THE POSSIBILITIES ILTR-WHEEL BRAKING.
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URI:VG THE past few years four-wheel brakes have attracted a vast amount of attention, and
have become very popular on pleasure. cars, both here and in America. The system has not, however, received nearly so much ,attention in the commercial-vehicle world, the truth of this contention being shown by the last heavy vehicle exhibition at Olympia, in which only about half a dozen chassis were,equipped with brakes acting on all four wheels: Before entering into a description of the various 'ways in. which these brakes can be applied, it will be as well to consider to what particular types of commercial vehicle four-wheel braking would prove of the greatest advantage.
First of all, it should he made clear that, contrary to common belief the weight of a vehicle has really iothing to do with the question of whether it should be fitted with brakes on two wheels or on four. It is speed which is of the greatest importance when deciding between the two systems. That this statement is correct will be evident from a brief consideration of the theoretical principles involved when s moving vehicle is brought to rest.
The energy which it possesses when in motion depends upon two factors, namely, the weight and the.. square of the speed. Now the first of these factors also governs the adhesion available between the tyres and the road, so that, although a heavy Vehicle possesses greater energy at a given speed. than one of lighter construction, the braking adhesion available is increased in like proportion. Speed, however, is very important because the energy which must be dissipated in bringing a certain lorry to rest from a speed of, say, 20 m.p.h. is no less than four times greater than that which obtains when the speed is only 10 m.p.h.
It follows, therefore, that the decrease in stopping distance, which can be*obtained by braking on four wheels, instead of two, will be of most service in the case of commercial vehicles which habitually travel at comparatively high speeds. Of these, the'two most common examples are the light delivery van, suit able for conveying loads of 10 cwt. to 20 cwt., and the passenger touring vehicle whether it be a char-it-banes or the more elaborate motor coach. In. each case, the speed of which the vehicle is capable 'may very well amount to 25 m.p.h. or even 30 m.p.h., so tha.t a reduction in the distance in which it can be pulled up is of great importance. To emphasize this point we may refer to some authentic tests, carried out in America on a large touring ear weighing over one ton. The results can be set out as follow :— Stopping distance. Rear brake. Front brake. Both brakes.
80 ft. 68 ft. 39 ft.
Speed, 30 m.p.h.
From these figures it. will ho seen that, by applying brakes to all four wheels, the stopping distance is halved, as compared with the result obtained when only two wheels are utilized.
Assuming, then, that four-wheel brakes are really desirable on the faster types of commercial vehicle, it remains briefly to review the different ways in which this end can be accomplished. There are two important points to consider in the layout, the first being the means used to operate the shoes or band which acts upon the front-wheel drums, and the second being the way in which the front-wheel, rear-wheel and auxiliary brakes are coupled together • and -operated.
As regards the first part, of the subject, the actual operating gear must be so arranged that i steering s in no way affected by the application
. of the brakes, and movements of the front axle do not affect the operating gear or the brake shoes. It has been clearly shown that, provided a practical degree of centre point steering is obtained, by
canting the pivot, inclining the wheel, or both, steering will not be appreciably affected by braking the wheels.
True centre-point steering, in which the axis of the pivot meets the road at the centre of tyre contact, is not really desirable, because the steering becomes too light and the wheels are very prone to wobble. A better result is obtained when the pivot axis meets the road at a distance of roughly in. from the centre of tyre contact.
Many methods have been devised with the aim of enabling the driver to actuate the front-brake shoes, no matter what degree of lock may be given to the front wheels, Perhaps the best way of dealing with this part of the subject is to review the systems already in vogue, four of these being illustrated on this page.
The system patented by M. Perrot, the famous French designer, has been very widely used, and is a most successful one. It is applied in conjunction with internal-expanding shoes, the cant spindle of which terminates in a universal joint plated just outside the drum. A short shaft is coupled to this joint and is carried in a bearing secured to the frame at the inner end. This shaft carries a lever, which is actuated by the brake pull-rod, and embodies a sliding joint which, in conjunction with the universal, makes allowance for front-axle and front-wheel movements.
Another method of actuation, shown in one of the illustrations, is believed to have been originally brought out by the Italian makers of the Isotta-Fraschini pleasure ear_ It is also used
by other concerns, notably the AmericaneRickenbacker. In this case, the cam spindle is carried in bearings which are part and parcel with the front axle. The cam itself is of the universal type ; that is to say, the surfaces in contact with the shoes are curved, so that, when the front wheel is locked over, the shoes, in a sense, roll round the cam. It is, of course, necessary that the axis of the steering pivot should pass through the centre-line of the cam. This system is quite successful and is neat and effective.
Mention should also be made of the various mechanisms in which a wedge, operating the shoes, is actuated by a push rod, which passes through the centre of a hollow steering pivot. The upper end of this rod may carry a cam or quick-thread mechanism, which is actuated by a lever and rods.
Hydraulic and pneumatic methods of front-wheel brake operation also possess considerable advantages, and the ingenious design recently brought out by the Westinghouse Brake and Saxby Signal Co. is also shown on this page. A pneumatic cylinder of the ordinary Westinghouse pattern is mounted on a bracket pivoted to a frame, and actuates a short universally jointed rod coupled to the brake cam. The method of operation is almost self-explanatory, the sliding joint, universal and pivoted bracket allowing for front axle and steering movements.
The external-contracting band brake is very popular in America, so that it is not surprising to find this type adopted for the/Buick front-wheel brakes. The actuating lever is somewhat similar to that used on the Perrot brake, but carries an arm at the outer end coupled by a short rod to a lever which operates the contracting band.
Turning to the way in which the brakes should be coupled up, there are several important matters to consider. First of all there is the controversial point relating to thepruporCrou'of braking effortwhich should be ,allotted to the front wheels and the rear wheels. This, of course, can be arranged in any desired ratio, According to the leverages adopted in the system.
At first sight, One might be inclined to distribute; the braking effect in accordance with the axle loads; thus, if the back axle weight amounted to 60 per cent of the total weight, we might have the leverages so arranged that .40 per cent. of the pull was applied to the front-wheel brakes and 60 per cent, to the rearwheel brakes.
This proportion, however, would not give the maximum braking effect, for the simple reason that when a vehicle is decelerated the weight is thrown forward on to the front axle, and the distribution is not by any means the same as that which obtains when the vehicle is stationary. However, there is yet another factor to consider, namely, that it should hot be possible for the brakes to lock the front wheels, as this would cause complete lack of steering control. Hence, it may be preferable to sacrifice a little in braking effect rather than run the risk of locking the front wheels ; . this can be done by arranging the leverage so , that the pull on the rear-Wheel brakes is slightly greater than that -on the' front-wheel brakes., The' Writer is of the opinion that all four brakes should come into Operation simultaneously. it should be noted, however, that some people advocate bringing the rear-wheel brakes into action somewhat in advance, while others believe that a precisely opposite method is preferable. No tests seem to have been undertaken with the object of proving which method is the better.
These are innumerable ways in which the brakes can be coupled up to the hand lever and pedal, but
of these only four appear to have gained any great. degree of popularity. In what is, perhaps, the most common arrangement, a brake is provided on each wheel, and these are all coupled to the pedal. A simple cam arrangement is provided, so that the hand lever can operate the same pair of rear brakes independently.. While this arrangement is simple and satisfactory, it is open to question whether it really complies with the exact letter of the law, which calls for two independent brakes.
A mere complex, but somewhat safer' system consists of coupling the four-wheel brakes to the pedal, and providing an independent brake operated by the hand lever. This independent brake may be mounted on the transmission, or, alternatively, can be arranged to operate on a second pair of drums secured to the rear wheels.
The advantages of the transmission brake can be utilized in another manner by coupling it, together with a pair of front-wheel brakes, to the brake pedal, while using the hand lever for operating the rearwheel brakes. This system lessens the exertion required on the part of the driver, because the effort exerted by the transmission brake is automatically amplified by the reduction gear in the back axle.
Lastly, we may mention the increasingly popular hydraulic and pneumatic systems of brake operation. in America, the Lockhead hydraulie system is now used on a great many cars, while in this counfry Westinghouse pneumatic brakes have been successfully adopted for many commercial vehicles, particularly the tractor-lorry.
The pneumatic system possesses the very, great advantage of reducing the exertion of the driver to a minimum, and also ensures an equitable distribution of the braking effect.