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Road testing of friction materials is a complex, time consuming and expensive activity. This paper describes the collection of braking data from a test vehicle having four wheel disc brakes and development of a brake dynamometer simulation test for the Detroit Traffic Road Test. Several dual-ended brake dynamometer simulation tests were conducted in a programmed, unattended mode over weekends where about 1600 road test miles were accumulated each weekend. Feasibility of using dynamometer simulation for road tests was shown by obtaining a correlation in the disc pad wear in both vehicle and dynamometer tests. Temperature effects on wear were seen in both the vehicle and dynamometer tests. Therefore, disc pad temperatures must be closely monitored or controlled during the Detroit Traffic Road Test or its simulation and the effects of temperature must be included in determining disc pad life or to obtain a meaningful comparison between two friction materials.

Lower temperatures at the sliding interface of a disc brake will reduce thermal wear and fade tendencies of friction materials. One method for achieving these lower temperatures is to improve the thermal design of gray cast iron discs. The purpose of this investigation was to study design improvements of brake discs for trucks. To accomplish this objective, an analytical thermal model was developed. The model employed the lumped parameter approach, in which the disc was subdivided into a number of small volumes. The model specifically simulated disc temperature response during 80.5 km/h (50 mph) fade tests performed on a dynamometer. The thermal model was correlated with test data to verify and improve its accuracy, and then utilized to evaluate effects of geometry changes. Results showed that mass concentration in the disc faces yields lower temperatures at the friction interface through 10 successive snubs.

Commercial “organic” friction materials were obtained from three different manufacturers, and were evaluated for their frictional properties. In all cases, the friction force (F) was found to be a power function of the normal load (P) and sliding speed (V) at a fixed temperature, F = K·Pa·Vb at T1, where K is the coefficient of friction which is constant and independent of the load and speed, and a and b are one set of parameters at the temperature T1. Usually, the exponent a ranges 0.80-1.25 and b from -0.25 to +0.25, depending upon the temperature. Thus, brake torque becomes a power function rather than a linear function of the line pressure. Brake fade is found to be governed by the three mechanisms-load fade, speed fade, and temperature fade.

Past work has shown that brake discs constructed of a copper alloy containing 1% chromium can significantly reduce temperatures at the sliding interface. The purpose of this investigation was to demonstrate this fact further, and specifically to determine how copper discs should be configured to provide desired temperature response with a minimum amount of material. To accomplish this objective, an analytical thermal model was developed of a disc design for heavy trucks. The model employed the finite difference approach, in which the disc was subdivided into a number of small volumes. The model specifically simulated disc temperature response during 50 mph fade tests performed on a dynamometer. The thermal model was correlated with test data to verify and improve its accuracy, and then utilized to evaluate the effects of material and geometry changes.

The effects of surface roughness of brake drums on coefficient of friction (brake torque) and lining wear were investigated using typical commercial grey cast iron drums and typical commercial linings. Sample and inertia dynamometers were used. The surface roughness is found to significantly affect the coefficient of friction (brake torque) and lining wear, though the effect is dependent upon the lining compositions. The significance and implications of these findings are discussed.

Automotive friction materials are composites containing three kinds of components: an organic binder, fiber for reinforcement, and property modifiers. At low braking temperatures, the wear rate of the friction materials is controlled primarily by abrasive and adhesive mechanisms. At higher braking temperatures, the wear rate increases exponentially with increasing temperature due to thermal degradation of the binder and other components, and the exponential wear rate is frequently accompanied by brake fade. Thus, one method of reducing thermal wear and fade tendency is to lower the temperature at the rotor/friction material interface. Since the rate of heat transfer from the interface is mostly dependent upon the conductive and convective modes, a rotor of high thermal conductivity will have a significant advantage over a rotor of low conductivity, if the heat capacity remains the same.

The friction and wear characteristics of automotive friction materials are strongly dependent on the composition and microstructure of the rotor surface. In this study we investigated the compositional and microstructural changes occurring in the surface layers of cast iron brake rotors during dynamometer tests with a typical organic friction material. Rotors were studied in the as-manufactured, lightly ground and sanded, and as-burnished conditions, as well as after 30 stops from 60 mph at a deceleration rate of 15 ft/s2. Optical and scanning electron microscopes were used to examine the surfaces. Minimum disturbance of the microstructure was found in the sanded surface, but the as-manufactured and burnished surfaces exhibited considerable disturbance. After the 30 stops the pearlite was transformed locally into martensite. Composition analysis of the burnished rotor surface showed high magnesium content.

A drag dynamometer was used to investigate the influence of rotor properties on the wear of automotive brake linings. The effects on lining wear of temperature, surface roughness, thermal conductivity, microstructure and composition were studied quantitatively, and the mechanisms governing lining wear were elucidated. Lining wear at high temperatures increases exponentially with increasing temperature, and decreases exponentially with increasing thermal conductivity of the rotor. The wear increases parabolically with increasing surface roughness of the rotor. Also, coupling of a lining with a rotor having compatible composition and microstructure is very important for controlling the lining wear.

A drag dynamometer was used to evaluate the performance of automotive brake drums made from four kinds of materials with different thermal conductivities. In the order of decreasing thermal conductivity they are chromium copper, aluminum/cast iron composite, cast iron, and nickel-aluminum bronze. All of the drums were of the standard configuration used in SAE J 661a, or closely approximated it. The drums were run in conjunction with three types of lining materials: nonabrasive, moderately abrasive, and highly abrasive. Temperatures near the lining/drum interface, coefficients of friction, and lining wear were measured and compared. For a given amount of work done, the temperature near the drum surface was found to be lowest for the chromium copper drums, with progressively higher temperatures in the aluminum/cast iron composite, nickel-aluminum bronze, and cast iron drums. Relative lining wear and coefficient of friction varied with the type of lining tested.