Brake Tests

149 Similarly, a 25 per cent, increase in braking power, from 144 per cent, up to 180 per cent, results in the same proportionate decrease in length of stop, viz.: from 896 feet to 787 feet, which is a 12 per cent, decrease as at the lower braking power. 326. An analysis of the curves on Fig. 84 shows that for single car breakaway stops the relation between the percentage of braking power and length of stop can be expressed by the following equation: in which, S == the length of stop. K = a constant determined by the character of the air brake equip- ment, brake shoes and brake rigging. p == percentage of braking power corresponding to the cylinder pressure obtained. x == a fractional exponent, depending upon the effect of the per- centage of braking power on the brake rigging efficiency and the coefficient of friction of the brake shoes. 327. This law is found to apply to both of the curves in Fig. 84 and that it holds for still lower braking powers than here shown was proven by both road and laboratory tests at low percentages of braking power (50 to 60 per cent.), the results of which satisfied the relation which had previously been found to exist between stops under similar conditions, but at percentages of braking power ranging between 90 per cent, and 180 per cent. 328. Referring to Fig. 84 the equation for the line showing the best stop is:— 1107.5 • p0.681 and for the line showing the average stops, 1169.2 • p0.583 329. An approximate expression for the variation of percentage of braking power and length of stop with the electro-pneumatic equip- ment, is that, for an increase of five per cent, in the braking power, the stop is decreased two per cent. Wheel Sliding. 330. In obtaining data on wheel sliding, slides under 15 feet were not recorded, it being considered that any sliding less than this is un- important.