Much of the information required for the selection process will be readily available. However some of the trailer specifications may not be easily found, in which case it may be necessary to have the fully loaded trailer weighed.
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.
The following information deals with the information you will need to choose an appropriate vehicle / trailer combination, and where to find it.
ATM and GTM
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 that is 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 built since August 1989 are required to have a plate listing, amongst other things, the trailers Aggregate Trailer Mass, although some trailer plates will also show its Tare Weight (the unladen weight of the trailer) and its GTM.
You must ensure the towing vehicle has sufficient capacity to tow the trailer when it is fully laden.
Trailer Ball Load
Ball Load is the amount of weight the fully laden trailer imposes (vertically) on the tow bar of the tow vehicle. Like other towing specifications, it must not be exceeded.
It's 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.
Ball Load is a function of the trailer’s axle position and the manner in which the trailer is loaded.
- 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, or with special ball mass scales.
- Ball Load is often around 10% of the trailers ATM however this should not be relied on for every trailer. Where no reliable information is available, the fully laden trailer must be weighed to determine this specification.
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 and stability.
Safety and vehicle durability are also important factors. The maximum trailer load will be specified to ensure the combination is controllable at all times and that it will not adversley affect or 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 relating to towing or additional equipment required.
This information is essential to the selection process. The specifications provided represent the absolute limits the vehicle can safely tow.
While towing specifications and recommendations are contained in the vehicle owner’s manual, to make comparisons easier, RACQ Technical Advice can provide this information for a range of popular vehicles.
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. 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 in the tow vehicle and trailer (including passengers).
Tow vehicle Axle Load
Some manufacturers provide Axle Load specifications for their vehicles. Most commonly this becomes important where the vehicle is 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 rearrange the trailer’s load.
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.
Need more help?
If you need more explanation of this information, or if you need to know the towing specifications for a particular vehicle contact our Motoring Advice Service.
Further reading can be found under Towing section of our motoring fact sheets.