Brake Tests

284 which were frequently obtained in consecutive tests under identical con- ditions. A further investigation of the effect of air and water cooling was carried out by taking samples of metal from one brake shoe and heating them to temperatures of 400 degrees, 800 degrees and 1200 degrees Fahrenheit and then cooling. A study of the metal before heating and after cooling did not show any difference in the structure which could be attributed to the difference in the methods of cooling. Relation of Machine to Road Tests. 549. It has always been observed that stops with a car or train were much longer than brake shoe machine test stops under similar conditions. Some of this difference in stopping distance could be accounted for by the relatively low efficiency of the car brake rigging. There are other sources of difference, however, which are inherent and of such a nature that their individual and combined effects could not be measured with the means available during these investigations. The determination of a factor which will express the difference between the performance of a brake shoe in machine tests and in road tests is of importance, because with such a factor established it will be possible to predict within the limits of ordinary variations of brake shoe per- formance the probable braking performance of a car or train under any given set of conditions, knowing the performance of the brake shoe, the equivalent conditions of the brake shoe testing machine and the constants or difference factors which must be used to compensate for the difference between machine and road tests. 550. The difference due to the performance of the brake rigging can be fully allowed for if the running rigging efficiency can be deter- mined, but in the first place the determination of the running efficiency would involve the making of road tests which it is desired to avoid, and in the second place all means employed up to this time to measure the running efficiency of the brake rigging have been rendered more or less unreliable by the erratic movement induced within the measur- ing apparatus. The measurements of standing efficiency, however, have given results which are consistent within themselves and with theoretical deductions and will therefore afford a convenient and satis- factory basis for establishing the proper allowance to be made for the characteristics of any given brake rigging. It is recognized that there may be a difference between the performance of a brake rigging when standing and when running, but for the reasons just mentioned, the standing efficiency is the most convenient to obtain and whatever difference may result from the motion of the train will be taken care of by the ratio or difference factor established from the comparison of road and machine tests, as will be explained. 551. After making due allowance for the known standing rigging efficiency, the car stops calculated on the basis of the observed mean