Brake Tests

161 require an ideal brake shoe and a controlling mechanism which would automatically adjust the retarding force of the brake, so that it would be at all times the maximum which could be used just short of pro- ducing wheel sliding. 360. The shortest 80 m.p.h. stop was made, with conditions the same as mentioned above, in 1,422 feet. This is equivalent to an average retarding force of 310 pounds per ton. 361. From the data of stops made with locomotive alone and single car breakaway stops it is possible to calculate the approximate length of stop which would be obtained with a locomotive and train of twelve cars equipped with the electro-pneumatic brake, by substituting in the formula, explained in Par. 406 to 411. 362. Calculated by this formula, the best 60 m.p.h. train stop that could have been obtained with the means available during these tests is about 800 feet and the best 80 m.p.h. stop about 1,570 feet. 363. The shortest 60 m.p.h. train stop with a locomotive and train of twelve cars was 1,021 feet. This was made with high braking power on the locomotive and No. 1 clasp brake, electro-pneumatic equip- ment, 180 per cent, braking power and plain shoes on the cars. 364. The shortest 80 m.p.h. train stop was made in 2,197 feet with high braking power on the locomotive and with No. 1 clasp brake, electro-pneumatic equipment, 150 per cent, braking power and plain shoes. 365. These stops do not indicate that the performance mentioned is better than would have been possible with other combinations of brake rigging and brake shoes, because the particular combination of the factors, which gave the shortest stops mentioned above, were not always present in tests made with other types of brake rigging, brake shoes and percentages of braking power. Check Runs—Twelve Car Trains. 366. In previous brake tests the average of two, or at the most three, stops under a particular set of conditions was thought sufficient to establish the average performance of the train, but observations during those tests and a consideration of comparative records revealed many instances of variations in performance which could not be accounted for by any known differences in the equipment, adjustment, or manip- ulation. A study of previous tests from this point of view led to the conclusion that variable factors existed, of which no accurate record has been obtainable, nor indeed was sufficient data available to show definitely the extent of such variable performance as was indicated. The determination of the amount and cause of such variable per- formances as might result under supposedly constant conditions was made the subject of special study in these tests. For this purpose