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 draw bar or 'A' Frame form the attachment points on the trailer.

The Towbar

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 weight rating is essential. Further, the load capacity of the towbar and the trailer coupling must be equal to or exceed the loaded mass of the trailer.

Note: Towbars should not protrude dangerously when your trailer is not connected.

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 Ball Mount or Tongue

The ball mount, also known as the tongue, is the section of the towbar to which the towball is attached. It is usually a flat 75mm wide, 16 to 20mm thick steel bar, which may be either straight or curved to achieve the correct coupling height. If the ball mount or tongue obscures the number plate it must be removed from the towbar when the trailer is not attached.

The Tow Ball

Tow balls suitable for weights of up to 3,500kgs must be 50mm in diameter and should comply with Australian Standard 4177-2. The tow ball must be a one-piece element, the shank of which should be 22.2mm in diameter. The top face of the sphere should be clearly stamped with the capacity (3.5 t) and tow ball diameter (50). The tow ball unit must be fitted to the vehicle with a locking washer and an appropriately sized nut. According to the Australian Standard 4177.2 the manufacturer's name or trademark must also be stamped on the flange of the tow ball.

The Coupling Body

The coupling body is the section that is attached to the 'A' frame of the trailer. It forms a socket for the tow ball and provides the necessary pivot point between the trailer and the towing vehicle. Coupling bodies commonly in use can range in capacity from 750kgs to 3,500kgs. They must be marked with their capacity, as well as the manufacturer's name and the size of the tow ball for which they are suitable. It is important to ensure that the coupling body's capacity exceeds or is at least equal to the fully laden weight of the trailer.

Coupling Height - 50mm Ball Couplings

Ball couplings used on trailers with an ATM of up to 3.5 tonnes must comply with Australian Standards and be installed so that the height of the centre of the body of the ball couplings is between 350mm and 460mm from the ground when laden.

The Trailer's 'A' Frame (Drawbar)

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 foolish to add to the drawbar additional items (such as spare wheels, outboard motors, metal carry bins etc.) that will increase the load on the drawbar, without the specific approval of the trailer and towbar manufacturers. Increasing the downward load on the trailer drawbar will also increase the tow-ball weight on the towbar.

Safety Chains

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 draw bar 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.