Brake Tests

237 improving shoe condition, from the time the train was first started, when the shoes did not have proper bearing, until the shoes had reached the full bearing on all of the wheels. 445. The influence of shoe bearing on the length of stop is more fully referred to in the data obtained on the check runs which have been discussed in Chapter VI (Par. 366). Cracked or Slotted Shoes. 446. A consideration of the results of the warping of the shoe led to the conclusion that the warping could be largely eliminated by slotting or cracking the shoes so that they would be more free to con- form to the contour of the wheel. 447. As a matter of fact, this cracking takes place with either plain or flanged shoes after a number of runs have been made and while the effect of this cracking on the length of stop is not accurately obtainable from the data of the road tests it was made the subject of further investigation in the laboratory tests of brake shoes. 448. To afford at the outset the shoe flexibility which comes about from the gradual cracking of the shoes during continued use, a number of plain shoes were slotted and applied to a single car equipped with the No. 2 clasp brake. The resulting stops of the car so equipped did not show the advantage of this shoe modification owing to the limited number of tests made. It was decided not to carry these investigations further, however, because laboratory tests were under way and offered a more convenient means of accurately determining the effect of slotting or cracking shoes. PLAIN Shoes—ONE-HALF AREA. 449. As a further study of the influence of bearing area on the performance of brake shoes, tests were made with the same shoes as in the slotted shoe tests but with the ends broken off and their area re- duced by 50 per cent. The first stop (test No. 0120) was made in 1031 feet. This was almost as short as the shortest stop made under similar conditions with the full area unslotted shoes (test No. 099, length of stop 1007 feet). Subsequent tests with these partial area shoes re- sulted in stops of 1210, 1190, 1193 and 1134 feet. All of these stops were with the brake shoes at a high temperature. These results tend to confirm the conclusion that the bearing area rather than the total face area of the shoe is the important factor in brake shoe performance, and that the bearing area on the first test with half area shoes was substan- tially as effective as that of the full area solid shoes which were undoubt- edly affected by warping to a considerably greater extent. Further- more, the much longer distances run in the four stops following the