The coupling must be strong enough to take the weight of a fully loaded trailer. There are five main parts involved in a trailer coupling: the towbar, the ball mount or tongue and the tow ball are all attached to the tow vehicle, while the coupling body and the trailer drawbar or 'A' Frame form the attachment points on the trailer.
The towbar is the framework attached to the back of the tow vehicle. For safe towing, a properly designed and fitted towbar with an adequate certified weight rating is mandatory. Further, the load capacity of the towbar and the trailer coupling must be equal to or exceed the loaded mass of the trailer.
If you bought a second hand vehicle with a towbar already attached, be especially careful. You need to make sure that the towbar is appropriate for whatever you intend towing. For example, although ideal for the previous owner’s box trailer, the towbar might be totally unsuitable for your caravan.
Unless a permanent part of the vehicle, it is compulsory for all towbars manufactured after 1 July 1988 to clearly and permanently display the maximum load rated capacity plus the make and model of vehicle for which they are intended, or alternatively, the manufacturer’s name, trade mark and part number. Check for this information to help you ascertain whether the towbar suits your needs.
The use of tools to engage or disengage is not allowed on couplings up to 3500kg.
Both parts of the coupling must be marked with the Manufacturers name or trademark, the words “use with model (identified model)” and the maximum allowable trailer ATM i.e. 3500kg at which the coupling is rated.
Weight Distribution Hitches (WDH) can increase the loading on the coupling. Before fitting a WDH it is advisable to check compatibility with the coupling. These devices should not be used in off-road situations.
Ball couplings used on trailers with an ATM of up to 3.5 tonnes must comply with Australian Standard 4177-3 and be installed so that when the coupling and towball are connected to the laden towing vehicle; the height to the centre of the tow ball above the ground shall measure between 350mm and 460mm (or be capable of being adjusted).
This is the front section of the trailer or caravan chassis to which the coupling body is attached with bolts, nuts and locking washers. Welding the coupling body is also permitted on trailers under 1000 kgs provided the manufacturer has specified that this approach is suitable and has provided welding instructions, which must be followed. The “A” frame or drawbar is required under the Australian Design Rules to be of sufficient strength for the specified trailer ATM, and must be able to be proven to do so by engineering calculation. It is therefore not advisable to add additional items to the drawbar. Increasing the downward load on the trailer drawbar will also increase the tow-ball weighton the towbar.
Excessive overloading of towbar ball weight will affect its performance and may void manufacturer’s warranty.
Safety chains are compulsory in all States and Territories of Australia. They must be strong enough to hold the trailer and prevent the drawbar from touching the ground, should the coupling fail or be accidently disconnected from the ball.
Trailers less than 2,500kgs ATM must be fitted with at least one safety chain of at least 9.5mm in diameter. Trailers over 2,500kgs ATM and up to 3,500kgs must have two safety chains. Chains must comply with AS4177-4 and have a size designation at least equal to the trailer ATM.
The chains attach the ‘A’ frame or drawbar of the trailer to the main towbar framework on the vehicle. The attachment must be made with ‘D’ shackles of equivalent strength to the chains. It is vital that the chains are attached to the main towbar framework and not to a detachable ball mount or tongue. Safety chains must be stamped with the chain’s capacity, the manufacturer’s identification and the digits 4177.
The chains should be as short as possible, leaving only enough slack to permit tight turns. If two are required they should be crisscrossed under the trailer tongue to prevent the forward end of the drawbar from hitting the ground if the coupling becomes disconnected.
Safety Cables of equivalent capacity to safety chains are also allowed on tow vehicles up to3,500kgs ATM.