Driver Knowledge Tests

Dog trailer coupling damage: how it happens and how to avoid it

A dog trailer (also called a full trailer in many countries), has at least one axle at either end of the trailer plus a drawbar at the front which connects to a bolt and pin coupling using a towing eye. These couplings are made by companies such as Ringfeder, V. Orlandi and Rockinger and added to the truck, whereas the trailer drawbar and towing eye will be made by the trailer manufacturer.

Jackknife bending

Even though the drawbar is made from steel, it’s possible to bend it through jackknifing the trailer. When turning the trailer, if the drawbar is pulled sideways further than the coupling will bend, it will create a bend in the drawbar and towing eye.

The red dotted line shows the normal position of the towing eye, which you can see has been bent to the left. This trailer is not usable until the drawbar is fixed. This repair cost around $5000 – the affected metal must be cut out and rewelded, then the trailer recertified.

This kind of jackknifing can also damage the coupling.

Newer trucks have jackknife sensors. If the drawbar moves the white levers on either side of the coupling, an alarm sounds in the cab.

Jackknife sensors (white) on either side of the coupling. The coupling isn’t showing signs of damage, but you can see the rust marks on either side, meaning it’s been used to its extent many times, which has worn the paint off it.

Jackknifing needs proper training.

The towing eye itself can be distorted. Shock loading it, or trying to pull loads which are too heavy, can stretch or elongate the towing eye, while backing in too quickly can dent or squash the towing eye.

Bolt-in and bolt-on towing eyes

It’s most common to have welded towing eyes – these are preferable – but there are some bolt-in and bolt-on towing eyes. Bolts need to be torqued properly. If bolts fail, it can shear the rest of them off.

8 bolts hold this bolt-on towing eye to the drawbar. Check none have come loose or rusted too much

Bolt-in towing eyes fasten with one bolt at the back.

Impact damage

Backing up to the trailer too quickly can cause impact damage on the funnel.

A triangle of metal has been busted out of the funnel due to some kind of impact. In this case, the drawbar had not been left high enough, and the weaker edge of the funnel bore the stress of the impact.

When backing up the trailer, first check that the towing eye is the right height for the coupling. Then bring it in slowly until you feel the resistance of the towing eye in the coupling.

Spring and chain damage

Longer drawbars are held up by springs and chains. Check the chain periodically for corrosion. The springs will gradually give and the drawbar will start to sag. You can pull the chain in one link to lift it up again.

Some units just have one spring and chain.

Cable damage

The supply lines to the trailer can be easily damaged if not stowed properly. In the image below, the trimatic connector has been left on the ground. Fortunately the EBS and hydraulic line are off the ground.

Check the lines for damage when you plug them into the truck.

Darren is an expert on driving and transport, and is a member of the Institute of Advanced Motorists

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