Brake Tests

272 522. The most that can be claimed for these straight lines is that they fit the results of the tests as well as any other lines that could be drawn, and that sufficient data was not available to warrant the plotting of curves instead of straight lines nor the extension of these curves beyond the speeds involved. 523. An extreme effect of speed is well illustrated by the dyna- mometer diagrams (Fig. 149), and on the braking force curves (Fig. 138). With constant brake shoe pressure, the force of retardation generated by the brake shoe begins to increase near the end of the stop and con- tinues to increase at an accelerating rate, the retarding force reaching its maximum at the end of the stop. This action of the shoe can also be accounted for by the average temperature of the working metal as influenced by the rapidly decreasing speed near the end of the stop. 524. The rate at which the working metal of the surface of the brake shoe must absorb energy is dependent on the speed. The average temperature of the working metal is affected by the speed in combina- tion with the ability of the brake shoe as a whole to radiate or conduct the heat absorbed away from the surface of the shoe. The temperature of the brake shoe as a whole is of course always greater near the end of the stop and would have some tendency to increase the average tem- perature of the working metal, but on the other hand, the rapidly de- creasing rate at which energy is delivered to the shoe for absorption, due to the increase in speed, reduces the tendency of heating to such a degree that the effect of a higher general shoe temperature is offset and the average temperature of the working metal reduced. 525. Confirming this it is always observed that vigorous sparking decreases as the speed begins to decrease more rapidly near the end of the stop and finally is replaced by ground off metal not incandes- cent.. This indicates that although the general shoe temperature is un- doubtedly higher near the end than at any other part of the stop, the average temperature of the working metal is correspondingly lower and consequently the force of retardation and coefficient of friction increases. This also accounts for the relatively high coefficient of friction always developed during the low speed tests. 526. The conditions which bring about the temperature changes mentioned are graphically illustrated in Figs. 138 to 140, in which the power curves referred to the scale of B.t.u.’s per shoe per second indi- cate a very high heat input into the working metal during the early part of the stop, this rate gradually decreasing to zero at the end of the stop. Comparing this curve with the retardation curve it is seen that the shoe metal seems to be incapable of exceeding a certain maximum resistance so long as the heat input is above that which can be expressed as 40 or 50 B.t.u.’s per brake shoe per second. But when the heat