Knowing Your Towing

Maximum Towing Capacity

Generally speaking, maximum towing capacities are roughly equivalent to or slightly greater than the vehicle unladen mass or at a maximum, 1.5 times the unladen vehicle mass. This means that when towing you are more than doubling the mass that the vehicle needs to not only transport up and down hills and around corners but also to bring safely to a stop.

A fully laden trailer therefore has a significant effect on vehicle stability and braking distances. Therefore by exceeding the maximum towing capacity or tow ball load that has been carefully calculated and tested by the vehicle manufacturer is to invite a potentially catastrophic loss of vehicle and trailer control.

Towing a caravan or trailer is regulated by state road traffic acts and while the towing regulations do vary slightly from state to state all of them agree that the maximum towing capacity must not exceed the lesser of the following:

  • Maximum towing capacity specified by the vehicle manufacturer

  • Maximum carrying capacity of the trailer

  • Rated capacity of the tow bar and coupling

  • Maximum carrying capacity of the tyres

Failure to comply not only compromises the safety of you and your family but it contravenes the state road traffic acts and could negate your insurance cover.

Make sure you research the full towing regulations in your own state and that of your destination as ignorance is not an accepted excuse for breaking the law.

The maximum towing capacity of Subaru vehicles varies depending on the model and on its engine and transmission capacity. Check out each model for current model specs.

For previous models please contact your Subaru Retailer or the Subaru (Aust) Pty Limited customer service centre on 1800 226 643.

Tow Ball Load

It is not only important to comply with the maximum towing capacity for regulatory and safety reasons but also to not exceed the maximum tow ball load. The maximum tow ball load is the load in kilograms on the trailer hitch when the fully laden trailer is connected to the vehicle.

          

The easiest method to measure the weight of a loaded trailer is to visit a public weighbridge before departing on your journey. The tow ball load can be calculated by weighing only the loaded trailer while connected to the vehicle and then weighing the fully loaded trailer without the vehicle connected. The difference in weight is the tow ball load. Alternatively, a set of scales of sufficient capacity can be placed under the trailer hitch via an axle stand.

It is important to carry out all measurements with both the trailer and vehicle fully laden. Remember fully laden means all of your personnel possessions, food/drink supplies and passengers included. It effectively means the total weight of the vehicle and trailer as you drive down the road.

Tow ball loads as a general rule in the Australian market are 10% of the Aggregate Trailer Mass (ATM). The ATM is the Maximum Trailer Mass (MTM) on the trailers wheels when attached to the vehicle plus the tow ball load. Trailer manufacturers specify the Maximum Trailer Mass (MTM) and tow ball load.

When loading a trailer it is imperative that the maximum trailer capacity is not exceeded and that the load is secure and balanced throughout the trailer to maintain the specified tow ball load. For example, positioning heavy water containers or toolboxes on the front ‘A’ frame immediately behind the towing hitch will cause a significant increase in the tow ball load.

So why is compliance with the maximum tow ball load so important? The stability of the trailer is dependant on the manufacturer’s correct trailer weight distribution. Excessive or too little tow ball load increases the likelihood of trailer sway and in a worse case even total loss of control. Also, too much tow ball load on the towing vehicle can cause excessive rear axle load that in turn reduces the weight on the front steering axle causing light and less responsive steering and vehicle control.

Tow Bars

An extremely secure connection between vehicle and trailer is of paramount importance due to the very high potential energy of the trailer as a result of its significant mass and the speed at which it travels.

Tow bars are manufactured according to an Australian Standard normally from heavy gauge steel plate and tubing and are designed to accommodate the vehicles maximum towing capacity with a significant safety margin. Not only is the tow bar’s structural design built to withstand extremely high loads but also of equal importance is the connection between the tow bar and the body of the vehicle. Unfortunately the Australian Standard for tow bar manufacture does not include testing for the interface between tow bar and the vehicle body.

No matter how strong and robust the tow bar, if the attachment to the body of the vehicle and the strength of the vehicle body at that attachment point is weak then the overall connection of trailer to vehicle is also weak and may be subject to failure. This may not be immediate but could result from longer-term fatigue failure.

It is for this reason that it is very strongly recommended to use a tow bar that has been approved by the vehicle manufacturer, particularly if the trailer is at maximum towing capacity. Only the vehicle manufacturer fully understands the strength and durability of the vehicle body, particularly when subjected to the extreme localised loads generated by a tow bar attachment.

Trailer Brakes

To be able to tow at the maximum permissible towing capacity the trailer must be fitted with its own braking system. Generally there are two types of trailer brakes: a mechanical override braking system that uses the inertia of the trailer mass to activate the trailer braking system and electrically operated brakes that are triggered by an electrical signal from the vehicles braking system.

The majority of modern trailers by far feature electric brakes and for these to be able to function a special trailer electric brake control module must be fitted to the towing vehicle. The big advantage of electric brakes is that there is no delay in the activation of the trailer brakes. This means that the braking of the trailer and vehicle is simultaneous and further more the amount of trailer braking force can be adjusted to suit the towing conditions.

The trailer brakes can also be activated independently of the vehicle by the driver should it be necessary to stabilise the trailer at any time. The use of electric trailer brakes however does require some training and experience to get the full benefit and to be able to use the system safely.

Electronic Stability Control

Modern vehicles and certainly all-current model Subaru vehicles are fitted with Electronic Stability Control (ESC). All Subaru vehicles fitted with Vehicle Dynamics Control (VDC), that includes ESC, have been tested and calibrated for towing under Australian road conditions.

You should make sure not to switch off VDC when towing as it provides a significant enhancement to the overall vehicle and trailer stability. For other vehicles, you should check with the vehicle manufacturer.

The only time it is recommended to turn off VDC is at low speed when towing a caravan or trailer uphill on a slippery surface. This is recommended because turning off VDC effectively is turning off the traction control component of VDC that limits engine power to prevent wheel spin. When pulling more than twice the vehicle weight, maximum engine power is often required to be able to climb up the hill. A typical example of this situation is towing a heavy boat up a slippery boat ramp.

Modern Subaru vehicles now include a Limited Slip Device (LSD) within the VDC package which remains on when pressing the VDC off switch. It works to transfer the power from the wheels that slip to the wheels that grips and therefore works to help to pull a heavy trailer up a hill by maintaining maximum possible traction.

Remember to turn VDC back on when you have successfully climbed the hill. By default, VDC is always on so when the engine is stopped and restarted VDC is again fully active.

Driver Training and Speed Limits

Towing with a trailer less than 4.5 tonnes in Australia legally does not require a special drivers licence or any training. However the effect of the trailer on vehicle stability can be significant, particularly when operating the tow vehicle at maximum towing capacity and in difficult road conditions at the speed limit.

It is therefore strongly recommended to get some practice or training from an experienced or suitably qualified person before undertaking a long trip in a fully laden condition.

Remember, as a general rule, the heavier the trailer the more difficult it is to control. In most states of Australia there is no special speed limit for towing, only compliance with the posted limit.

Happy towing!

Check out the range of Subaru accessories

Read and learn more about Subaru’s Vehicle Dynamics Control (VDC) system

 

Related articles: Choosing The Best Tyres

Subaru Share Media Options Car Advice