Factsheet 13c - September 2005

Semi-trailers

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A semi-trailer has one axle set, attached to the towing vehicle at one of these tow coupling positions:

• At or forward of the rearmost axle of the towing vehicle (for rigid vehicles with one axle in their rear axle set or where another trailer is the towing vehicle), or
• Not more than 300 mm behind the rear axis of the towing vehicle (for rigid vehicles with two or more axles in their rear set).

Note: Trailers attached to the towing vehicle behind the above positions are considered simple trailers (including Stinger Steer Vehicles).

The gross mass of a semi-trailer (including its load) is the mass transferred to the ground through the axle(s) of that trailer. Any transferred weight from a following attached trailer is to be included in the calculation of gross mass of the first trailer.

A heavy semi-trailer must have a rear axle set consisting of either:

• a single axle set, or
• a tandem axle set, or
• a tri-axle set, or
• a quad-axle set, provided the rearmost axle and the first or third axle in this set are steering axles and the semi-trailer does not form part of an A-train or B-train combination.

The maximum overall length for a truck and semi trailer combination (including load, but excluding collapsible mirrors) is 18.0 metres.

For a semi-trailer, forward distance means the distance from the rear axis to the centre of the kingpin. The maximum is 8.5 metres.

The maximum rear overhang for a heavy semi-trailer is 4.0 metres or 50 percent of the forward distance, whichever is less. The maximum for a light semi-trailer is 4.0 metres.

For semi-trailers, front overhang means the distance from the centre of the kingpin to the foremost point of the vehicle (including its load). The maximum is 2.04 metres radius arc ahead of the kingpin.

The maximum gross mass of a rigid vehicle and one semi-trailer is 39 tonnes, unless it has at least two motor driven axles in the rear set of the rigid vehicle.

Please note: Prohibited tow coupling position

Heavy semi or heavy simple trailers have a prohibited tow coupling position. Tow couplings for towing heavy semi or heavy simple trailers aren't allowed to be fitted to rigid vehicles between the following positions:

• For a rigid vehicle with one rear axle, from the rear axle to a point 700 mm behind that axle
• For a rigid vehicle with two or three rear axles, from 300 mm to 700 mm behind the rear axis of the rigid vehicle.