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Heavy trailers and combination vehicles (full, semi, simple, pole, A- and B-train) (Factsheet 13c)

Published: 08 2011

This factsheet details of the dimension and mass limits for specific vehicles and vehicle combinations.

Heavy trailers and combination vehicles

(full, semi, simple, pole, A- and B-train)

This factsheet describes the legal requirements in the Land Transport Rule: Vehicle Dimensions and Mass 2002 relating to the dimension and mass limits for heavy trailers (Classes TC (3.5 to 10.0 tonnes) and TD (over 10 tonnes), and heavy-vehicle combinations. Please refer to Factsheet 13 for general dimension and mass limits and towing requirements.

This factsheet also includes information about changes made by the Land Transport Rule: Vehicle Dimensions and Mass 2010 Amendment, in force from 1 May 2010. Dimensional changes have been highlighted to assist you to quickly determine how you might be affected, and indicate the previous limit.

For definitions of dimension and axle terms see Factsheet 13.

What are the general specifications for heavy trailers?

Width

The maximum allowable width (including the load) for full, semi, simple, pole, A- and B-train trailers is 2.5 metres (excluding side marker lights, direction indicators and the bulge towards the bottom of the tyre). The only extra width allowed is:

  • 240mm each side for collapsible mirrors
  • 25mm each side for ropes, lashings, straps, chains, connectors and tensioning devices that aren't permanently or rigidly fixed to the vehicle; or J-hooks (to secure stock crates or bins)
  • 75mm for hubodometers on a non-retracting, non-steering axle on each vehicle and on the axle that minimises the extra width
  • 75mm for central tyre inflation (CTI) system hoses.

Height

The maximum allowable height (including the load) is 4.25 metres. An additional 25mm above 4.25 metres is allowed for tarpaulins, covers and lashings, straps, chains, covers, and related connectors and tensioning devices that aren't permanently or rigidly fixed to the vehicle.

The height of the body or load of class TD trailers (those with a gross vehicle mass exceeding 10 tonnes) may be restricted by stability requirements – particularly the need for the vehicle to have a minimum static roll threshold (SRT). See Factsheet 13e, Static roll thresholds, for more.

All vehicles must be loaded in a safe manner, with a height appropriate for the type of load.

Ground clearance

Except when loading or unloading the vehicle, the minimum ground clearance for heavy trailers (ie those with a gross vehicle mass over 3.5 tonnes) is 100mm. The ground clearance also has to be at least 6 percent of the distance from the nearest axle to the point where the ground clearance is measured.

Projecting loads

A load is allowed to project outside of the sides of a trailer, as long as the maximum allowable dimensions are not exceeded. See Factsheet 13, Vehicle dimensions and mass for information about the maximum projections allowed and the hazard warning devices you need to attach to projecting loads.

Load sharing

All axle sets in a heavy trailer must have a suspension system that provides effective damping and shares the load between the wheels of the set. No tyre should carry a mass more than 10 percent greater than the mass it would carry:

  • if all the tyres in the set were the same width and the load was shared equally between the tyres, or
  • if a tandem axle set contained a twin-tyred axle and a large single-tyred axle ('super single') and was built to divide the load between the tyres in the set in either the 60 percent twin/40 percent large single ratio or 55 percent twin/45 percent large single ratio. (A vehicle with these axle sets must have an indelible plate fixed to it by the manufacturer – for more information see the Vehicle Dimensions and Mass Rule on our website

Retractable axles – heavy trailers

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

  • the retractable axle has an automated control to ensure that the axles that remain on the ground stay within the appropriate legal mass limits and manufacturer's limits
  • 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)
  • such retractable axles are certified for compliance with the requirements in both the bullet points above.

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

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 aren't part of an A-train or B-train may have a:

  • tandem axle set or tri-axle set with one steering axle, or
  • quad-axle set, if two of the axles are steering axles – 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 the NZTA, to confirm that the steering axles meet the above requirement.

Stability of heavy trailers

Class TD trailers must have a static roll threshold (SRT) when laden of at least 0.35g (where 'g' is the acceleration due to gravity). SRT measures the stability of the vehicle. Class TD trailers with a body height or load height over 2.8 metres above ground must be certified that they have an SRT of at least 0.35g. (See Factsheet 13e, Static roll thresholds.

How to use this factsheet

First, read the:

Then read the relevant requirements from the list below:

Where can I find out more?

Contact details

  • Email us: info@nzta.govt.nz.
  • Phone our contact centre: 0800 699 000.
  • Write to us at: NZ Transport Agency, Private Bag 6995, Wellington 6141.