The brake judder phenomenon is one of the main concerns in the automotive industry related to vehicle comfort and market complaints. Many factors contribute to the appearance of judder, the most common one being the brake disc thickness variation or uneven rotor shape deformation. However, this fact is usually difficult to illustrate, as the deformation takes place during the transient phase of braking and temperature evolution; the disc recovers its initial thickness once it cools down, and the measurement does not add valuable information about the specific braking period or cooling transition. Therefore, a study of the real disc behaviour during these phases is necessary to understand if any unexpected shape is appearing, which might be the final root cause of the excitation of judder vibration. Two different tests were applied: cold and hot brake judder. During the cold judder, the disc was always starting cold and at a controlled DTV, which corresponds to the disc’s natural shape and only affected by the wear conditions. This test configuration only considers the measurement during single snubs, with the corresponding heating and further cool down. On the other hand, hot judder was performed by repeating consecutive snubs, heating up the discs to a high temperature and assessing the roughness in those ranges. In this case, not only the DTV evolution during the stop is important, but also the change between each snub, according to the temperature increase. The study showed very revealing and interesting results regarding the disc shape evolution in real testing conditions. The DTV and the order breakdown of its curve are generated during the brake application and release phases, showing a clear deformation process due to the friction and heating. The cooling also shows a disc shape recovery, coming back to its initial condition once the temperature decreases. The vehicle judder assessment (subjective rating criteria) is also taken into account, being worst rated when the amplitude of the DTV is reaching higher values during the snubs; this helps to better characterize the vehicle roughness and its triggering caused by the disc shape fluctuation. This study only takes into consideration one vehicle sample, performed during a specific test. The study could be improved by comparing multiple samples and disc designs, in order to understand how they are evolving by means of the application of the brake. This study reveals an advanced methodology and testing capability, rarely performed in the industry. The measurement of the brake disc thickness variation on-vehicle, during real operational conditions, and its correlation with the brake judder subjective evaluation, is a clear improvement in the brake NVH field. 2 Brake judder appearance is a big concern for many OEM’s related with vehicle comfort. The characterization of this event, when caused by disc deformations, is a key factor when trying to identify possible countermeasures. Disc re-design, or other ways to control disc deformation, is one of the possible solutions to achieve complete vehicle vibration attenuation.
Bernat Ferrer, Applus IDIADA, Spain; J.J García-Bonito, Alberto Claverías, Laura Ortiz, Fabio Squadrani,, D.H. Jeon