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Meeting Present-day
Page 115
Page 116
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The Problem of BRAKING NEEDS
Braking as Presented to Chassis Manufacturers, with an Outline of the Four Main Systems now Employed
MUCH has recently been written about safety on the roads, but little helpful criticism has been offered. Surely among points of first importance in this respect is the efficiency or otherwise of the brakes of . all classes of vehicle—from lightweight motorcycles to heavy commercial vehicles.
To the uninitiated the solution to the problem of providing good braking may seem to resolve itself into the provision of large drums and shoes with a. big leverage to the pedals. There is, however, a lot more in it than this.
It will be readily agreed that when a vehicle weighing, perhaps, 10 tons, has to be stopped in an emergency from a speed of 30 m.p.h., much power must be absorbed in the process. Actually, the retarding force would amount to around 11,000 lb. if the vehicle vAre brought to rest in approximately 2.1 seconds, assuming a constant rate of retardation. Now this retarding force of 11,000 lb. has to be exerted primarily by the driver, and it is obvious that without some form of mechanical aid the case would be hopeless.
Before reviewing the systems at present in vogue, it would seem oppor
tune here to interpose a few remarks concerning braking efficiencies and what they mean. The force of gravity is a useful standard by which other forces can be compared, and it is easy to call this force 100 per cent. and to compare others as ratios.
One of the accompanying drawings illustrates the phenomena associated with 100-per-cent. braking efficiency. If a bus, travelling at $0 m.p.h., be pulled up in approximately 30 ft., the retardation would be at the same rate as the acceleration of ea stone falling freely from, say, a cliff, the retarding force operating on the bus being exactly equal to its own weight, assuming, of course, that a constant rate is maintained. In practice this seldom occurs, as the efficiency usually rises as the speed decreases.
Now, such retardation as is connected , with 100-per-cent. braking efficiency is extremely violent and, accordingly, is undesirable, for it causes personal reactions to which the human frame is unaccustomed. One has, therefore, to seek a compromise that will give retardation compatible with traffic needs, but which is comfortable for passengers and safe where goods vehicles are concerned.
Another point is that tyre adhesion plays a big part in the ultimate possible braking efficiency, and if the design of drums, shoes and servo mechanism were to be arranged to give an exceptionally high efficiency under, say, full load conditions, it is almoet certain that the vehicle would become dangerous when the load was removed —obviously the weight on the rear axle would be considerably reduced, thus affecting adhesion. This condition is exaggerated by the fact that, during retardation, the centre of gravity of the vehicle virtually moves forward, so that the front wheels take a greater load during braking (and the rear wheels a lighter one) than when a vehicle is stationary.
Matters are further complicated by the fact that the hand or auxiliary brake, in current design, usually takes effect on the rear wheels, and, although this layout is quite justifiable with existing braking efficiencies, it would be by no means a blessing were much higher efficiencies the general order. We are referring, of course, to current chassis in which the service brake operates on both the front and the rear wheels.
It is difficult to give an average figure for the load carried by each axle, when a vehicle is empty or par tially or fully laden. The weights naturally vary enormously, so that if it be intended to avoid skidding with the rear wheels when the brakes are vigorously applied with the vehicle empty, the effectiveness of the retarding mechanism has to be kept reasonably low, and the driver can then use the hand brake (if it operated on only the rear wheels) when the vehicle is fully laden.
When descending steep hills, all the foregoing problems are aggravated to some extent, yet even on a vehicle having a brake efficiency as low as 30 per cent, it should be possible to hold the machine on a slope having a gradient of 1 in 3.2.
Reviewing existing systems as applied to modern chassis, one is first D57 of all confronted with thern wide divergence in types, and in the manner in which the shoes are coupled up to the pedal and hand lever. Broadly speaking, there are four distinct types of brake at present in use : the straightforward system with a servo motor, a system with a servo action produced by the shoes themselves, an hydraulic application, and the combination of hydraulic application with a servo.
The first is very popular, for it provides a light pedal pressure even when pulling up in an emergency. A development of the type is that produced by the Clayton Dewandre company, where a master servo cylinder is coupled by vacuum .lines to further cylinders attached to the front wheel hubs. This cuts out a lot of linkages and thus reduces maintenance work to a considerable extent. Another is the Marelli, in which a bellows is employed instead of a cylinder.
The Westinghouse brake, introduced late last year, is particularly suitable for trolleybuses, a two-stage compressor being driven by the engine, or in the case of a trolleybus, an electric motor. After passing through a cooler, air is delivered to a container at a pressure up to 450 lb. per sq. in. The application cylinders are of small bore and comparatively long stroke, and are attached to the axle assemblies in the normal way, the piston rods acting directly oa the brake-cam levers. Although the volume of air used is small, adjustment is provided in the form of extensible pisten rods. In this system there is no mechanical coupling between the pedal and the shoes within the drums ; there is, of course, a mechanical coupling in the Dewandre and Marelli systems.
Perhaps the best example of the selfservo or self-energizing type is the
Bendix arrangement, in which the whole brake gear is mechanical and is provided with a separate means for adjustment at each wheel, so that the braking effect can be balanced. In this system the shoes are fully floating, the primary and secondary shoes being made up so that a large servo action to help the driver is afforded. Furthermore, the friction material covers an arc which represents about 75 per cent. of the inner periphery of the drum. The hand lever is interconnected with the pedal system and is provided with a surplus range of movement.
The third type is represented by the well-known Lockheed hydraulic system. In this layout the oil in a master
cylinder is put under pressure by the pedal action, and by means of communicating pipes this pressure is transmitted to cylinders with pistons actuating each pair of shoes. The whole system is, of course_ self-contained and the effectiveness is exactly proportional to the driver's effort, for the pressure rise in the master cylinder—and, indeed, throughout all the pipe lines—is merely a matter of leverage and the thrust on the extremity of the pedal. There is, of course, no mechanical coupling, but a great feature of the system is that equal pressures are applied to all the shoes.
Finally, we come to the hydraulic system with a servo action. Instead of the driver operating the piston in the master cylinder direct, the applied
pressure is first taken through a servo motor—usually of the vacuum type— which increases it to a considerable /extent.
The systems outlined are representafive of their types, but, quite obviously, fhey do not include every make. As we have proved during our road trials of various vehicles, there is at the, present time a high standard of braking efficiency on all British chassis. In respect of the properties of the actual shoe facings, great strides have been made by the makers, an outstanding example of such improvement being that the friction fabric loses markedly less effectiveness through developed heat than was formerly the case.