Brake Tests

285 coefficient of friction in machine tests do not check with the stops obtained in road tests, but are invariably shorter. This discrepancy must be due to inherent difference in conditions, and having the data of sufficient number of comparative road and machine tests as in the present investigations, it is possible'to determine the average ratio of the road and the machine performance after allowance has been made for the standing brake rigging efficiency. This ratio will then furnisli a factor involving both the inherent difference between machine and road tests and the difference between standing and running efficiency whicli can be used in calculating the probable car or train stop after having determined the other controlling factors from machine tests and meas- urements of the car. 552. The results of tests of standing efficiency are plotted in Fig. 76 and have been discussed in Par. 293. Applying the brake rigging efficiency thus determined and the stops determined in road tests at different percentages of braking power as shown by curve “C,” Figs. 169 and 170, curve “B” has been obtained as the probable relation between per cent, braking power and length of stop with 100 per cent, rigging efficiency. Tlie difference between curve “B” and curve “A” which shows the relation between per cent, braking power and stop as obtained on the machine tests is then proportional to the ratio or factor discussed in Par. 540. The following table shows various values for this ratio which have been derived from the figures shown:— BRAKE Shoe Type of BRAKE Ratio Between Road and Machine Stops CALCULATED on the Basis of 100 Per Cent. Rigging Efficiency Mean 60 Per Cent. B. P. 180 Per Cent. B. P. Plain Slotted or Broken. Flange Slotted or Broken Averages No. 3 Clasp.... No. 3 Clasp 1.280 1.250 1.290 1.210 1.270 1.290 1.265 1.250 1.280 553. An illustration of the method of using the machine and road test ratio described is as follows:— Let us assume that it is desirable to know the performance of a certain car in a breakaway stop from 60 m.p.h. at 150 per cent, braking power and that the standing rigging efficiency of this car has been measured. We will further assume that this measurement of rigging efficiency is expressed by the solid line curve on Fig. 76 and that slotted or cracked flanged brake shoes will be used. Consulting the curve