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Authoritative Paper by Mr. F. J. Field on Improvements in Shoe Adjustment, Distribution of Braking Force and Facingmaterial Impregnation AMONGST the improvements that have been effected, during the past twelve months, in brake systems, the Lockheed dual master cylinder, the Dewandre R.P. automatic adjuster and the Bendix Auto control were quoted as being of special note by Mr. F. J. Field, M.I.A.E., of Ferodo, Ltd., in a paper read last week before members of the Institute of the Motor Trade, Edinburgh.
The first, he pointed out, provides the equivalent of two independent systems, and thus removes the cause for a criticism sometimes directed at the hydraulic principle; the second maintains almost perfect adjustment and equalization throughout the useful life of the shoe facings and thus removes the need for making adjustments by hand ; the third, whilst allowing normal braking forces to be distributed equally between the front and rear wheels, ensures that more powerful forces are distributed more or less proportionally to the weight transference from the rear to the front wheels. All three of these devices were described in this journal on their respective inceptions.
Discussing the high quality of the three proprietary complete brake assemblies commonly incorporated in their products by chassis makers, the author, who appeared to B20
be referring in this matter to the private car more than •' the commercial motor, remarked that the services of the . expert and specialist brake engineers might well be ex. tended towards the improvement of front axle and steering layout.
In connection with the self-energizing-shoe principle. Mr. Field made some interesting remarks about the shoe factor. This is the numerical relation between braking force and shoe-tip pressure. In the case of the non-servo cam-operated brake, he gave this factor as 0.83; for semiservo brakes of the Lockheed type, 1,15, and for Bendix Duo-servo brakes, 2.5. Facings having a friction value of 0.4 were suitable for the first and second of these, but 0.3 was a value better suited for shoe facings in the lastnamed class.
He considered that a vehicle having a gross weight of anything up to 5,1 tons could be adequately braked by semi-servo or full-servo brakes of the self-energizing type, but, for machines of greater weight, vacuum or compressed-air assistance was desirable. Under the heading "Accidents," he remarked that these were due " largely " to the failure to apply brakes in time, and brake makers could not be held responsible. Neglect in the upkeep of brakes, however, was a very fruitful cause of accident. There were 22,000 cases in which police action had been taken in 1934-1935.
The root of the trouble, Mr. Field pointed out, was that the "Transport Authority" had not set a standard of braking efficiency, although the low figure of 32 per cent. had been recommended. As it was, the only method of judging braking for purposes of legisative regulation was by determining that a vehicle could be brought to rest in " a reasonable distance."
A curious legal decision was quoted to illustrate this point. The police evidence, in the case in question, was simple. They agreed that 40 ft. from 20 m.p.h. was a reasonable stopping distance for a lorry with four-wheel brakes ; but because the vehicle under consideration had two-wheel brakes the court considered that 80 ft. was a reasonable distance in which to stop, and accordingly dismissed the case.
Having paid a tribute to the excellent work that had been and was being done by the numerous makers of brake-shoe facing materials and of drums, the author described the essential properties of the former as resistance to wear, and constant friction value throughout a wide range of speeds, pressures and temperatures. The former depended to a big extent on the drum surface, the latter largely upon the bonding, namely, the impregnating ingredients, which he described as "by far the most important element in brake lining."
He stressed the importance of using correct types of facing and praised the shoe-exchange system, introduced by the makers of proprietary brakes, which be hoped would eventually supersede the refacing of shoes.
He held that a low frictional coefficient was preferable to a high one, and expressed the view that a too light brake pedal was definitely undesirable.
As examples of marked progress in facing manufacture, Mr. Field quoted the substitution of zinc-alloy wire for brass and recent developments in the technology of chemical impregnants, but admitted that they were still seeking a practicable means for overcoming the problem of accidental lubrication by oil or water.