Brake Tests

274 EFFECT OF WIDTH AND LENGTH OF SHOE ON MEAN COEFFICIENT OF FRICTION. 530. Tests were made under clasp brake conditions to determine the effect on the coefficient of friction when the shape of the nominal bearing area is reduced. The results of these tests are shown in Figs 164 to 166. It will be noted that the half area full width shoe D shows a slightly higher mean coefficient of friction than the half area full length shoe C throughout the range of braking powers. The curves for shoes C and D lie between the curves for the plain shoes A and B. 531. On the basis of the available bearing surface the half area shoes should show a lower coefficient of friction than the full area shoes, but by modifying the shape of the shoe the effect of warping has been reduced to such an extent as to offset the loss due to a smaller nominal bearing area. 532. That the tendency to warp is reduced on the half area shoes is shown by the results of the intermittent tests (Fig. 155), where it will be noted that the ratio of the actual to the available bearing surface was consistently higher for the half area shoes than for the full area shoes. 533. The curve for the reduced area flanged shoe “G” lies be tween those for the flanged solid and slotted shoes "E" and "F," and indicates the same effect of warping characteristics as found for the plain shoes. It is °f interest to note that although the area of flanged shoe “G” was the same as the plain slotted shoe “B” (which gave the best results for plain shoes) the performance was better than that of shoe "B." Therefore, on the basis of equivalent available bearing surface a shoe of the flanged type will give a higher mean coefficient of friction than a plain shoe either solid or slotted. One Versus Two Shoes Per WHEEL. 534. The gain in brake shoe performance with two shoes per wheel (clasp brake conditions) as compared with one shoe per wheel (stand- ard brake conditions) is most clearly shown in the comparison of the brake shoe machine test results. This difference is less easily deter- mined from the results of the road tests because of the presence of other actors not present in machine tests. r 535. Curves showing the average results of tests of various types o brake shoes under both single shoe and clasp brake conditions are shown in Figs. 165 and 166. A comparison of the values of mean co- e neient of friction for standard and for clasp brake conditions shows a