Weight Limits and Axle Loading
This domain tests the difference between gross weight, axle weight and tyre capacity - and whether you can move a load to make a vehicle legal rather than simply lighter.
The limits that apply
- Gross vehicle weight and gross combination weight from the registration document and the manufacturer's plate.
- Axle and axle-group limits from the same documents and the Ministry schedule; a tandem bogie limit is never simply double a single-axle limit.
- Tyre load ratings, which can be exceeded before the axle limit is reached.
- Weighbridges and roadside checks enforce these; overloading brings fines, may invalidate insurance, and can ground the vehicle until the load is reduced.
Load distribution
- Keep weight between the axles, biased towards the drive axles for traction, but never so far back that the steering axle goes light - light steering means poor grip and longer stops.
- The fifth-wheel position sets how weight splits between tractor and trailer.
- Braking and descents transfer weight forward; a load that is legal when parked can overload the front axle on a steep descent.
Worked example
A three-axle rigid: front axle limit 7.5 t, rear bogie 20 t, GVW 26 t. At the weighbridge: front 5.5 t, rear 22.5 t, gross 28 t. The load is 15 t. Wheelbase 6 m. Reasoning: rear is over by 2.5 t and gross by 2 t. Shifting the load 0.6 m forward moves 15 × 0.6 ÷ 6 = 1.5 t to the front, giving 7.0 t and 21.0 t - front now legal, rear still 1 t over, and gross still 28 t. Remove at least 1 t of cargo and re-weigh.
Traps that cost marks
Assuming half a bogie limit applies to each side; ignoring tyre ratings; forgetting that gross weight does not change when you move a load.