Everything you need to know about the humble ball coupling

Philip Lord — 30 January 2017

The humble ball coupling is one of the most overlooked components of a caravan. This bit of gear is very reliable but it’s also vital for your safety, as well as your rig and your passengers. Used improperly, or if not maintained, the results of a coupling not doing its job has disaster written all over it.

It is hard to come by precise information about the early history of caravan couplings. But according to research by members of www.vintagecaravans.com – a website dedicated to pre-1970s caravans – it appears that ball couplings, or balljoint couplings as some early variations were called, replaced the yoke and pin type of coupling to tow trailers behinds passenger vehicles in about the 1930s, and they serve as the basis of what is used today (there were several variations to the theme, such as the 1930s to 1950s Don captive ball coupling).

Today, there are a few different coupling sizes. In the USA, the standard coupling size is 2in (50.3mm); in Europe and Australia the ISO standard 50mm coupling is the prevailing size, while the 1.75in (44.45mm) ball is also used in some countries such as New Zealand.

The 50mm ball coupling is the mainstay for trailers up to 3500kg ATM. If the trailer is designed to weigh more than 3500kg, other tow couplings are used, such as the pintle hook arrangement favoured by the military for light truck trailer-towing requirements.


The basic 50mm ball coupling is a very simple device. It consists of the body casting, locking handle and a spring-loaded locking tongue entering the main casting at 45 degrees. Also attached is an adjusting bolt and locking nut designed to take up excess clearance and accommodate wear of the ball or coupling, ensuring a secure fit.

According to Al-Ko (which makes ball couplings and many other caravan components), while the legal requirement is for the cavity to be between 50-51mm spherical diameter, it makes its couplings closer to the 51mm maximum to ensure that a 50mm towball fits. 

The coupling can be welded to the A-frame or pre-drilled with holes so that it can be secured with bolts.

The Australian Design Rules require a positive locking of the coupling with provision for a second independent device – which means the coupling must automatically lock onto the ball and there must be further provision to secure the plunger from disengaging. The plunger is secured to the pull handle by a pin and has a spring fitted to its shaft to ensure the positive locking requirement for ball couplings.


A ball coupling does not need much maintenance, but it is important to cover off two elements. Firstly, the coupling cavity should be lightly greased so that it can swivel freely and avoid excessive wear on the towball and coupling. Secondly, any excessive slack in the coupling when it is secured on the ball should be taken up with the adjusting bolt.

The friction type of coupling should never be lubricated and the towball must be secured to the tongue with a locking plate so that it does not swivel. A standard towball cannot be used either, as it lacks the flats on the ball plate to key into the locking plate. Eventually the friction pads will wear beyond their specified service thickness and will require replacement.


The Australian Design Rules specify certain features that a coupling must have to be road-registered legal for use in Australia.

Trailers up to 3500kg ATM must have a quick release coupling designed to be engaged and disengaged without the use of tools. It must be of a positive locking type with provision for a second independent device. The locking must be readily verifiable by visual inspection.

The ADR 62/02, under clause 12.4.1, defines the markings required on the coupling to be considered legal for use. They are as follows:

Ball couplings used on trailers with an ATM that does not exceed 3.5 tonnes must comply with Australian Standard AS 4177.3-2004 Caravan and light trailer towing components Part 3: Coupling body for ball couplings.

Ball couplings on towbars are required to be installed so that the height of the centre of the body of the ball coupling is between 350mm and 420mm from the ground when laden (Refer to ADR 62/01). Alternatively, if complying with the requirements of ADR 62/02 the maximum height of the centre of the body of the ball coupling may be increased to 460mm. However, the ball may be installed at any other height, provided it is also capable of being adjusted to at least one height within the 350-460mm range.

The coupling body used on trailers should be designed to be compatible with these heights.

Alternatively, ball couplings may instead comply with ECE R55/- as Class B coupling heads.

A coupling body complying with AS 4177.3 must be marked with:

a. the manufacturer’s name or trademark;

b. the mark ‘50’;

c. the maximum rating for the coupling body in one of the following, as applicable, 750 kg; or 2000 kg; or 3500 kg;

d. a code to indicate the serial number, batch, production date, or similar;

e. (i) the words ‘DO NOT WELD’ if the coupling body is manufactured from non-weldable materials;

e. (ii) the words ‘WELD ONLY’ if the coupling body is specifically designed to be attached by welding only

A coupling body complying with ECE R55/- must be marked with:

a. the manufacturer’s name or trademark;

b. the mark ‘B’ or ‘B50-X’;

c. the maximum ‘D-Value’ rating for the coupling body. This corresponds to Australian Standards based ratings as follows: 750 kg; (D-Value of at least 6.4); or 2000 kg (D-Value of at least 14); or 3500 kg (D-Value of at least 20); and the mark ‘S’, followed by the permissible static vertical load in kg.

Caution: The Australian Standard and ECE standard contain 50mm ball coupling articulation limits. These angular limits are not intended to cover off road operation. Trailers for this type of operation should be designed accordingly.

The European Community standard noted in this legislation (ECE R55) is intended to cover the friction couplings typically used on European and English caravans.


The main variation to the standard 50mm ball coupling is the friction coupling (also called the stabiliser coupling). The friction coupling is common on European vans that typically have a 4-5 per cent towball mass. The friction coupling uses small pads that, when the coupling is secured, press against the towball on different points to provide some resistance to movement. This is so that the coupling itself provides some benefit to trailer stability.

The other commonly-used couplings are the various offroad styles, of which there are many different designs. Tregg and others use a pin and poly-block coupling with increased longitudinal rotation over a standard 50mm ball coupling, while Al-Ko has recently introduced a 50mm ball coupling for offroad use. 

The full feature appeared in Caravan World #522. Subscribe today for the latest caravan reviews and news every month! 


ball coupling component caravan rig coupling


Philip Lord

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