Historical Notes: Brakes
The pioneer motorist, at least prior to 1900, regarded his brakes as being extremely important, but for a different reason from that which we should advance today. The roads of his day carried but a minute fraction of the present-day traffic, and the "routine slowing down" or the "emergency stop" of our day and generation were virtually unknown to him. Instead, he viewed with something approaching terror the prolonged main road decline, and considered that the primary duty of his "stoppers" was to prevent the dreaded "runaway" backwards no less than forwards. His progress was therefore, in this respect, not unlike that of the horse-drawn conveyances that surrounded him, and it was not unnatural that the first automobile brakes should closely resemble those fitted to the aforesaid conveyances. They were, in fact, mere blocks which scraped upon the iron-tyre with which the driving wheels were shod. Thus, in God's fresh air, and under the powerful pull (or sometimes push) of the motorist's right arm, did work change itself into heat and cope manfully with the situation. But it needed but a fractional increase in the maximum operational speed of the automobile thus equipped to produce a disaster of the first magnitude down a respectable slope, and many such tragedies there were too, sad to say. Anyone who has attempted a smart descent of a suburban pavement seated upon a home-made soap-box trolley can well visualise the danger. "There is, perhaps, no essential feature," writes Mr. Beaumont in 1902, "which has so commonly been badly designed and badly made as the brake and its gear." The band brake followed, but this necessitated some form of lining, which rapidly took fire or overheated and was very often utterly useless in the reverse direction. Indeed, the more "selfwrapping" the band brakes were - and quite a few had a surprising amount of servo assistance implicit in their design - the more did the lowering of the sprag become an important feature of an ascent, and the very necessary ritual of changing down appropriately and controlling the said descent become the hall-mark of the good and careful driver. Even after the fairly general adoption of band brakes, it was still considered, when iron or solid rubber tyres were fitted, correct to provide spoon or block brakes as the emergency alternative, but the coming of the pneumatic tyre made it essential that all the main braking surfaces should be at the hub itself, in spite of the fact that this was considered to place additional loads upon the wooden spokes and the general construction of the wheels then in common use. The road surface, too, had much to do with the problem of stopping, and an average of co-efficient of friction for the macadam roads of those days was 0.3 for a steel tyre and 0.6 for a solid rubber one, and this, in itself, meant that too powerful a system quickly made for a pair of locked rear wheels. In spite of this, however, the truth slowly dawned that "the stress put upon a brake to effect a stop in a given distance increases as the square of the increase of speed," quoting Mr. Beaumont again, and that something had better be done about it. It is perhaps better to forbear any lengthy description as to what happened to the braking systems already dealt with, if it happened to rain when the necessity for stopping took place! Sufficient to say that at this early stage in the motor-car's history a complete brake failure was not in any way considered a disgrace so far as the manufacturer was concerned, and the danger was considered to be all part of the fun of motoring. Linings in the block type of brake were made variously of cast-iron, wood (either twisted elm or boxwood) and india-rubber, while bands were usually covered with leather or camel-hair, or else were themselves steel, contracting onto the iron drum; 1903 was, perhaps, the year that saw the start of the general adoption of the beginning of the internal expanding shoe brake that has remained, in principle, at least, a standard fitting ever since. Mercedes, Brush, Panhard, Chalmers, Humber and Beeston-Humber were early in the field, and by 1905 there were only, according to Mr. A. E. Berriman's tables in The Autocar, Mobile, Enfield, the 10-h.p. Vulcan, Swift, Thornycroft, Delaunay Belleville, Standard, Albion and the small Humber left amongst those using "external" back brakes. The layout of those days was, of course, that the hand lever operated the rear brakes, whilst the pedal operated the transmission brake. It was considered normal practice to drive on the foot brake for ordinary purposes, reserving the hand brakes for the emergency stop, the long descent, or the parking. As Mr. Pemberton remarks. "a good driver uses his brakes alternately upon long hills. He knows that even the latest brakes which are not water-cooled may overheat." The "Raybestos" type of lining was becoming increasingly popular around 1911, but even at this stage it may amuse to learn that it was very common practice to oil brakes in those days, especially the cast-iron type. Due to faulty oil seals they often oiled themselves, but it looks strange to read in The Auto, with reference to the asbestos-lined brake: "Particularly when new, and if at all small for their work, brakes such as these are apt to be fierce unless well lubricated," and their summing up in an elementary article on brake maintenance includes these words. "The main point, at any rate, which it may be useful to bring home is that drivers need not be afraid to lubricate their brakes except in cases where express instructions are otherwise issued by the makers of the particular vehicle in question." Verily, times have changed.
Motor racing, as "Baladeur" has so often told us in his altogether delightful way, no less than the coming of the tarred road surface, was the chief proving ground for four-wheel brakes. The idea, as is well known, was not new, but with loose surfaces the rule rather than the exception, and with only the 1914 traffic levels to contend with, it was not until 1924 that the idea began to find real favour and we began to see those little red triangles on the back of Morris Cowleys. Nowadays one needs a red triangle on the front of an Edwardian.
As with all new ideas, there were objections. The principal dangers alleged against four-wheel brakes were, the risk of the front-wheel skid, always viewed with some horror, and the disturbance of the steering gear due to brake effort, and so seriously did some people take these points that some manufacturers left the front and rear brakes uncoupled for quite a long period. There was, however, something altogether majestic about some of the classic early four-wheel brake layouts, the Perrot being perhaps the best example, but now that the roads improved so that technical folk began to talk about the possibility of the value of co-efficient of friction reaching unity, or even exceeding it, especially when balloon tyres became popular, the increasing difficulty of locking wheels threw the deficiencies of human pedal pressures and non-assisted linkages well to the fore. As is now well known, some of the first four-wheel braked heavy cars were positively lethal even if you possessed enormous strength, and in spite of the fact that some manufacturers, notably Rolls-Royce, pursued their own individual approach to the problem, by the middle 'twenties it was clearly apparent that good four-wheel braking was a specialist job, best kept out of the hands of the motor manufacturer himself, but that revolution took some six years, between 1924 and 1930, to materialise. During those years the individual maker struggled on, employing one of the standard linkage layouts, and overcoming the inherent failings of these layouts by servo assistance if driven to it. Jolly good some of those systems were too, and it is indeed interesting to see how they each overcame the common problems of compensation and "effort distribution." The Dewandre servo deserves special mention as a standard aid that flourished satisfactorily during this period, striving manfully to cope with many layout faults, notably that of lack of torsional stiffness in shafts, upon which most of these standard layouts specialised unless they were impossibly heavy.
An idea of the relative popularity of these layouts may be gained from the following "stocktaking" done at Olympia by Maurice Platt in 1928 :-
Details of these types will be given in Part II of this article.
Before coming to the "specialist" brake era, it is right to pause to consider what the brake lining manufacturer had to face. Obviously increased braking power meant increased loadings, but the inevitable tendency to "squash" under load was overcome by the special dies and hydraulic machinery then used in their manufacture, although the cheaper products still oozed bitumen and went "flabby" under the new and very much revised conditions imposed. They also took a lot of the blame for squeaky brakes which ought by rights to have been laid at the door of thin and badly designed drums, Today, the Bendix, the Lockheed and the Girling are the three main proprietary brakes that survive, and rightly so too. The self-wrapping servo, as exemplified by Bendix, found an early expression in the Perrot brake, but has since been developed to an extraordinary degree. The hydraulic system had to face early criticism because, so the "wide-heads" said, there was extreme danger if a pipe broke or even leaked, in which case air must inevitably enter the system. In addition, they said the new system demanded a special fluid, since oil varied its viscosity with the temperature of the day !
But revolutionary though hydraulics might have seemed, there was another logical approach open and Mr. A. H. Girling took that approach. Most of the weaknesses in the layouts of the 1924-1930 period, he seems to have said to himself, lay in the "spring" and "inefficiencies" in the linkages due, in particular, to lack of torsional stiffness in the cross-shafts, and to the inevitable losses of their bearings and cam layouts. He therefore produced, in the early 'thirties, the masterly system which bears his name, based upon rods in tension only, which bisected the shoes by the hardened wedge-and-roller principle. At once the need for the complicated servo disappeared and all its attendant snags of lack of progressive control and "feel" were overcome. True, time was to show that after many miles of practical experience with the early examples the Girling rods wore at the yokes and rattled horribly, but there can have been few greater examples of a reversal to pure first principles than the Girling brake.
As so, with strange mixtures of these three basic types, we move to the modern era where, we are given to understand, the linings actually scrape against the drums all the time and words like "hydromechanical" enter the braking dictionary. Likewise we hear of disc brakes and other advances that no doubt tell us of even better and better performances to come. When the modern girl of 18 can "drop anchor" at 90 plus withont the slightest qualm and pull up all square at that, there must have been a revolution indeed, but somehow one cannot help wondering whether, in spite of all the progress, modern motoring does not suffer somewhat from the psychological point of view. This is not to belittle the braking technicians' efforts in any way whatsoever, but do we not "drive" better automatically if we are aware of the limitations of our braking ?
However - this is 1953 . . . - "A.B.C." (To be continued.)