Failure to abide by the towing regulations, including maximum loads, may result in a fine, or in the case of an accident, refusal of the insurance claim, and the possibility of further legal action.
Trailer mass terminology explained
ATM, GTM and Tare Weight
The maximum weight of a trailer is specified as either its Aggregate Trailer Mass (ATM) or Gross Trailer Mass (GTM).
ATM is the combined weight of the trailer and its full load when it is not coupled to a tow vehicle. GTM is the weight of the fully loaded trailer imposed on the trailer’s axle when it is coupled to the tow vehicle. GTM will always be less than ATM as some of the trailer weight is transferred to the tow vehicle when the trailer is coupled to it.
All new trailers (including those that are home and individually built) built since August 1989 are required to have a plate listing, amongst other things, the trailer’s Aggregate Trailer Mass, although some trailer plates will also show the Tare Weight (the un-laden weight of the trailer) and the GTM.
Trailers built before August 1989 may carry little or none of this information. For pre August 1989 trailers with no plates or documentation, determining specifications can be difficult and you may need to seek professional assistance from a trailer dealer or engineer. The type of braking system
fitted to the trailer may provide some guidance to its likely ATM if other information is unavailable.
You must ensure that the towing vehicle has sufficient capacity to tow the fully laden trailer.
Trailer Ball Load
Ball Load is the amount of weight the fully laden trailer imposes (vertically) on the tow bar of the tow vehicle. Trailer Ball Load is not a specification defined by the trailer manufacturer – it is the actual weight imposed on the rear of the tow vehicle and as such is a function of the trailer’s axle position and the manner in which it is loaded.
While there is no requirement to list a trailer’s Ball Load, it can be measured at a weighbridge by disconnecting the fully laden trailer from the tow vehicle and resting only the trailer’s draw bar (via the jockey wheel) on the scales. Alternatively, some caravan dealers have special ball mass scales for this purpose.
Ball Load is not usually a critical issue with small trailers unless the tow vehicle has a low Ball Load specification, as is the case with many small cars and some European vehicles. However, heavy trailers such as horse floats and caravans often impose quite high Ball Loads on the tow vehicle.
For caravans it’s important to load the van in the same manner every time once the correct Ball Load is established, as changes in loading patterns can have a dramatic effect. Loads should never be arranged behind a trailer’s axle in an attempt to counter excessive Ball Load. This can be very destabilising and potentially dangerous. It’s important to realise that like other specifications, the maximum tow ball load must not be exceeded.
Payload is the trailer’s carry capacity. It’s the difference between its Tare Weight (un-laden weight) and its ATM. Payload is important for all trailers, however it is critical to campers and caravans, many of which have quite limited carrying capacity to start with. Modifications and additions to the trailer can very quickly eat into its carrying capacity and result in an overloaded trailer, or the inability to legally carry the really important equipment.
Additional water tanks and gas cylinders are commonly fitted to campers and caravans and can add significant extra weight. As a guide, each additional litre of water carried adds 1kg (excluding the weight of the water tank and necessary fittings) while each full 9kg gas cylinder adds approximately 18kg (plus the weight of the fittings).
Tow vehicle specifications
A vehicle's towing capacity is determined by its manufacturer and is based on factors such as the design of the vehicle, the vehicle's rear axle load, the capacity of its tyres and the effect the laden trailer will have on the vehicle's attitude, stability, durability and overall safety.
The maximum trailer load will be specified to ensure the combination is controllable and that it will not significantly shorten the life of the vehicle's body and mechanical components.
Vehicle handbooks generally provide the following information:
- the maximum weight of the trailer without brakes, that can be towed by the vehicle,
- the maximum weight of a trailer with brakes, that can be towed by the vehicle,
- the maximum tow ball load, and
- any conditions (such as speed) relating to towing or additional equipment required
The maximum towing weight specified by the vehicle manufacturer equates to the trailer’s ATM. However some vehicle manufactures confuse matters by specifying towing capacity in terms of Gross Trailer Weight, which is another way of saying it, refers the trailer’s Aggregate Trailer Mass. Do not confuse the term Gross Trailer Weight with Gross Trailer Mass as in this regard they are different things. ATM and GTM are defined above
as well as in the relevant legislation.
Tow ball load
Tow Ball Load is the proportion of the trailer weight that is applied to the rear of the tow vehicle. A vehicle’s Ball Load specification will be found in the vehicle’s handbook and will be listed as a weight in kilograms or as a percentage of the trailer’s ATM.
Ball Load specifications are often around 10% of the maximum towing mass specification however, this isn’t always the case – some may have a lower Ball Load while others may have a higher Ball Load. European vehicles in particular can have quite low allowable Ball Loads in relation to their towing capacity. Too little Ball Load can cause the trailer to become unstable and too much can be detrimental to the tow vehicle’s durability as well as affecting its stability, steering and braking.
Gross Combination Vehicle Mass (where given) is the maximum allowable weight of the trailer, tow vehicle and the load (including passengers) in the tow vehicle and trailer.
Tow vehicle Axle Load
A Maximum Axle Load specification is sometimes given for vehicles, such as utilities, that are capable of carrying a load over the rear axle in addition to the load imposed by the trailer (the Ball Load). In cases where the total of the Ball Load and the load in the vehicle exceeds the allowable Axle Load, it will be necessary to reduce the load carried or, in some cases it may be possible to rearrange the trailer’s load, though it will be important to ensure trailer stability is not adversely affected by moving the load too far rearwards.
Some vehicle manufacturers impose reduced speed limits when towing. This may be across the board or it may be when the weight of the trailer exceeds a certain limit. Where such limits apply, this typically means that the vehicle is restricted to a maximum of approximately 80km/h, so it’s important to factor this into your purchase decision, particularly if you are intending to tow a trailer long distances.
Tow bar specifications
All tow bars made after July 1, 1988, and many made before this date, will have a plate attached that lists the maximum towing weight, the maximum ball load, and the make and model of vehicle the bar was designed for.
The vehicle’s specifications will always be the maximum the vehicle can legally tow, even if the tow bar is rated for a higher load. However if any of the tow bar specifications are lower than those given for the vehicle, the tow bar’s specification will override the vehicle’s specifications.
It is common to find tow bar specifications that differ from those given for the vehicle. This usually occurs where the bar is made for a number of different models in the range, or where light and heavy-duty tow bars are offered for the vehicle.
When dealing with older unmarked tow bars, it can be difficult to determine the bar’s capacity. We recommend in this case that you discuss the matter with a reputable tow bar manufacturer who may be able to provide professional advice about its suitability for the application.
Trailer information, design rules and legalities
Vehicle Standards Bulletin 1 is the 'rule book' for Australian trailer builders. It contains all the information you need to know about light trailer dimensions, lighting, brakes and much more.
Need more help?
Owner’s manuals will contain the towing specifications you’ll need to make the right decisions. However if you’re in the process of buying a vehicle, trawling through handbooks to find it can be a tedious process. To make the selection process a bit easier RACQ Motoring Advice (link below) has a list of towing specifications for a range of popular tow vehicles and can assist RACQ members with specific enquiries. Copyright laws prevent us from making it available on line though.
Be warned that towing specifications can change from model to model, year to year and even within the same model range. So it’s vital that you always confirm that the information obtained coincides with that provided in the vehicle’s handbook. If you have any doubt about the accuracy of the information, we recommend confirming it with the vehicle’s manufacturer.
If you need more explanation of this information or if you need to know the towing specifications for a particular vehicle please contact RACQ Motoring Advice or email an enquiry.