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.
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.
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.
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 adversely affect or significantly shorten the life of the vehicle's body and mechanical components.
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.
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.