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RECENT IMPROVEMENTS IN BRAKE LAYOUT.
Page 106
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How a Simplified Arrangement has Enabled Efficiency to be Improved.
IT is particularly interesting to study the striking difference which has occurred during the past few years in the layout of the braking systems on the majority of our commercial chassis, and especially in connection with those utilizing four-wheel brakes.
Only a short time ago the systems were, in many cases, most complicated, involving several cro.;s-shafts, relays, rocking levers and many types of compensating device—some extremely crude. The provision made for adjustment was often quite unsatisfactory, many. almost inaccessible points had to be lubricated and the cross-shafts were mounted in straight and rigidly held bearings which caused binding when any considerable frame flexion occurred.
Now, in nearly every chassis, the layout includes a single cross-shaft• of ample dimensions mounted in spherical bearings and with the front and rear rods running direct to the brake-operating cams, in most cases these rods being unbroken and having no relay levers.
Much of the credit for the improvement is due to Clayton Wagons, Ltd., Abbey Works, Lincoln, 'and, in particular, to the company's technical adviser, Major S. A. Currin. it is the result of some tv:o years' work in getting the makers to come into line and to employ a brake gear which will give good results, even without a servo, • so that when the latter instrument is also employed the braking is exceptionally efficient.
While at the Berlin Show we examined the braking of quite a number Of chassis, and in nearly every case where a. servo was employed it was of comparatively large dimensions, the reason being that the layouts, in most instances, were so inefficient that the large servo was essential.
When first Major Cumin went into the whole subject be came to the conclusion that it would be necessary either to redesign the brake gear throughout or to provale a servo three to four times as large as had been intended, but, unlike the policy followed in Germany, he chose the former solution. For a long time he fought strenuously against the principle of compensation, and we believe that we are correct in saying that there is Lot now a single heavy chnesis being constructed in this country with four-wheel 'brakes having compensation. This is very satisfactory in every way.
Some foreign chassis even have a servo fitted to the hand brake, which seems absurd and unnecessary, because with a proper brake design it is quite easy with the hand brake to give almost the same retardation figure as does a pedal-operated brake assisted by a servo. With it foot brake, the designer is limited to a very definite leverage giving a maximum pedal-pad movement of approximately 5 ids., which corresponds to a leverage of between 45 and 50 to 1, which is as high as is considered advisable, but, in the case of the hand brake the leverage can be taken to as high as 130 to 1, which means about 15 ins, travel on the end of the lever.
The essence of satisfactory braking is simplicity and robust construction coupled with the correct proportioning of the leverages to the front and rear axles. This matter of leverage is rather a complex question, because in conjunction with it has to be taken into account the flexing of the axle on the vehicle springs, .