Heavy combination vehicle definitions and specifications

Combination vehicle

A towing vehicle in combination with one or more trailers or other motor vehicle that is being towed.

Gross mass limits

The combined gross mass of an A-train must not exceed 39 tonnes. The combined gross mass of a B-train must not exceed 39 tonnes, unless it has:

• at least two motor-driven axles in a tandem or tri axle set
• three motor-driven axles in a tri-axle set.

The combined gross mass of any other combination of a rigid vehicle and two trailers must not exceed 20 tonnes.

Oscillating axle

A pair of short axles in the same line across the vehicle, each axle being:

• mounted with two wheels on it (with each wheel at least 500 millimetres apart across the vehicle), and
• able to oscillate, that is, one wheel can rise while the other falls.

Oscillating axle

Overall length

For a rigid vehicle and full trailer combination (including load, but excluding collapsible mirrors), the maximum overall length is 20 metres.

The maximum overall length for a truck and semi-trailer combination (including load, but excluding collapsible mirrors) is 19 metres*.

* A semitrailer with a quad axle set and two steering axle is limited to 18 metres (and must have been first registered before 1 December 2017).

Rear steering axles – heavy trailers

Heavy full trailers, heavy simple trailers and heavy pole trailers must not have rear steering axles.

Heavy semi-trailers that are not part of an A-train or a B-train may have:

• a tandem axle set or tri-axle set with or without one steering axle, or
• a quad-axle set with one rear steering axle capable of turning in both directions through an angle of at least fifteen degrees, or two steering axles if first registered before 1 February 2017. If two of the axles are steering axles, these must be either:
  • the two rearmost axles, which must be able to turn in the same direction through an angle of at least 15 degrees, or
  • the foremost axle and the rearmost axle, which must be able to turn in opposite directions through an angle of at least 15 degrees.

A quad-axle set must be certified by a heavy vehicle-certifying engineer, or other vehicle inspector or inspecting organisation specifically approved by Waka Kotahi NZ Transport Agency, to confirm that the steering axles meet the above requirement.

Rear trailing unit distance

The distance between the rigid vehicle’s point of attachment on the first trailer and the rear of the combination, including its load. The maximum distance is 14.5 metres, and it applies to all combinations of rigid vehicles and trailers.

Retractable axles – heavy trailers

Heavy trailers may have retractable axles in the rear axle set if:

• the retractable axle has automated control to ensure that the axles that remain on the ground stay within the appropriate legal mass limits and manufacturer’s limits, and
• forward distance and rear overhang limits are complied with, both when the axle is on the ground and when it is retracted (note that the position of the rear axis may change depending on whether the axle is retracted or in contact with the ground), and
• such retractable axles are certified for compliance with the requirements in both the bullet points above.

Note: you can only operate a vehicle with an axle in a retracted position if the vehicle is unladen.

Semi-trailers

A trailer where:

• part of the weight of the trailer and its load is borne by the towing vehicle, and
• the coupling is not behind the rearmost axle of the towing vehicle or, if the towing vehicle is a rigid vehicle and has more than one axle in its rear axle set, is no more than 300 millimetres rearward of the rear axis of the towing vehicle.

Stability of heavy trailers

When laden or unladen, heavy trailers with a gross mass exceeding 10 tonnes must have a static roll threshold (SRT) of at least 0.35g (where g is the acceleration due to gravity). SRT measures the stability of the vehicle.

Trailers with a body height or load height of 2.8 metres or more above ground are also required to be certified that they have an SRT of at least 0.35g.

When the trailer goes for its certificate of fitness (CoF) inspection, the information from the SRT certificate will be entered on a new certificate of loading for that vehicle.

Trailer:truck mass ratio – heavy trailers

For a rigid vehicle (truck) towing one heavy trailer, the ratio of the gross mass of the trailer to the gross mass of the truck at all times must not exceed 1.5.

This combination doesn’t apply if the speed of the combination vehicle doesn’t exceed 40km/h or if the vehicle is operating under an overweight permit with a vehicle axle index above 1.1.