Towing Parts

Towing Parts

The Tow Bar

TowBarThe 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.

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.

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

The Ball Mount or Tongue

ball-mountThe ball mount, also known as the tongue, is the section of the towbar to which the towball is attached. It is usually a fl at 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.

If you have purchased 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.

The Tow Ball

tow-ballBased on Australian Design Rules & Standards, tow balls suitable for ATM weights up to 3500kg:

  • Must be 50mm in diameter, and stamped on top of ball as a legal requirement.
  • Must be a one piece element, the shank of which should be 29mm in diameter.
  • Must be fitted to the vehicle with a locking washer and appropriately sized nut.
  • Must have the manufacturer’s name or trademark stamped on the flange of the towball.


With the tow vehicle loaded to GVM, the towbar/ball on the towing vehicle must be capable of being mounted (adjusted) to any one height within the range of 350mm to 460mm (from the ground to the centre of the towball).

The Coupling Body

coupling1The 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.

Regardless of coupling capacity, the 50mm ball must still comply with the capacities outlined under the heading tow ball.

Off-Road Couplings (NON 50MM BALL TYPE)

coupling-2Off-Road couplings are designed for use where high degrees of articulation are required. Some use a separate pin to connect, whilst others use a built in locking mechanism. Many have polyurethane components to absorb shocks.

All of these couplings are required to incorporate a positive locking mechanism plus a separate means of retaining this mechanism in the locked position.

This locking must be readily verifiable by visual examination.

Both parts of the coupling must be marked with the Manufacturer’s 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.

Note: The coupling must be strong enough to take the weight of a fully loaded trailer.

The Trailers 'A' Frame (Drawbar) & Safety Chains

a-frameThis 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 1000kgs 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 not advisable to add additional items to the drawbar. Increasing the downward load on the trailer drawbar will also increase the tow-ball weight on the towbar. Excessive overloading of the 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 accidentally 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.

Safety Cables of equivalent capacity to safety chains are also allowed on tow vehicles up to 3,500kgs ATM.

It is vital that the chains are attached to the main towbar framework and not to a detachable ball mount or tongue.

The chains attach the ‘A’ frame or drawbar of the trailer to the main towbar framework on the vehicle. The attachment must be fit for purpose of equivalent strength to the chains.

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. Western Australia is the only state that requires two for those under 2.5 tonnes.