Mr. Narcís Molina Montasell, Applus IDIADA, SPAIN
Dr. Juan Jesús García Bonito, Applus IDIADA, SPAIN
Mr. Amadeu Martorell Branchat, Applus IDIADA, SPAIN
Ing. Fabio Squadrani, Applus IDIADA, SPAIN
Research and /or Engineering Questions/Objective:
Brake creep groan and brake squeal are very distinct vibration and noise problems exhibited by brakes under certain braking conditions. Creep Groan is a low frequency (< 100 Hz) vibration induced by a stick-slip condition associated with the friction/velocity characteristic of the friction material. Creep groan can evolve into a resonant condition when a dominant suspension resonance is excited. This situation can induce high vibration levels both in the suspension and in the vehicle body, producing vibration annoyance.
On the other hand, squeal noise is a phenomenon that occurs at high frequencies (>1 kHz) and involves complex modes of the brakes, such as flexural vibrations of the brake disc. The excitation of the disc modes can be caused by the variability in the friction force due to surface roughness and to the bending modes of the pads. Squeal noise transmission is basically airborne. In general, creep groan and squeal are studied as independent events with no mutual interaction.
This work presents an investigation of how these two distinct brake phenomena can be related to each other during the operational use of a disc brake, as they can appear concatenated in time.
The vehicle under study is a medium-duty commercial vehicle equipped with an air disc brake system. The creep groan and the squeal dynamics occurring at the front axle are characterized in operational conditions by means of 14 tri-axial accelerometers on the brake caliper and 28 on the vehicle front axle. This experimental set up allows the time and frequency analysis of the evolution of the creep groan triggering, charging, and its unstable stage and the effect that the latter event might have on the activation of squeal generation. The analysis uses operational vibration data of the suspension and the brake system of the vehicle under creep groan and squeal conditions in combination with vibration measurements of the brake assembly only in a brake-dyno under squeal conditions.
This study confirms previous findings that show that the development of creep groan exhibits three stages: the triggering phase, the charging phase and the unstable stage. It is shown that the unstable stage involves the axle beam, the leaf springs, both front wheels and their respective brake calipers. For the brake under study, it is shown that during the unstable creep groan phase, the brake caliper housing exhibits a high deformation that limits the correct control of the pads positioning and orientation with respect to the disc. This situation induces a high mobility of the pads with little position control. The results suggest that this lack of control in the correct positioning and support of the pads, caused by the unstable creep groan phase, can facilitate the generation of squeal.
Limitations of this study:
The number of tri-axial accelerometers used in this type of analysis is considerable and thus it requires using a multichannel system for efficient tests.
What does the paper offer that is new in the field in comparison to other works of the author:
The paper reviews and confirms the existence of three main phases in creep groan generation for discs brakes and introduces an interpretation of how the final unstable stage of creep groan, apart from the typical vibration annoyance associated to it, can also be a squeal inducer for some brake caliper designs.
This work shows that, for some disc brake caliper characteristics, the evolution stages found in operational brake creep groan, i.e., triggering, charging and instability phases, can occur concatenated with brake squeal. This association seems to be related to the fact that the unstable phase of operational creep groan produces high acceleration levels in the brake system that hinders the correct positioning and orientation control of the brake pads inside the caliper housing. This lack of control affects the duration of the unstable stage of creep groan and can promote the initiation of other noise problems, such as squeal.