Variation of the retarding forces on the brakes of automotive vehicles for a given pedal position or a given pedal pressure is frequently due to the ordinary wear of the brake-linings or other parts and may readily be prevented by periodic inspection and adjustment. Sometimes, however, sudden and serious reduction in retarding ability occurs when the brakes have been applied for comparatively long periods with short cooling-intervals or when the brakes have been wetted or oil has reached the linings. Laboratory tests of braking materials have shown that a marked increase in temperature will generally result in a reduction in the coefficient of friction of asbestos textile brake-lining materials and that oil and water have a similar effect of a temporary character. In the present paper laboratory tests have been paralleled by a series of experiments undertaken with a view to securing data on the working conditions of the brakes of a passenger-car in actual service, particular attention being given to the nature and magnitude of the causes that lead to variations of braking ability. Three series of tests were made: (a) with the brake-linings dry, (b) after the linings had been soaked with water and (c) after the linings had been dried and had been treated with liberal potions of oil.
A detailed description of the testing apparatus is given, two features of which were the thermocouple set-up, consisting of eight copper-constantan couples, one in each brake-shoe, and a decelerometer for measuring the deceleration obtained in feet per second per second as the car was brought to rest. The results of the tests are explained and are depicted graphically in a series of curves.
The conclusions reached are that (a) with linings onsidered representative of those in general use in automotive vehicles, the apparent coefficient of friction drops with rise of temperature; (b) the drop in the apparent coefficient of friction with rise of temperature may be largely, if not wholly, due to the influence of the temperature rise upon the saturant; (c) the drop in the apparent coefficient of friction with rise of temperature seems to be less for hard, dense linings than for others; (d) no typical difference in performance with change of temperature was apparent between the woven asphaltic-saturated linings and the rubberized, or vulcanized, folded and stitched linings; (e) the tendency of the apparent coefficent of friction to rise rapidly seems to be greater for the rubberized or vulcanized linings after being water-soaked than for the solid woven asphalt-saturated linings; neither increase nor decrease of the coefficient of friction with water-soaking was apparent; and (f) the coefficient of friction of all linings tends to become less after they have been oil-soaked, and their tendency to recuperate from such a condition is not rapid.


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