Tow Legal - Tow Safe

Trailer Braking Systems

Some of the trailers examined during the event(s) were in excess of 30 years old. When they were first in use, a typical tow vehicle would be a Morris Oxford, weighing in excess of 1,500 kg, developing (perhaps) 60 horse power, allowed to tow at 40 m.p.h.

Today, we are seeing vehicles weighing less than 1,200 kg, developing almost 200 horse power, allowed to tow at 60 m.p.h.

30 years ago, many trailers were built using re-furbished rear brakes from family saloon cars. The trailer coupling was basically a horizontal plunger, working against a spring, that activated a combined transmission/ handbrake lever that connected to the brake via a wire rope. This wire rope was attached to what would have been the handbrake mechanism in the re-furbished brake assembly. Quite often, only one axle of a twin axle trailer would be equipped with brakes. It is easy to understand why these old brakes are often ineffective in todays' motoring environment.

The braking system currently required by legislation is hydraulically damped, auto-reverse and it is required to be fitted to all wheels of the trailer. What was a horizontal plunger within the coupling is now called a draw-tube; mounted within it is a hydraulic damper which minimises brake surge and snatch. The coupling assembly is fitted with a separate handbrake lever, to which a break-away cable is attached. Modern brake linkages are invariable rod & bowden cable, with full compensation to ensure equal force is applied to each of the brake assemblies.

The actual brake assembly within the wheel hub is a complex affair. For increased efficiency, it is a twin leading shoe design. This means that both brake shoes in each hub work very effectively when the wheel is rotating in the ahead direction. When the wheel rotates in reverse, the brakes are inefficient, so the trailer can be pushed back against the inefficient brakes (hence auto-reverse). With the older type of brakes, it was necessary to engage a reversing lock or lever to prevent the brakes from locking up when the trailer was reversed.

When the trailer hand brake is applied, either when parking the trailer, or by virtue of the break-away cable having operated, the brake mechanism within the hub reverts to a single leading/single trailing shoe system, as soon as the wheel tries to rotate backwards (which prevents the trailer running away backwards when parked facing up a hill).

Break-away Cables and Secondary Couplings.

Having read the foregoing, it will be appreciated that the mechanics of the brake system are comparatively complex. One of the problems frequently encountered during the event(s) when undertaking a brake effectiveness test, was that the brakes had been adjusted out of bias.

In nominal terms, the "road braking" function of the coupling operates the first 50% of brake rod travel. The next 30% of travel is utilised by the forward parking brake function, the last 20% of travel is required to set the brake in reverse park mode.

As the brakes wear, the driver becomes aware of a decrease in trailer braking effect. Upon inspection, he/she can see a brake adjustment (bottle-screw) at the front of the trailer, just aft of the coupling. By tightening this adjustment, the trailer brakes appear to become more effective; but, if a 15% adjustment has been made at the front of the spectrum there will only be 5% left to set the brake in reverse park mode at the other end of the spectrum - hence the brakes being "out of bias". The inevitable result is that the next time the trailer (caravan etc.) is parked facing up a hill, it will run away backwards as soon as it is detached from the towing vehicle, even with the hand brake lever fully applied !

There is a correct procedure for adjusting auto-reverse brakes. It is not particularly difficult, but it does involve jacking the trailers wheels clear of the ground in order to adjust the brake hub mechanisms, before crawling under the trailer to adjust the bowden cable ends and set the compensator(s) before adjusting the length of the main brake rod. It should be stressed that the test method used at these events does not test brake efficiency, it tests brake effectiveness, by imitating "panic braking".

A ratchet strap is used to close the coupling, which will apply the trailer brakes in "road braking" mode. The driver is then asked to drive the vehicle combination forward it should not move (other than reluctantly). The driver is then asked to reverse the combination it should reverse after initial resistance. The trailer handbrake is then fully applied and the driver asked to pull the combination forwards, it should not move (other than reluctantly); finally, the driver is asked to reverse the combination, it should not move (other than reluctantly). If these goals are met, the brakes are (in TTAS' opinion) working effectively.

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