A New SAE Practice for Brake Natural Frequency and Damping Measurements by Eric Frank

As the world of Brake Squeal analysis continues to turn, it poses some interesting problems relative to the field of Noise and Vibration analysis. It is one of the few phenomena that is relatively unmapped, or more to the point, unmappable. This sound, a result of the contact between pad and rotor, can be difficult to isolate, repeat and identify. Brake Squeal has evolved from a natural occurrence to a subjective predictor of brake pad quality. It is for these reasons that a clear test specification is essential to compare like parts within each company and across the industry. Although one currently exists, it is clear that some refinement is needed.

Though some mystery still lies within Brake Squeal, it is clear that the structural characteristics of the pad and rotor are the key contributors to predicting objectionable noise. Since this paper focuses on the pad itself, the modal analysis and damping were chosen as criteria. The paper's authors began by contacting 12 companies and asking them to identify the first three bending modes and damping for each of 9 brake pads (which were 3 different pads of varying type from 3 different manufacturers).

The participants were first asked to conduct this test using their normal intra-company procedure, based on the existing SAE spec. The initial results were widely scattered, as was expected. Although the test procedures were essentially the same (accelerometers on the brake pad, impact hammer), there was wide variation in the results for both modal identification and damping. These differences were attributed to variation in both measurements and analysis. After clarifying some of the test specifics, some of the participants were asked to repeat the test. There was some improvement in correlating the modal results, but there was clearly still some discrepancy. The analogous damping results were still uncorrelated. After making revisions to the proposed procedure (accelerometer mounting technique, mass loading requirements, window type) a subset of three of the original participants repeated the test. This yielded excellent correlation for both modal and damping results between participants.

Looking at the author's final results, which were based on a subset of three of the initial 12 participants, the question is begged: Will the outstanding correlation continue when applied to a larger population? The next rounds of testing will tell. Based on the variation of results from initial to final testing, it is clear that the current spec needed refinement. As the NVH community continues to advance and grow, it is essential to review these specifications. As the best procedures may have been used at the time of the specs birth, they can easily be outdated and passed up by newer, more efficient technologies. Care must be exercised to ensure that everyone's results are comparable. Even in the hands of experts, tools aren't always used in the same fashion.

Review of paper 2005-01-2315 - A New SAE Practice for Brake Natural Frequency and Damping Measurements, authored by Jim Thompson, Link Engineering Company and Eric Denys, Material Science Corporation.