Mass, Dimension and Loading Requirements
The three dimensions of compliance
Heavy vehicle compliance rests on mass, dimension and load restraint. General mass limits apply unless you operate under a concessional scheme. Key general limits include a steer axle limit, a tandem axle group limit and a tri-axle group limit, plus a gross vehicle or combination mass limit. Dimension limits cover width, height, length and rear overhang. Loads must not overhang dangerously and must be marked if they project beyond the allowed distance. Load restraint must withstand forces in all directions, not just downward; the load must be restrained against forward, rearward and sideways movement.
Why it matters
Overloading increases braking distance, damages roads and bridges, and can void insurance. Poorly distributed mass can overload one axle even when the total is legal, so axle group limits matter as much as gross mass. Height affects clearance under bridges and power lines; width affects lane use and pilot vehicle requirements.
Traps and rule of thumb
Questions often confuse gross mass with axle mass, or assume a legal total means every axle is legal. Another trap is thinking a heavy load only needs to be tied down; it must be restrained against shifting in every direction. Rule of thumb: check each axle group, then the gross mass, then width, height and length, then restraint. Worked example: a load that is within gross mass but puts too much weight on the drive axles is still overloaded and must be redistributed.