What are the limits for semi-trailers?
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 300mm 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 is a steering axle, and the semi-trailer does not form part of an A-train or B-train combination. Note that the first or third axle in this set can also be steering axles, and may lock in the forward direction at speeds above 30km/h or in reverse gear.
The 2010 Amendment Rule allows semi-trailers to be 1 metre longer to cater for intermodal (rail and shipping) containers. This was broken down to 700mm in forward distance, which actually reflected the on-road performance of these vehicles, and 300mm to the rear overhang.
The maximum overall length for a truck and semi-trailer combination (including load, but excluding collapsible mirrors) is 19 metres (previously 18 metres). Quad semi-trailers with two rear steering axles are still limited to 18 metres.
For a semi-trailer, forward distance means the distance from the rear axis to the centre of the kingpin. The maximum is 9.2 metres (previously 8.5 metres).
The maximum rear overhang for a heavy semi-trailer is 4.3 metres (previously 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 2.04m 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 may not be fitted to rigid vehicles between the following positions:
- for a rigid vehicle with one rear axle, from the rear axle to a point 700mm behind that axle
- for a rigid vehicle with two or three rear axles, from 300mm to 700mm behind the rear axis of the rigid vehicle.
