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 DRIVING/TOWING
TRAILER CONNECTIONS EXAMINED
The right connections are vital

 

Safe, appropriate and legal coupling to a trailer involves many factors and all trailer couplings should comply with the Australian code of practice for light trailer construction. We take you through the maze.

Trailers are split into different weight categories by what is termed Aggregate Trailer Mass (ATM). This is the total mass of the laden trailer, when carrying the maximum load recommended by the manufacturer, and includes any tow ball load imposed onto the towing vehicle.

Couplings must be of a positive locking type with provision for a second independent device. The locking must be readily verifiable by visual inspection.

Trailers having an ATM of up to 3.5 tonnes must have a quick release coupling which is designed to be engaged and disengaged without the use of tools.

By far the most common trailer coupling in Australia is the one that slips over a 50mm tow ball. Ball couplings used on trailers with an ATM of up to 3.5 tonnes must comply with the Australian Standard AS 4177.3-1994 and be installed so that the height of the centre of the body of the ball coupling is between 350mm and 460mm from the ground when laden. (This height regulation used to be 420mm maximum.)

Off-road style couplings would appear to be exempt from height restrictions, but it's a grey area that seems to sit between the Australian Standard and ADR62/02.

A coupling body complying with AS 4177.3 will be marked with: its manufacturer's name or trademark if Australian made; the mark ‘50’; the maximum load rating of the coupling body (750kg, 2000kg, or 3500 kg); a code to indicate the serial number, batch, production date, or similar; and the words ‘do not weld’ if the coupling body is manufactured from cast iron or other non-weldable material.

All trailers without breakaway brakes must be fitted with safety chains. Trailers with an ATM up to 2.5 tonnes must have at least one safety chain complying  with AS 4177.4 –1994. Trailers with an ATM over 2.5 tonnes and up to 3.5 tonnes must have two safety chains, rated at 3500kg and complying with AS 4177.4-1994

The chain must be permanently attached to the trailer - shackles are not permitted at the trailer connection. For trailers up to 3.5 tonnes ATM, the safety chain attachment can be by welding. The weld must extend around half of the circumference of the link and the adjoining link must have free movement.

The safety chain attachment must be located as near as practicable to the coupling and where two points of attachment are required they must be mounted one on either side of the centre line of the drawbar.

The primary purpose of safety chains is to prevent the trailer separating completely from the towing vehicle in the event of a coupling failure. Their secondary function is to prevent the drawbar of the trailer digging into the road surface.

 

Different couplings

The traditional 50mm tow ball compatible coupling body that is fitted to the vast majority of trailers and caravans has its limitations on rough ground. However, that doesn’t completely rule it out as an off-road trailer coupling, because we know of many people who use normal ball couplings with success. The trick is understanding the ball coupling’s limitations and not exceeding them.

It’s pretty easy to visualize the limitations of a ball coupling if you look at a coupled vehicle and trailer, side-on. The forward edge of the cast coupling body is quite close to the tow bar tongue and won’t tolerate much vertical angularity between the towing vehicle and the trailer before it touches the tongue. If that happens there’s a chance that the front edge of the cast coupling body will break, or the upward pressure on the coupling body retaining collar will break.

Ball couplings with swivels between the coupling body and the mounting plate on the trailer drawbar have more rough terrain tolerance than fixed types. The most common swiveling-type coupling bodies are those that operate override trailer braking systems.

The AL-KO and Hyland couplings take the swiveling coupling body a stage further, by separating the ball receiver from the coupling body and mounting it in a yoke that allows the ball receiver to pivot fore and aft. Because these couplings have articulation vertically and horizontally they overcome the rough terrain limitations of the traditional ball coupling, while preserving its quick-hitch advantage.

The other traditional quick-hitch coupling is the pintle hook and eye, of military origin. (Pintle is Old English for penis!) The pintle hook is a pin with a locking latch that accepts an eye fitting that’s mounted on the trailer drawbar.

A variation of the simple pintle is one that swivels around its vehicle mounting. Although popular in the trucking world the pintle has fallen out of favour with smaller trailer makers, mainly because there’s limited vertical angularity in the designs that don’t have excessive free-play between the eye and the pin. Those with free-play suffer from trailer ‘kick’ and ‘lag’ and are noisy when hauled over rough ground.

Non-traditional couplings that are now the most popular camper trailer types in Australia don’t use towballs. These poly-block couplings have two swiveling points: one in the vertical plane and one in the horizontal; and a fat polyurethane bush to absorb road shock.

These couplings are similar in principle to a universal coupling, allowing wide angular movement in all directions.

The most popular of these is the Treg poly-block, but others include the Trigg, Nathan, AT35 and Orac. The Nathan and Orac have less vertical movement restriction than the others and offer the greatest theoretical articulation, but in practical, off-road towing terms they’ll all do the off-road towing job.

Coupling and uncoupling these double-swivel types isn’t as straightforward an operation as it is with a ball coupling. We’ve towed dozens of them and each has its quirks. On flat ground coupling and uncoupling isn’t too difficult, but if the ground is uneven the job can take a few, at times frustrating, minutes. It can also be messy, as you fiddle around with greasy pins.

The upside is smooth towing performance, without trailer thumps and bumps being transmitted to the tow vehicle.

 

Coupling Care

Couplings take a hammering, even on good roads, so it’s important to check them regularly for wear and tear. You cannot replenish coupling grease too frequently.

Ball couplings in constant use need a regular clearance screw adjustment check. Shiny patches on the ball may indicate damage to the coupling body or trapped foreign material.

Pins on swivel couplings need to be removed and the bushes cleaned, before regreasing and reassembly, especially after trips that involve dust and water.

Shackles and chains need regular inspection – rust is a clear sign of potential weakness. When you’re not towing, don’t leave shackles swinging on the tow bar eyes: take them off so they don’t wear in the bow curves.

 

Electrical Connectors

Most of the trailer problems we come across in the bush concern wheel bearings and electricals. Quality wiring and fittings, and regular maintenance can eliminate electrical dramas.

Trailers and towing vehicles must have electrical connectors that comply with Australian Standard 2513-1982 ‘Electrical Connectors for Trailer Vehicles’.

Three types of seven-pin connectors are specified in the Standard and twelve-pin connectors are also specified.

Because of interchangeability problems that may arise, it is recommended that pin five (blue wire) in seven-pin connectors be used only for service brakes. If auxiliary circuits are required, a twelve-pin connector is preferable.

Trailer electrical connections have as tough a life as mechanical couplings and so need as much care. Plugs and terminals are exposed to the elements and should be cleaned and sprayed with an electrical contact fluid after each trip.

Weak points are where wiring passes through bodywork holes, the grommets in the back of terminals and plugs, and terminal-to-wire contacts.

Problems with globes can largely be eliminated by opting for LED lights.

 

Trailer Brakes

The nationally approved upper limit for unbraked trailers is a GTM rating of 750kg.

Trailers not over 2000kg GTM must have brakes that operate on at least one axle. 

Brakes other than override systems must be able to be operated from the driver’s seat.

Trailers over 2000kg GTM must have brakes on all wheels. In addition, trailers over 2000kg GTM are required to have brakes that apply if the trailer becomes detached from the towing vehicle and the brakes must remain applied for 15 minutes after separation. This requirement means that the braking system of two-tonne trailers is most likely to be either all-electric or electro-hydraulic, with a battery on the trailer. 

Electric brakes are popular with caravan makers, but many off-road and boat trailers have override brakes.

In an override system the trailer coupling shaft is free to slide a short distance in and out of its housing, in accordance with the towing vehicle’s acceleration and deceleration forces. When the towing vehicle decelerates the coupling shaft operates either a cable pull, to apply mechanical brakes, or a master cylinder push rod, to operate hydraulic brakes.

Top shelf hydraulic trailer brake actuators have integrated electric power packs and react to the towing vehicle’s stop-light circuit, rather than the delayed override reaction.

All-electric brakes react to the towing vehicle’s brake action, via the stop-light circuit and braking timing can be varied by a manual controller.

Weight Distribution Bars

These spring-steel bars are designed to level-out a vehicle and trailer combination and have become more widespread as caravans and boat trailers have increased in size in recent years. These bars act to distribute weight from the towbar to the front axle of the towing vehicle.

They’re not suitable for off-road camper trailers that require significant articulation between 4WD and trailer.



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