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True trackin —too true'
Page 52
Page 53
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by R. D. Cater, AMInstBE.
INDIVISIBLE LOADS are not, as is most often thought, necessarily the heaviest loads seen on the roads. And neither is the movement of indivisible loads restricted to operations by professional hauliers alone. Many ownaccount operators spend the whole of their efforts moving their company's products from place to place in lumps that, while they might not be heavy, are very large and most decidedly indivisible.
While legislation passed in recent years has permitted heavier and larger vehicles to be used on the public highways, little thought has been given to the many ancient buildings that make up the bulk of business premises up and down the country. Not only are these restricted from the owners' productive-effort point of view, but they constitute a very real restriction on the size and type of vehicles with which they can be served. Access to many buildings is virtually impossible with many long trailers unless they are equipped with some form of steering bogie such as the Hoynor True Track which I have just tested.
Since 1966 I have carried out evaluations of three different types of steering semitrailer. In each case the manufacturer had aimed at producing a vehicle offering improved manoeuvrability for a given length. One of the trailers I tried was the Dyson equipped with an Italian Viberti self-tracking rear axle, which Dyson manufactures under licence in the UK. While this unit cuts down the effective swept circle over which a vehicle turns, it is a following rather than a steering axle and it is not effective when reversing. .
A second unit tested was a prototype designed by Mr. Richard Petty, a Yorkshire haulier who knew very well what was needed but did not, unfortunately, produce a very saleable design. This trailer, a tanker, was definitely steered but the mechanism and the suspension onto which it was fitted was unnecessarily complicated and the design never got further than the prototype stage. Both reports appeared in CM September 2 1966.
About 20 months after these tests, the Danbury-based company of Hoynor Ltd. announced that it had introduced a steering trailer called the True Track which it would produce to order on lengths of from 40 to 100ft. The steering system is applicable to single-, tandem-, threeor four-axle trailers and to assess its usefulness my tests included a series of manoeuvres carried out with a 40ft-long trailer hauled by a Scania Vabis 110 9ft 6in. wheelbase tractive unit.
The tests took place in and around the factory premises of the vehicle's owner. This firm has added it to the fleet to permit large fabrications it manufactures to be carried to site locations in fully assembled condition: extreme accuracy is essential for the firm's products and on-site assembly is therefore unacceptable. Even with the new unit in service the situation of the factory does not permit manufactured units longer than 50ft to negotiate its entrances and exits. But as tight as the firm's own conditions are, the real benefits of the new vehicle were said both by the transport manager and the driver assigned to it. to be in the easy negotiation of sites and side and back streets, choked with parked vehicles, encountered on almost every delivery made.
The Hoynor True Track employs an extremely simple steering mechanism. A revolving kingpin—equipped with a wedge-shaped extension on its lower flange and what serves as a steering arm on its upper flange—is bush-mounted in the rubbing plate of the semi-trailer. The wedge-shaped extension mates exactly with the lead-up tails of the fifth-wheel and so when the tractive unit is turned the kingpin turns in the turret. The movement of the steering arm in relation to the trailer frame is transmitted through a series of drag links to the trailer axle(s) where, . via Ackermann steering mechanism, it is led to the wheels.
A sliding block is incorporated in the steering arm which allows a variable ratio to be applied to the steering. In this way it is possible to enjoy the best of both worlds in that the steering mechanism is utterly positive and yet produces completely accurate tracking of the trailer around the turning circles scribed by the tractive unit. The purely mechanical linkage—rather like a multiplication of that used on the conventional eight-wheel rigid—engenders accurate alignment over long periods. There is only one part of the mechanism that can produce slack—the sliding block—and there only a few thousandths of an inch is possible. The kingpin can travel through a 180deg arc and ample manoeuvrability for the tractive unit is allowed.
To prove the usefulness of the unit I asked its driver to negotiate some of the more difficult areas at his employer's works. I was fortunate in finding an artic of similar length belonging to BRS and fitted with a fixed bogie. It was put round one of the only corners it could negotiate, as a comparison. The tractive unit had to scribe a radius some 20ft larger than the True Track outfit with its tractive unit to permit the trailer bogie to clear the corner of the building.
After the worst conditons inside the works had been sampled, we took the machine out through the main gate onto the public roads. Parked vehicles and narrow streets highlighted the attributes of the outfit which could be driven into congested areas quite simply without having to pay considerable attention to what the back end of the trailer was going to do. To reverse the vehicle one has to drive it rather like a very small artic.
The slightest degree of reverse lock will cause the trailer steering to become effective, when the back end must then be regarded as the front end.
In the design of the True Track, Hoynor has succeeded in producing a vehicle which will allow a driver, working alone, to handle long and clumsy loads of either great or little weight—it can be built for gross weights of up to 100 tons—with comparative ease. It dispenses with the need for steersmen and places the vehicle completely under the control of the driver.