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Compressibility is a common quality metric for friction materials. In addition, it is typically used as an engineering parameter for brake system design and performance. Compressibility (or elastic properties) of the friction material can effect brake roughness, pedal feel, and noise performance. A characterization technique is presented to determine the cyclic compressibility (over ± 1 kN) as a function of preload, temperature, frequency and time. The initial motivation was related to modeling of brake roughness, but applications to pedal feel and brake noise are also explored. For a given semi-metallic material, changing the temperature from 20 to 300°C or the preload from 8 to 4 kN both halve the cyclic compressibility. Less significantly, a change in frequency from 20 to 1 Hz reduces the cyclic compressibility by 10%. Differences between linings are also considered.

A method for ultrasonic measurement of friction material elastic constants was evaluated through a round-robin process. The purpose of the study was two-fold: 1) to formulate and evaluate a standard test method (SAE J2725) for measuring elastic constants using ultrasound and 2) to quantify the measurement variability when the testing method is applied by multiple operators and instrumentation. The study involved measurement of 6 different friction materials by multiple operators at 5 different laboratories. All participants measured the same 6 samples. The friction material sample set ranged in density from 2.3 gm/cc to 3.2 gm/cc, the in-plane modulus varied from 11 GPa to 25 GPa, and the through-the-thickness modulus varied from 3 GPa to 7 GPa. All measurements were carried out at ambient temperature in accordance with procedures outlined in the draft SAE test specification (J2725).

North American drivers particularly dislike wheel dust (brake dust on their wheels). For some vehicle lines, customer surveys indicate that wheel dust is a significant concern. For this reason, Ford and its suppliers are investigating the root causes of brake dust and developing test procedures to detect wheel dust issues up-front. Intuitively, it would appear that more brake wear would lead to more wheel dust. To test this hypothesis, a gage was needed to quantitatively measure the wheel dust. Gages such as colorimeters were evaluated to measure the brightness (L*) of the wheel, which ranged from roughly 70-80% (clean) to 10-20% (very dirty). Gage R&R's and subjective ratings by a panel of 30 people were used to validate the wheel dust gages. A city traffic vehicle test and an urban dynamometer procedure were run to compare the level of wheel dust for 10 different lining types on the same vehicle.