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• Tyres Govern Trailer Design

14th December 1951
Page 45
Page 45, 14th December 1951 — • Tyres Govern Trailer Design
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Latest Developments Reviewed : Causes of Snaking: Man ceuvring Semi-trailer Outfits : Braking on Heavy Trailers

TRAILER builders usually had to

shape their designs to suit the tyres that Were available; said Mr. A. Marenbon, a director of. R. A. Dyson and Co., Ltd., when he delivered his paper, "Trailers and -Semi-trailers," at the Institution of Mechanital Engineers (Automobile 'Division), in London,: on Tuesday. It was only during :recent years, he added, that the .production of suitable tyres had enabled British manufacturers of trailers to produce pnetrinatic-tyred 16-wheeled. machines in the 100-ton class.

Probably the largest-capacity pilaumatie-tyted trailer yet to be built was the American Rogers 300-tonner, which was of relatively simple . construction. It had two lines of four four wheeled compensated bogies' on each carriage, the complete trailer having 64 wheels, It was 39 ft. long and 1/ ft. wide.

. Trailer Outfits in America

In the United States two out of every three vehicles, of over 5-ton capacity were trailer outfits of one kind or another. American registration figures revealed a total of 3m. trailers, including caravans..

It should trot be taken for granted, he said, that a trailer could not have a power unit or have its wheels driven. The speaker referred to en Austrian concern, for example, which had recently introduced a farm trailer in which the front axle was driven from the tractor. There was also an American semi-trailer, the axle of which Was driven by a self-contained petrol engine.

Mr. Marenbon dealt at length with the tracking of trailers and semi-trailers.

Italy, he said, seemed to be the on!y country in which the fully tracking semi-trailer was used to any extent and he quoted the Orlancli, which had steerable stub axles inter-connected• to the fifth wheel. • The natural tendency of the axles of a tandem bogie was to run straight ahead.. Hence, if they Were permitted to have some side movement, during a tarn they would follow a. Curve of greater radius than if they were fixed. Accordingly, "eat-in would be less. and there would be a greater degree of tracking, with a reduction in tyre scrub. The Continental approach to the 'question was, however, toprovide Ackerman-steered rear wheels on the tandem bogie.

Referring to trailer turntables, Mr. Marenbon said that the old idea of using a king-pin with central bolster plates and concentric wheel plates or turntable rings, was gradually heir's! abandoned. A popular modern method was to use large-diameter interlocking rings with balls or rollers interposed, and to dispense with the king-pin. Less friction in the turntable assisted manhandling and helped to prevent snaking.

Dealing with some of the causes of snaking, the speaker referred to lateral flexibility in the draw-hook or loose ness in the fit of the drawbar eye; friction or play in the trailer steering; trai!er overrun; too short -a drawbar and too short a trailer wheelbase. In the ease of a two-wheeled trailer, snaking was reduced if the centre of gravity came well forward of the axle.

The usual type of hook or jaw was not considered as being entirely satisfactory. Some American concerns had produced shockless ,couplings in which a buffer, actuated by vacuum or air pressure, exerted a predetermined pressure against the drawbar eye when once it was in position.

Referring to the reversing of a semitrailer outfit, Mr. Marenbon said that the shorter the tractor Wheelbase the easier was the outfit to manceuvre, and the longer the semi-trailer wheelbase the less was the •tendency of the outfit to fold up. It was Preferable, but not essential, for the semi-trailer wheelbase to exceed that of the tractor.

Braking According to Load In this country, ease of loading was sometimes achieved by making the rear axles of a trailer detachable, but in the United States it was common practice to leave the wheels in position and to detach the lower deck from the forward end.

Mr. Marenbon considered it desirable that four-wheeled trailers of a gross weight of more than 6-7 tons should be equipped with four-wheel brakes. A valuable feature would be a device which enabled the power applied to the irailer brakes to be reduced when the trailer was unloaded and the towing vehicle loaded.

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People: A. Marenbon
Locations: London

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