Brake Tests

163 repeated tests, under as nearly as possible the same conditions in cartel to detect influences which may not be apparent without such checking and yet would have an effect on the value of the comparisons, value or the 88 89 and A graphical record of all of the check runs is shown in Figs. a™dabk 90. These plots indicate clearly the variations which are un- Voidable, except by the most careful provision for the constancy F a actors which have an effect on the stop. The brake shoe bearing is the most difficult factor to control and at the same time it is ing most potent in producing variations in brake performance. 371. Another example of the importance of the brake shoe con dition and the manner in which it can be affected is afforded by test No 150 This stop of 1,040 feet was the shortest 60m.p.h.train stop made with ;.1P • •’ aking power. It was a check run with the single shoe ra erigging and was not only the shortest made with this train but was shorter than any check run with trains equipped with clasp brake gg g. An inspection of the record will show that this stop was made the dor peculiarly favorable conditions. While it was the first run of (9) day, at rail condition was good and the test having followed ni"e (9) tests at a' (90 per cent.) brakinS Power insured the best possible t oe bearing. The favorable shoe bearing was further con- uted to by the light service applications made during the move ment of the train from and to the test ground and by the standing n over night. In this connection, it is of interest to that tests Nos. 82 and 92 were also short stops (1,091 feet and 1,076 feet espec ively) and that each of these was also preceded bv several at 90 per cent, braking power. All of these tests were several runs e brake shoe condition was, as a whole, satisfactory on ade : 1 shoe brake train. ausractory on the single 372. On the other hand the longest stops of the check runs with any arrangement of brake rigging were also made with this same train (single shoe rigging) after the shoe condition became unsatisfactory to many shoes running partially off the wheel, thus showing that for a given set of conditions other than that of the LES S th 8 shortest and also the longest stops of the entire series or courtine ts were brought about by variations in brake shoe conditions alone quality 3./ From the preceding it follows as a matter of course that the quality of the comparisons of air brake equipment and brake rigging may be considerably affected by the performance of the brake Being may kotowierle of brake shoe Performance under various conditions, there- fore, becomes of prime importance in order to properly interpret the results of road tests. Furthermore, the knowledge gained during the road tests indicated the desirability of a more searching laboratory investigation along several original lines. Such investigations were made and are discussed in Chapter VII.