Brake Tests

252 RESULTS OF TESTS—EFFECT OF BEARING AREA ON RESULTS. 492, The measurement of brake shoe bearing area, as described in Par. 488, did not prove to be as valuable in connection with the study of its effect on mean coefficient of friction as was desired, as the dia- grams taken were only able to show the total shoe surface which had been in bearing contact at some time during the stop. The mean co- efficient of friction is a function of the mean average of all the bearing areas which may have existed at each instant of the stop and, there- fore, this average should be considered if a true relation between bear- ing area and mean coefficient of friction is to be established. The diagrams shown in Figs. 151 to 154, indicate the total bearing area which came into contact during the stop but they do not give any in- formation as to its value at any instant of the stop or its average value throughout the stop. 493. The unsatisfactory nature of the determination of bearing area was appreciated and as a means of obtaining additional information a special series of “Intermittent” tests was made in the following manner:— 494. The wheel was brought up to the speed desired and the shoes applied in the usual way. As soon as full pressure was obtained the shoe was released and the diminished value of the speed at the instant of release was noted. The bearing area of the shoe was then measured. This operation was repeated five (5) times, each application being made at an initial speed equal to the final speed of the preceding application, starting with an initial speed of 60 miles per hour and allowing the wheel to stop during the application of the pressure for the fifth time, after which the shoe was released, removed from the wheel and its final bearing area measured. 495. A number of tests were made at both 120 per cent, and 144 per cent, braking power on various types of shoes well worn to the wheel. The results of the tests are shown in Figs. 155 and 156. The shoe bearing area is expressed in per cent, of the total face area of the shoe, and is plotted against the speed of tlie wheel, noted at the instant the shoe was released. 496. From Figs. 155 and 156 it is apparent that the bearing area varies widely throughout the stop, the bearing area being small at the beginning but increasing to a maximum at or near the end of the stop. The results, however, are only indicative, because there is necessarily some difference between the conditions of the intermittent and regular