Brake Tests

263 513 The principal factor in producing high friction for any given braking' condition is the frequent shifting of the bearing area from the heated to the cooler spots over the face of the shoe. With a shoe in such a condition that the bearing area cannot shift, the wheel is forced to wear the highly heated and ineffective shoe metal rapidly away, and successively expose cooler metal which is better able to offer resistance and absorb energy. This condition existed for the reduced area shoes of both plain and flanged types, the rate of wear being higher for these shoes than for those of full area. 514. The frequent shifting of the shoe surface in contact is de- pendent upon:— (A) The Fit of the Shoe to the WHEEL. The fit of the shoe to the wheel has an important influence on the facility with which the contact area can shift. If the shoe fits well, according to the ordinary understanding of this expression, it necessarily cannot bear on the wheel equally at all portions of its surface. But when wear takes place only a comparatively small amount of metal has to be worn off the bearing spots in order to bring cooler spots into contact, and consequently a good fit of the shoe will guarantee against the possibility of the bearing area being held concentrated at any par- ticular spots after they have become heated to the point where the metal breaks down rapidly and offers a comparatively poor resistance. (B) The Flexibility of the Shoe. The effect of warping is clearly shown by the comparative per- formance of a solid shoe of any type and a similar shoe slotted so as to make it more flexible. In every case the slotted shoe has a decided advantage. With a warped shoe bearing area cannot shift read- ily and consequently the average temperature of the working metal is high and a correspondingly low mean coefficient of friction is obtained. During the tests of the solid shoes, a shoe would often be observed to be warped in such a manner that it touched the wheel only at each end while at the center of the shoe the two surfaces were 4 in. apart, although the full braking power was applied to the shoe to force it against the wheel. Under such conditions the temperature of the bearing area would rise until the red heat had penetrated the shoe fully I in. In such cases the bearing area would not shift to a cooler part of the shoe until sufficient metal had been worn off to offset the effect of the warping. The uneven heating effect caused by the concentration of the bearing area at the two ends of the shoe would in turn cause the shoe to warp so that the ends of the shoe would be drawn away from the wheel and the bearing area again held for a comparatively long time at one spot at the center of the shoe. Such tests invariably resulted in longer stops or lower mean coefficient of friction than was the case with the same type of shoe slotted.