Brake Tests

160 CHAPTER VI. GENERAL DISCUSSION OF STOPS. 353. This chapter contains the results of tests made on the stopping of vehicles from speeds of thirty to eighty miles per hour, when equipped with various types of air brakes, brake rigging and brake shoes. 354. The detailed data of all the tests are shown on pages 349 to 390, but for more convenient reference all tests have been arranged in consecutive order on graphical log sheets, pages 325 to 348, which shows the significant data pertaining to each test. The data of comparative runs has been grouped in general average tables, pages 300 to 317, with average values indicated wherever it was possible to average two or more tests made under similar conditions. Diagrams showing the graphical comparisons of general averages are given in Figs. 93 to 100. Various other graphical comparisons of stops are shown (Figs. 101 to 114), in which the length of stop is plotted for different percentages of braking power at one speed, and different speeds with one percentage of braking power. 355. To illustrate the relation between speed, brake cylinder pressure, per cent, braking power, length of stop, time of stop, slack action in the train, rate of doing work during the stop and average retarding force during the time the brake is applied, plots of a large number of tests under different conditions are shown (Figs. 117 to 137). 356. Before giving a description or analysis of these detailed tabulations and diagrams, it will be of interest to call attention to some of the salient features of the tests. 357. The shortest 60 m.p.h. emergency stop was made with a single car (locomotive not attached) with the No. 3 clasp brake electro- pneumatic equipment, 180 per cent, braking power, and flanged brake shoes. The car was stopped under these conditions in 725 feet. The average retarding force for this test was 332 pounds per ton. This is equivalent to the resistance offered by a 16.6 per cent, grade or, in other words, the resistance which stopped this car in 725 feet from 60 m.p.h. was equivalent to that which would be encountered due to a grade on which one end of a P-70 car (80 feet long) would be 13.3 feet higher than the other end. 358. This stop of 725 feet at 60 m.p.h. establishes a new record for a railway car stop and was made with a modern heavy passenger equipment car. 359. Assuming a rail adhesion of 25 per cent., the shortest possible stop which could be obtained, by utilizing this adhesion to its maximum throughout the period of braking, would be 481 feet. This would