DYNAMOMETER TEST OF BRAKE-DRUM HEAT IN DUAL WHEELS 260051
Premature failures in the summer of 1925 of inner-dual motorcoach tires and tubes on 20-in. wheels in which the brake-drum was directly under and close to the rim caused the rubber companies to undertake development work on tire beads, flaps and tubes; but no solution of the problem was to be found by a change of the rubber compound. The need for definite information regarding actual tire temperatures developed in road service led to extensive joint tests in Florida early in 1926 by a tire and a wheel company. In these tests a brake-drum temperature was maintained as nearly as possible constant at 475 deg. fahr. above atmospheric temperature by successively accelerating the car and applying the brake. When it was judged that this temperature had been reached, stops were made to take thermocouple readings of the temperature.
Realizing that tests based on constant brake-drum temperature would not give as true a comparison of the heating tendency of different wheels and the effectiveness of heat-dissipating devices as tests based on constant brake-horsepower input, the B. F. Goodrich Co. undertook a series of tests with a Sprague dynamometer tire-testing machine. These were continued over a period of 3 months with apparatus described by the authors. Comparative tests were made on four different wheels, of which two were disc wheels, one 20 in. and the other 24 in. in diameter. The other two were steel-spoke wheels of different types but both 20 in. in diameter. Two ventilating fans were supplied with one of the latter wheels. Two cast-steel brake-drums, similar in other respects, but one having surface corrugations extending from the flange part way across the outside of the drum and the other having substituted for these corrugations a strip of Goodrich superheat packing 1/32 in. thick, were also used in making the tests.
The method of conducting the test and of taking temperature readings from the drum and the tire beads is described. In all tests 6-in. dual tires were mounted on the wheels and inflated to the standard pressure of 80 lb. To obtain comparative data, variables were introduced by running the wheels at various road speeds, with and without standard tire-load, with the different drums and with and without the fans fitted, and, in the case of one of the steel-spoke wheels, by mounting a fan separately from the wheel and directing its current on the drum at different angles.
Data derived from the tests and presented in tables and charts lead, among others, to the conclusions that (a) “burned beads” cannot result from internal heat due to tire flexing but must be attributed to brake-drum heat; (b) excessive flexing due to under-inflation, heavy loading or high speed will accentuate the effect of brake-drum heat by decreasing the temperature gradient between the beads and the tread; (c) increase in the speed of rotation for a given brake-horsepower input increases the cooling properties of a wheel; (d) for the heat-dissipating devices tested, substantially the same relative improvement in cooling effect on all wheels was noted; (e) forced ventilation by external fans is indicated as an aid for dissipation of brake-drum heat, and the slower the speed of a motorcoach is, the greater should be the effect; (f) on the basis of constant brake-horsepower the wheels show a wider temperature difference than on the basis of constant brake-drum temperature but in the same order; and (g) comparison on the basis of constant brake-horsepower more nearly reflects the relative cooling properties of the wheels in actual service.