuming.
2. The client pressures you to keep driving to save time. Explain your obligations and the realistic consequences of proceeding overloaded.
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The delivery deadline does not override mass compliance. As the driver you are responsible for ensuring the vehicle is not overloaded before it travels, and an overloaded heavy vehicle is a defect you must not drive on a public road.
Practical consequences of proceeding:
- Enforcement exposure. An overloaded vehicle detected at a weighbridge or by mobile scales attracts substantial penalties, and the excess can trigger a prohibition notice that stops the vehicle where it stands - worse for the client than a short delay.
- Safety and handling. Excess mass on the steer axle reduces steering authority and increases stopping distance; an overloaded drive group raises tyre and brake temperatures, raising the risk of a blowout or brake fade on a descent.
- Mechanical damage. Sustained overload accelerates tyre, suspension and brake wear, and can void insurance cover if an incident is traced to the overload.
The defensible course is to stop, adjust the load (redistribute or offload the excess), re-weigh, and then advise the client of the revised arrival time. Documenting the re-weigh protects you if the load is later questioned.