I have seen a tipping vehicle carrying sand and gravel
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which appears to be a four-wheeler converted to a six-wheeler, the third axle of which is located ahead of the driving axle. This can be lifted off the ground and the vehicle often runs partly laden with the axle raised.
Is this legal and has it any advantage apart from eliminating tyre scrub? Would it not have been preferable to extend the chassis and mount the third axle at the rear?
A Employing a lift-up third axle in this way
is legal so long as, when it is raised, the driving axle is not overloaded. The main advantage to an operator is probably the additional adhesion of the driving axle when the tipper is running unladen on slippery sites with steep gradients.
When the vehicle is being operated off the public road, the axle could be lifted when the vehicle was laden without breaking the law. But this would result in severe overloading of the tyres with a consequent danger of serious damage.
If a lift-up axle were fitted behind the driving axle, raising it from the ground would increase the rated overhang of the vehicle to more than the legal maximum.