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Regulator issues suzi coil warning for drawbar trailers

In a recent safety bulletin, the NHVR has highlighted the dangers of using coiled brake system air hoses (suzi coils) on drawbar type trailers such as dog, pig or tag type trailers that use an ‘A’ type coupling system.

“Plastic coiled air hoses (commonly called suzi coils) are a popular choice for supplying air to trailer brake systems. Their ability to stretch for several meters makes connecting with a variety of different trailers easier. Braking systems that rely on air are designed so that in the event of an unintentional decoupling, the air hose supply line breaks, reducing the air pressure below a certain threshold and activating the trailer’s emergency brake system,” said the bulletin.

“This system works best on trailers where the coupling method is via a ‘B’ or fifth wheel coupling system as this style of trailer is typically fitted with an axle group located to the rear of the unit. The design of these trailers results in the front of the trailer dropping to the ground as soon as it decouples, breaking the air hose supply line and allowing the emergency brake system to activate almost immediately. By contrast, trailers with ‘A’ type coupling systems (e.g. pintle hook, ball) are typically fitted with an axle group to either the centre, or at both the front and rear of the unit. The design of these trailers means that in the event of an unintentional decoupling there may be a delay in the activation of the emergency braking system.”

The NHVR safety bulletin gives examples of correctly and incorrectly fitted hoses.

The NHVR pointed to a number of decoupling incidents where suzi coils were used as air supply lines to trailers with ‘A’ type coupling systems.

The design of the trailer, together with the use of suzi coils, it said, resulted in the suzi coils stretching, allowing the trailers to travel significant distances before the emergency brake system has activated.

“In some extreme cases these trailers have drifted into other lanes with a total lack of emergency brake application due to the extent that the suzi coils have stretched,” it said.

To reduce this risk, the NHVR is recommending that suzi coils are not fitted to trailers with ‘A’ type coupling systems and are fitted only to trailers with a ‘B’ or fifth wheel coupling systems.

The NHVR added that trailers using an ‘A’ type coupling should use traditional rubber hoses meeting the SAE J1402 or equivalent standard as air supply lines to the braking system, as rubber hoses are not designed to stretch. This means that in the event of coupling failure, the emergency braking system can be applied more quickly.

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