Brake Tests

151 331. Tests were considered as having excessive wheel sliding when the sum of all slides was 3,000 feet or over in twelve car train tests and 250 feet in single car tests. These arbitrary figures were chosen when an analysis of the wheel sliding data developed that the sum of all slides, when less than 3,000 feet, was not sufficient to have any material effect on the length of stop and was usually made up of relatively short slides on a number of pairs of wlieels, but when more than 3,000 feet, several pairs of wheels usually slid for the greater part of the stopping distance making the sum of all slides well above this figure. 332. The detailed tabulation of the wheel sliding which occurred during all the tests is shown on pages 322-324. 333. The table on page 152 shows the number of tests made at each percentage braking power with each type of brake rigging and the number of tests in which the wheels slid 15 feet or over. 334. With the No. 1 clasp brake it will be noted that wheel sliding occurred at low as well as at high percentages of emergency braking power. These tests were run between February 10th and March 5th with a comparatively low air temperature prevailing. This tem- perature, accompanied by the high humidity which is characteristic of the location of the test track, brought about rail conditions which were largely responsible for the sliding during this period of the tests. This bad rail condition was especially marked in the first tests of the day, which tests, with few exceptions, were electro-pneumatic emer- gency applications at 150 per cent, braking power. 335. Out of a total of ninety, 150 per cent, emergency tests made with the No. 1 clasp brake, 62 tests developed no wheel sliding at all. Of the 28 in which there was wheel sliding, 13 were the first runs of the day, usually made at about 8.30 a. m. Previous to the running of the first test of the day there was a period of two hours during which the track had not been run over by other trains. This permitted an accumulation of frost or moisture on the rail during that interval. Few tests, made subsequent to the first run of the day, showed excessive wheel sliding, and it may be concluded that the rail condition referred to was chiefly responsible for the sliding that occurred. 336. The tests with the other types of brake were made later in the spring when the weather was more favorable to good rail condition, and it will be noted that in these tests there is a marked decrease in excessive wheel sliding. 337. An analysis of the percentage of runs with wheel sliding at various percentages of braking power shows that with plain shoes the amount of wheel sliding depends rather on the rail and weather con- ditions than on the percentage of braking power.