Modern Conception of the Semi-trailer
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Interesting Tests Which Led to Considerable Improvements in Braking, Steering and Weight Distribution
IN our issue dated October 20 we published a description of a largecapacity body, made by the Capital Motor Co., Ltd.. mounted on a semi
trailer. The latter is one of a large number which has been produced by the British Trailer Co., Ltd., Farm Lane, Fulham, London, S.W.6, during the past two years. Whilst -it is suitable for use with any 4-5-ton shortwheelbase tractor chassis, it was designed, primarily, for use with the Fordson Thames model .9-ft. 10-in. wheelbase chassis, which was, at the time, more easily available than other makes.
Consultations were started with the engineering department of the Ford Motor Co., Ltd., with a view to producing an efficient articulated vehicle of its class—a machine that would be a unit and not classed as a tractor and a trailer.
Following road tests, much useful data were obtained with regard to braking, steering and weight distribution and, in consequence, radical alterations were made in the prototype design. As a basis, the company used its Four-in-Line semi-trailer which has twin oscillating axles mounted on large-diameter trunnions located between the twin-spring suspension. The latter unit is interesting and has the primary function of providing flexibility under laden or unladen conditions but, at the same time, it enables a true oscillating axle to be mounted.
Four hubs with four sets of Girling brakes are employed, giving double the braking effort of that obtained with a normal single-axle type. Dual vacuum servos are employed—one to each pair of wheels.—and the size of the reservoir employed provides for about six brake applications without the engine running.
Under test, and with a gross load of 13 tons, a brake efficiency of 50 per cent. was obtained, which means stopping in a distance of 27 ft. from 20 m.p.h.
As skidding of the trailer wheels was experienced when running light, a special form of linkage had to be employed which would obviate this without reducing braking efficiency with the trailer laden.
It was also found that, for the best results, both in braking and steering. the king-pin of the turntable of the semi-trailer had to he more than 1 ft. forward of the rear axle and, at the same time, this allowed correct weight distribution in relation to the three axles and the Ford Motor Co.'s axle weight limits.
With the Four-in-Line system each tyre is always carrying its full share of the load, irrespective of the condition or contour of the road and, as each wheel is free to rotate independently, scrubbing or stuffing is virtually eliminated.
The practical aspect of this is shown in increased tyre life, mileages of 50.000 and over having been recorded for a set. In consequence of the reduced rolling resistance of the four-wheels-inline arrangement, as exemplified in the present design, it can be proved that there is a saving in fuel, and a decrease in the wear and tear on the tractive unit. Comparisons would be made against a semi-trailer fitted with a normal type of twin-wheeled single axle.
The makers' automatic hitch is common to both the permanently coupled and the detachable types of semi-trailer, but, in the latter case, a steerable front support, with jockey wheels, which can be raised or lowered by screw gearing, is fitted.
The hitch mounting on the chassis of the tractive unit has been specially designed to avoid structural alterations as it is appreciated that this enhances its value when, eventually, it is offered in the used-vehicle market.
With regard to the semi-trailer chassis frame, the original design was built with a 6-in, crank located 7 ft. from the front, leaving 13 ft. of the frame at the lowest possible level with the object of providing a low centre of gravity.
In common with most manufacturers, the British Trailer Co., Ltd., has learnt a lot from its war-time experience in the quantity production of all types of semi-trailer, and this will be put to good use in the post-war period.
The trailer maker wishes to acknowledge the assistance received from both the Ford Motor Co., Ltd., and Clayton Dewandre Co., Ltd., in carrying out the tests to which reference has been made.