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Trailer talk

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The drawbar is an integral and critical part of any trailer. It serves the function of housing the coupler so that it can be towed and it maintains the balance of the trailer. The drawbar improves the rigidity of the chassis and assists in keeping the trailer running correctly in a straight line behind the tow vehicle.

Typically there are quite a few types of drawbar design but the A frame drawbar is most commonly used in South Africa and the rest of the world. A robust drawbar needs to have good strength both vertically and horizontally and be able to cope with the often excessive forces that are delivered to this area, especially when heavily loaded. The drawbar is under constant strain with the materials being constantly compressed and twisted. For this reason, the drawbar on any trailer needs to have strong integrity to deal with the constant forces placed upon it. The materials need to be able to cope with these stresses without developing hairline cracks and the like which can quickly develop into more severe and even life threatening accidents.

A-FRAME DRAWBARS

The A frame drawbar is designed to support the outer edges of the trailer and reduces chassis flexation when the trailer is unevenly loaded. This is the most used type of drawbar design due to the strength this frame offers the trailer. The distance of the drawbar between the trailer and coupler can be extended if extra bracing is utilized to ensure the rigidity of the material stays intact and is not compromised in any way.

It is interesting to note that tests show that a longer drawbar offers more stability and a smoother ride to the trailer. Jackknifing is more common to trailers with shorter drawbars. Reversing a trailer with a longer drawbar will also be a bit easier offering better return response. Drawbar length is determined by the distance from the centre of the trailer’s front cross member to the centre of the towbar ball coupler.  The majority of trailers make use of fixed length drawbars, but there are extendable drawbars options on some trailers. Most manufacturers design their drawbars to suit the size and weight of their particular style of trailer based on their experience and testing of the optimum length for that particular trailer so that it offers good stability, turning response, axle positioning and reversing response.

DRAWBAR TOO SHORT?

If the drawbar construction is too short there is a good chance that the front of the trailer, or its leading edge could clip the tow vehicle in tight turns. While a shorter drawbar will offer improved towing with the trailer following the tow vehicle nicely, reversing will show opposite results and become over sensitive.

Shorter drawbars are most often used in off-road trailers because they track better behind the tow vehicle when put into tight cornering. In off-road trailers, the shorter drawbar also reduces the trailer’s propensity to bottom out when encountering uneven and rocky ground.

ACTUAL DRAWBAR LENGTH

A trailer’s drawbar needs to be at least half the width of the tow vehicle. Most vehicles range between 1.55m to 2.0m so the length of drawbar needs to be at least half of this distance plus approximately an added 250-350mm) to gain the optimal level of clearance when turning and increase the stability to the trailer.

ERRATIC TRAILER MOVEMENT

Trailer fishtailing can be very dangerous and is often the cause of major accidents, but it is nearly always avoidable. Trailer sway is determined by the snaking movement from side to side of the towed trailer and if not brought under control can lead to violent swinging of the trailer with deadly consequences. Trailer sway is often caused by the load being placed too far rearward of the axle or when a totally uneven load is packed in the trailer. Also, trailers that bear as load with a high centre of gravity are more prone to trailer sway. It is thus important to keep the load as low as possible maintaining the lowest centre of gravity for optimal towing. Poorly matched tyres on the trailer can also accentuate trailer sway.

TOWING ON THE LEVEL

Towing a trailer is always best when the tow vehicle’s towbar and the trailer are level, ensuring the trailer sits level when being towed. If the trailer is coupled too low or too high on the towbar, it can seriously affect the towing dynamics of the trailer. If the trailers nose is pointed up, it increases load to the rear and when hitting bumps it could even act as a pendulum with dire consequences. Towing on the level is always the safest and best option. If the weight in the trailer is too far forward, trailer pitching can be experienced which adds weight to the towbar of the tow vehicle, particularly in instances of heavy braking in downhill situations, causing the trailer to nose dive and thus lifting the front of the vehicle in extreme cases.
By Walter Ellis

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