Driving tests

Trailer coupling and drawbar damage: how to prevent it

A truck with a drawbar trailer or full trailer (also called a dog trailer in Australia), will be connected using a drawbar and a bolt and pin coupling. Power and air is provided using suzi coils. A full trailer needs to be coupled correctly otherwise there’s the risk of damage.

First, you must learn how to connect a truck and trailer properly. The drawbar’s towing eye fits inside the bolt and pin coupling unit and the pin drops through the eye. These mechanisms can be damaged by not lining them up properly, going too quickly, not having the coupling ready and treating the suzi coils poorly.

Cables and air lines (suzi coils)

When the trailer is parked, keep any air lines and cables draped over the drawbar or inserted into any dummy sockets – as long as they are not on the ground in the dirt and puddles, they will be fine. Getting dirt into the connectors causes wear and damage. Leaving the cables on the ground means they can be trampled on or run over and this wears out the outer sheath.

Cables have been left on the ground where they can soak in the puddles. At least the hydraulic and EBS cables aren’t on the ground.

If the coupling comes undone and the cables are still connected, this will damage them. The cables must not drag on the ground or hang down where they will rub on the road or become snagged on anything.

Damage to the towing eye

The towing eye is the round hole that the pin drops through. Reversing too quickly into the coupling can squash the towing eye, while shock-loading it can elongate it.

A squashed towing eye won’t accept a pin, while an elongated towing eye will vibrate while driving as the pin moves around in the enlarged eye.

Damage to the drawbar

In a jackknife, the truck and trailer exceed the permitted angle and this can bend the drawbar. Jackknife sensors can warn a driver if this is about to happen.

The force has bent the towing eye and the A frame – you can see the crease where the metal has buckled.
Top view of a drawbar that’s bend to the right.

Damage to the bolt and pin coupling

Reversing into it too quickly while not lined up can break the funnel. Reversing too quickly when it is lined up can damage the plate at the back of the coupling.

This funnel was damaged either by misaligning the towing eye with the funnel and backing in too quickly, or reversing into a low, solid object

When no connected, the coupling can be damaged by reversing into low walls or other obstacles at that height.

Damage to the plugs and sockets

Before you plug anything in, check that it doesn’t have any dirt or stones caught in the plug or socket as these can bend pins or get into the hydraulics. Don’t pull on the cable itself, hold the plug. Don’t force a plug in – if it doesn’t want to go in, inspect the plug and socket to find out why.

Recap on how to protect your trailer coupling

  • Line up the towing eye with the funnel
  • Don’t back in too quickly
  • Store your cables off the ground
  • Take care of your cables and plugs
  • Don’t exceed the limits of the coupling, i.e. be careful of jackknifing the trailer
  • Don’t shock-load the coupling
  • Follow the recommended safe coupling and uncoupling procedure outline in the trailer coupling course linked above (or go to the Courses page).
  • Always report and damage to the coupling as it could fail if driven.
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Darren is a member of the Institute of Advanced Motorists and the NZ Motoring Writers' Guild

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