Brake Tests

246 470. After every test (except those at 30 m.p.h.) the wheel was cleaned by making two stops with cleaning shoes from a speed of 40 miles per hour. The first cleaning shoe was sand filled and was used for the purpose of removing any rough spots of brake shoe metal which might have adhered to the test wheel. The second cleaning shoe was of a special composition which polished the wheel and removed any sand or grit remaining. After the 30 m.p.h. tests, only the composition pol- ishing shoe was used. This program for the cleaning of the shoe was determined upon after many experiments had been made to find a method for cleaning the wheel which would insure the most consistent and uniform wheel surface conditions. 471. An important difference between the conditions of machine and road tests was that of the wheel treads. During the road tests the tread of the wheel is continually rolling on the rail but during a machine test the wheel is subjected to the action of the brake shoe only. It was impractical to produce any condition which would be equivalent to rolling the wheel tread during machine tests. The effect of this differ- ence in wheel surface conditions is one of the factors which go to make up the difference between machine and road tests described in the dis- cussion of the relation of road tests to machine tests. (Par. 549.) 472. The brake shoes to be tested were at all times arranged to come on test in a regular order so that all shoes were given time to cool and were at atmospheric temperatures at the start of every test. 473. Preliminary stops were made for the purpose of wearing the shoes in to the best possible bearing on the wheel. OBSERVATIONS. Time and Length of Stop. 474. The time of stop for all tests was taken by an observer with a stop watch. The length of the dynamometer card was exactly pro- portional to the distance of the stop of the test wheel. The card was accurately measured in every case and the necessary factor applied to give the true length of stop. The initial speed of nearly every test varied slightly from the speed desired and in such cases the actual distance of the stop was equated to an equivalent value for the desired speed assuming that the stopping distance is directly proportional to the square of the speed. Speed. 475. The dynamometer card mechanism was thrown into gear three or four seconds before the shoe was applied to the wheel. The record obtained for time and distance just before the application was used as the basis for calculating the actual speed of that test.