BRAKES: CURRENT DESIGNS AND TENDENCIES.
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Four-wheel Braking, Servo Motors and the Various Methods of Application in Vogue.
THE most interesting development in connection with braking which has occurred during the past few years is the ever-increasing use of brakes applied to the front wheels, and so marked has this tendency become that one of the results of the recent deliberations of the Coroners' Association was a resolution to the effect that this Association would approve of any proposal designed to make four-wheel brakes compulsory. Similar views are evidently held by those who took part in the conference On London's traffic accidents last week, as this assembly went so far as to agree that the general use of four-wheel braking ought to be encouraged.
The Value of Braking All Wheels.
The advantages of applying the brakes to all the wheels of a four-wheeler, instead of only to the rear pair, have often been discussed in this journal, and, from the safety point of view, there are two of outstanding importance, these being (a) a material reduction in the distance required to bring the vehicle to a stop from any given speed, and (b) the greatly reduced tendency to side-slip when applying the brakes on a greasy surface. It is a mistake, however, to give the impression that all classes of vehicle would equally be better off with additional brakes on the front wheels.
The advantages to be gained by four-wheel brakes depend to a considerable extent upon the weight distribution, the conditions being most favourable when the front and rear-axle weights are not widely different. In the case of a vehicle such as a four-ton lorry, however, as much as 75 per cent, of the total load may be taken by the rear wheels, and the reduction in stopping distance made possible by the fitting of the front brakes is then not so very marked. The rigid-frame six-wheeler is another important ease, and it may be doubted whether such a vehicle, which is already invariably fitted with brakes on all four wheels of the rear bogie, needs an additional pair upon the front wheels.
Turning to the brakes themselves, the general trend is to fit them to the wheels rather than to -the transmission, and in the former location internal-expanding rigid shoes, placed within a drum and operated by a earn, are almost invariably used. Some difficulty has been experienced with such brakes in connection with the rapid development of coaches, buses and similar vehicles designed to run on pneumatic tyres, owing to the fact that the brake drum is of nearly as great a diameter as the rear-wheel rims and heat is liable to be conducted to an extent sufficient to raise the temperature of the rim to danger-point—i.e., dangerous to the rubber of the tyre. This difficulty is causing some designers to consider the possibility of returning to the use of a transmission drum for the service brake: others are tackling the problem by arranging for special ventilating duets between the brake drum and the wheel rim.
Gradual Simplification of Brake Gear.
Brake-operating gears are, on the whole, becoming simpler, which is all to the good, as a reduction in the number of pivots and joints tends to reduce friction and, therefore, pedal pressures. At the same time, those makers who have adopted four-wheel braking have necessarily added considerably to the complexity of their brake-operating gears and the friction which must be overcome when applying them. In consequence, various ideas are in course of development for the reduction of the pedal pressure to a point unlikely to cause driving fatigue.
For example, there is a hydraulic system, which has c42
become quite popular in America, in which a fluid— usually oil—is displaced from a master cylinder when the pedal is depressed and is led through pipe-lines to cylinders and plungers placed adjacent to the brakes. An analagous, although very different, system, which has been applied to British-built commercial vehicles with great success, in addition to those made in America, is, of course, represented by the Westinghouse power brake, 4n which the shoes are applied by air, or, rather, gas, acting under pressure against diaphragms and operating pull-rods.
The gas from the engine is stored automatically under pressure in a reservoir, and is admitted to the diaphragms operating the brakes when the driver moves a valve, which may be either hand or pedal-controlled. Such a system is particularly convenient for application to tractor-trailer vehicles, as it removes all the difficulties otherwise involved in using brakes on the rearmost wheels.
Utilizing Further the Low Pressure in the Manifold.
'Another way of getting over the difficulty of increased friction in a four-wheel-braked vehicle consists of utilizing the partial vacuum created by the engine in the inlet manifold. This is done very cleverly in systems such as the Dewandre and Westinghouse vacuum types. The depression of the brake pedal opens a valve which allows the engine rapidly to exhaust the air from a cylinder and atmospheric pressure then drives a piston into the cylinder, so producing the necessary pull on -the brake gear.
Matters are so arranged that the piston faithfully follows the movement of the brake pedal, and the pressure which is actually being put upon the brakes can be felt by the driver. Furthermore, in the event of the engine stopping or the servo system failing from any other cause, the driver can still apply the brakes directly by the pedal, although increased foot pressure is then required.
Reverting to the front-wheel brake, the actual mechanism employed for operating the shoes takes various forms, all of them being so arranged that the Steering is not adversely affected by braking and vice versa. The three systems in most prominent use at the time of writing are the Perrot, the Alford and Alder (Rubury Patents) and the Kirkstall. In each case the brake cam is operated through a universally jointed spindle, the joint being located either near to or in line with the axis of the steering pivot.
Differential Braking of Front Wheels.
In the Alford and Alder and the Perrot designs special provision is made so as to obtain an automatic differential action between the two front brakes when cornering, the idea being that as the front wheels are locked over so more braking should be thrown on the inner wheel and less on the outer one. On the whole, this would seem to be conducive to increased safety. when the roads are exceptionally greasy, but in other circumstances practical experience would appear to suggest that there is not much to choose as regards controllability between the vehicle with differential brakes and one on which the brakes are always applied equally.
In a brief survey, such as this, it is hardly po.ssible to do justice to so Important a subject. We have not been able, for example, to touch upon brake design as affecting reliability and freedom from failure. However, sufficient has been said to show that real progress has been made during the past few years, both in improving controllability and in stopping distance.