Brake Tests

277 decided advantage for the clasp brake throughout the entire range of braking powers. The gain in favor of the clasp brake with slotted shoes amounts to about 40 per cent, at a braking power of 180 per cent, and 100 per cent, at a braking power of 40 per cent, an average gain for the whole range of braking powers of about 70 per cent. This gain is partly due to the different braking power conditions necessarily used as explained in paragraph 465, but after all allowances are made it is still much larger than between the single shoe and clasp brake road performance. But it should not be overlooked that in road tests the results are affected by the combined performance of the brake shoes and the foundation brake rigging. The difference in the efficiency of the clasp and single shoe rigging may offset the gain which might be expected from difference in coefficient of friction and as neither of these factors could be observed uninfluenced by the other, a satisfac- tory comparison of the mean coefficient of friction under the two con- ditions in road tests is impossible 536. When the action of brake shoes under standard and clasp brake conditions are considered in the light of the influence upon the working metal, the amount of energy to be absorbed per unit of work- ing area, and the average temperature of the working metal, it is ap- parent that the clasp brake must have some advantage over the stand- ard brake both in mean coefficient of friction and shoe wear. 537. The clasp brake provides two brake shoes to do the work which the standard demands of one shoe and this difference may be considered from these standpoints:— First. The clasp brake shoe has but one-half the wheel load and consequently one-half as much energy to absorb. Second. The clasp brake shoe is working at only one-half the shoe pressure at which the standard shoe must work under the same braking power. Third. The available area for the same amount of energy to be absorbed is double. The important fact here is that it is only the available shoe surface and not the actual working area which is double. The tendency of a shoe to warp will be more or less influenced by the pressure forcing it against the wheel. 538. It follows, therefore, that on account of the lower pressure per shoe the clasp brake is at a disadvantage from the standpoint of warping when compared with the standard brake. However, with clasp brakes the intensity of the local heating of isolated bearing spots is not so great on account of less energy to be absorbed per shoe and con- sequently the cause of warping should be somewhat decreased. This tends to minimize the warping effect under clasp brake conditions. These are the chief factors which result in the performance of the brake shoe under clasp brake conditions, showing less improvement over that