Brake Tests

268 on one brake shoe variations in the results are to be expected and it is therefore imperative that deductions be based on the results of a num- ber of tests. 517. The variation in the results of successive machine tests will of course be greater than in road tests because in the case of a car or train an average is obtained from the action of all the brake shoes in- volved. The Effect of Speed on Coefficient of FRICTION. 518. The results of both road and machine tests on brake shoes have shown that the coefficient of friction under any condition of brak- ing power is dependent upon the initial speed, and the coefficient of friction tends to decrease as the speed is increased. This condition can be expected from the fact that the total energy to be absorbed during a given stop is always proportional to the square of the initial speed. The effect of the higher speeds is to increase the rate at which the tem- perature of the working metal changes, and although the actual bearing areas will shift more rapidly, the greater tendency to a general heating of the shoe reduces its ability to conduct or radiate heat from the face of the shoe. For these reasons the average temperature of the working metal is consistently higher at higher speeds, and the coefficient of friction correspondingly lower. 519. If the foregoing correctly describes the effect of a continually changing speed on the instantaneous values of the coefficient of friction during the stop it follows that the mean coefficient of friction is going to vary with the average temperature of the working shoe metal. Con- sequently, since the rate at which energy is absorbed by the brake shoe is higher and the total amount of energy absorbed greater for high speeds and the converse of this true for stops from lower speeds, it follows that the mean coefficient of friction should vary inversely with the initial speed. 520. The relation between initial speed and mean coefficient of friction for various shoes and braking conditions indicated by the test results is shown on Figs. 162, 163 and 164. These curves show the char- acteristically lower mean coefficient of friction for tlie high initial speeds. In considering the significance of these curves the conditions under which the data was obtained must be taken into account. That these conditions have an important bearing on these results is evident from the marked difference between the results obtained under single and clasp brake conditions. 521. It should not be supposed that because straight lines have been drawn that this relation holds outside the range of speeds tested because we would not expect to find a zero mean coefficient of friction at any finite speed however high.