News Article

RULES ARE RULES..........BUT ARE THEY?

For many years one of the accepted rules of towing trailers has been the “85% rule”. This broadly states that the gross weight of the trailer should be no more than 85% of the weight of the towing vehicle. No doubt there were sound reasons at the time for promulgating this advice amongst caravanners and the like, but is it still relevant today?

The “85% rule” was undoubtedly formulated to improve the stability of the towing combination, but nowadays we have a number of highly effective stability devices which make continued adherence to this rule questionable.

BPW is a leading supplier of axles and chassis for trailers and knows a thing or two about trailer stability. They offer a range of axles which in themselves improve towing stability, as well as a special stabiliser coupling, and an electronic stability control unit which automatically senses sudden sideways movements and applies the trailer brakes.

The BPW V¬Tec range of axles uses a V¬shaped axle beam to increase the tyre/road contact through corners. This improves tyre adhesion, thus helping to ensure the trailer follows the towing vehicle through the bend.

When a tandem axle combination is required, such as with larger caravans, BPW has a neat way of improving stability by introducing load compensation.

One of the problems with tandem axle trailers is the fact that when driven over rough terrain, or very uneven road surfaces, all the load can be transferred to just one of the axles (see diagram). This can lead to severe over¬loading.

The engineers at BPW have come up with a clever design to prevent this. They have connected the two axles together in such a way that as one axle moves vertically up or down, the other moves in the opposite direction by way of compensation. Thus ensuring that at all times (subject to the limit of axle travel) both axles remain in contact with the ground.

BPW uses two torsion bar axles connected by two link rods as shown in the diagram. As one axle moves upwards or downwards in response to say a speed hump or a pothole, the other axle will automatically move in the opposite direction. So at no time is one axle lifted clear of the ground.

Apart from axles, BPW also offers a stabilising coupling which helps to reduce both swing and yaw. It uses two friction elements housed in the coupling head and which clamp onto the tow ball. The coupling is easily connected to the towing vehicle and is a very effective safety device.

Two years ago BPW introduced its iDC, an electronically controlled unit which senses sudden or violent sideways movements of the trailer and automatically applies the trailer’s brakes. Designed primarily (but not exclusively) for caravan applications it consists of a sensor which is attached laterally to the caravan axle and which is linked to a control box fitted to the towing hitch. The control box houses an electric motor which is connected to the caravan’s braking system and will apply the brakes when a suitable signal is received from the axle sensor. This unit comes as standard on several larger caravans and can also be retrofitted.

One further important factor affecting trailer stability but one which is often overlooked, is nose weight. Get this wrong and no amount of stability devices will ever truly compensate. Here again BPW can help. They offer a jockey wheel which incorporates a nose weight indicator. Again, aimed mainly at the caravan market, this device is both simple and effective. Before coupling up, just check the nose weight indicator on the jockey wheel and redistribute weight in the caravan accordingly.

So, with all these stability devices available, is the “85% rule” still relevant?

Legally, the final say on the weight a vehicle can tow rests with the vehicle manufacturer. The “85% rule” has no basis in law and is probably now outdated due to the very considerable strides which have been taken to improve overall trailer stability. Caravan manufacturers, in particular Airstream, Buccaneer, Elddis and Fifth Wheel, are very conscious of vehicle stability and now include several, if not all, of the BPW products described above in their premium vehicles.

March 2011

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