Factsheet 13c – September 2005

General trailer requirements

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Width

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

• 240 millimetres (mm) each side for collapsible mirrors
• 25 mm 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)
• 75 mm for hubodometers on a non-retracting, non-steering axle on each vehicle and on the axle that minimises the extra width.

Height

The maximum allowable height (including the load) is 4.25 m. An additional 25 mm above 4.25 m 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 100 mm. 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.

There's no minimum ground clearance requirement for light trailers, but if the suspension has been modified so the ground clearance is less than 100 mm, it will need to be approved by a low volume vehicle certifier.

Load sharing - heavy trailers

All axle sets in a heavy vehicle, except a twin-steer axle set, 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.)

Read information on unsprung agricultural trailers.

Projecting loads

See Factsheet 13, Vehicle dimensions and mass for information about the hazard warning devices you need to attach to projecting loads that are within the maximum dimensions for a trailer.

Stability of heavy trailers

Class TD trailers (trailers with a gross mass exceeding 10 tonnes) must have a static roll threshold (SRT) when laden of at least 0.35 g (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 m above ground must be certified that they have an SRT of at least 0.35 g. (See Factsheet 13e, Static roll thresholds.)

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 NZ Transport Agency (NZTA), to confirm that the steering axles meet the above requirement.

Rear axis (RA)

If the vehicle has only one non-steering axle, the RA is at the centre of that axle.

If the vehicle has a tandem axle set with only non-steering axles in its rear axle set and each axle has an equal number of tyres on it, the RA is midway between the centres of the axles.

If the vehicle has a tandem axle set with only non-steering axles in its rear axle set and one axle has twice as many tyres on it as the other axle, the RA is positioned two-thirds of the distance from the lesser tyred axle towards the greater tyred axle.

If the vehicle has a non-steering triaxle set in its rear axle set, the RA is midway between the extreme axles of the set.

If the vehicle has one steering axle in its rear set together with one non-steering axle, the RA is at the non-steering axle.

If the vehicle has one or two steering axles in its rear set together with two non-steering axles, the RA is midway between the extreme non-steering axles of the set.

If the vehicle has one or more retracting axles in its rear set together with one or more non-retracted axles, the RA is midway between the extreme non-retracted axles of the set.

Definitions

Single axle set means either one axle or two axles have their centres spaced less than 1 m apart.

Tandem axle set means two axles have their centres spaced no less than 1 m and no more than 2 m apart.

Tri-axle set means there are three axles and:

• the centres of the first and third axles are spaced not less than 2 m and not more than 3 m apart, and
• all axles contain an equal number of tyres, and none of the axles is a single standard-tyred axle.

Twin-steer axle set means a tandem axle set with single tyres, where both axles are connected to the same mechanism in order to steer similarly.

Quad-axle set means a set of four axles where:

• the centres of the first and fourth axles are spaced no less than 3.75 m and no more than 4 m apart, and
• all axles contain an equal number of tyres, and
• none of the tyres is a single standard tyre.