Troy, Mich. — This is the fourth article in a series of four by Narcís Molina, Project Manager, Braking Systems in Applus IDIADA, about the relationship between brake creep groan and brake squeal in disc brakes.

Relationship of Brake Creep Groan and Squeal in Disc Brakes (Part 1 of 4)
Investigation of Relationship between Brake Creep Groan and Squeal in Disc Brakes (Part 2 of 4)
Relationship Between Brake Creep Groan and Squeal in Disc Brakes (Part 3 of 4)

This article investigates how two distinct brake phenomena such as squeal and creep groan can be related to each other during the operational use of a disc brake, as they appear concatenated in time.

The first part introduced the problem and the methodology; the second part presented the main results associated to creep groan, while the third article analyzed the observed displacements under operational groan. This fourth and last part presents some squeal-related results, and draws the main conclusions.

3.2. Squeal

Brake squeal is a sustained, high-frequency and friction-induced vibration. In the frequency domain, squeal is detected as one or more pure tones, between 1 and 18 kHz. As opposite to other low-frequency problems (e.g. groan), squeal is purely airborne; indeed, the driver perceives squeal as an acoustic annoyance.

Squeal is generally considered to be caused by the excitation of an unstable vibration mode of the brake system; excitation occurs at the friction interface due to a negative friction-velocity slope, the presence of doublet modes —modes of vibration that are close in terms of resonant frequencies, but present a spatial shift of 90°—, etc. There are other theories that could potentially explain the mechanisms to generate squeal, such as:

  • ‘Sprag-slip’, i.e. a geometrically-induced or kinematic-constraint instability that reaches a limit cycle.
  • ‘Hammering’ excitation induced by the uneven disc surface during its rotation –due to thermal distortion, wear, material deposition, etc. Noise is genereated by the action of the pad, when sliding against the hills and valleys of the disc surface.

3.2.1. Noise Characterization

The vehicle-based creep groan preliminary tests (Section 2.1.1) revealed that, under certain circumstances, the evolution of creep groan generation ended up with the appearance of squeal noise. Figure 27 shows, for the FL caliper accelerometer, interesting transitions from creep groan to squeal (and vice versa).

These results suggest that brake noises, despite being completely different in nature, can be linked in both directions transferring energy from one another. Further tests are conducted in order to understand the characteristics of this interaction.

Figure 27: Spectrogram of vibration of the FL caliper; Y-axes scale shows up to 15 kHz (top) and 200 Hz (bottom) in order to represent, respectively, the squeal and groan phenomena. Note the transitions from creep groan to squeal (and vice versa).

The worst-case scenario is found around 7.2 kHz, with peak noise levels of about 100 db(A).

3.2.2. Operational Deflection Shapes

Figure 28 depicts two frames of the ODS for the isolated brake assembly tested in the dynamometer. The squealing frequency is  8 kHz —note the frequency shift with respect to the vehicle-based test (Figure 27). Represented components include the caliper (housing and anchor), inner/outer pads and the pressure plate (yellow arrow —between the inner pad and the piston). From the visualization of the animation, it can be inferred that the leading edge of the inner pad (red arrows) has a high acceleration in a direction normal to the friction interface. This is presumably caused by a poor supportive action of the pressure plate.

Figure 28: ODS at 8 kHz, under squealing conditions. Shown components: caliper housing, caliper anchor, outer pad (left), inner pad (centre) and pressure plate (right, yellow arrows)

The animation also demonstrates that the pressure plate moves in anti-phase with the backplate of the inner pad, implying that both components impact in the premises of the leading edge.

These results suggest that the high mobility of the caliper housing is not only boosting creep groan occurrence, but also provides a noticeable poor pad positioning, which eventually leads to high frequency squeal.

3.2.3. EMA (Brake Parts, Free-Free)

Table 9 lists the natural modes of the inner and outer pads, along with the disc up to 12.5 kHz. Those closer to the squeal frequency, 7.2 kHz (vehicle test) / 8 kHz /dynamometer test), are identified in grey and illustrated in Figure 29.

Table 9: Natural modes of the inner and outer pads, and the disc
Figure 29: Closest natural modes to the squeal frequency for the inner pad (left), the outer pad (centre) and the disc (right)

4. Conclusion

This study confirms previous findings that show that the development of creep groan exhibits three stages: the triggering, the charging and the unstable stages (Table 10).

Table 10: Nature of the excitation over the three phases of groan

*Pad reorientation: sum of uncontrolled displacements and pitch.

The modal analysis and ODS (under groan conditions) of the front suspension and the brake are consistent in frequencies and shapes, as the creep groan progresses from the triggering to the unstable stage (see Figure 30). The main natural modes are:

  • 11.5 Hz. Deformation of the caliper housing with tangential, vertical and transversal displacements of the pads.
  • 17 Hz. Again, deformation of the caliper housing. Pads tend to translate vertically, remaining parallel to each other.
  • 23.5 Hz. In-plane, horizontal deformation of the caliper housing, causing a horizontal dragging of the pads.
  • 63 Hz. The caliper anchor follows the knuckle with good structural integrity, which at the same time rotates about the king pin axis. However, the central window of the caliper housing deforms, again dragging the pads —which rotate simultaneously about the horizontal and vertical axes.
Figure 30: Relationship between the ODS and the in-vehicle EMA for the creep groan investigation

At 63 Hz, under unstable groan, the caliper housing exhibits a high deformation. This situation induces a high mobility of the pads with little position control, which can facilitate the generation of the 7.2-8.0 kHz squeal as a combination of ‘sprag-slip’ and ‘hammering’. Since the pads move freely, they can form a suitable spragging angle that induces an out–of-plane diametral mode on the disc, which acts as a resonator to generate squeal. The fact that the pads also exhibit bending modes around 8 kHz (Figure 29) promotes the combination of ‘sprag-slip’ and ‘hammering’—the latest is forced by the leading edge of the inner pad when sliding against the disc surface. This ‘hammering’ effect is seen in Figure 28: both the inner pad and the supporting pressure plate move in anti-phase in the y-direction, normal to the disc surface. Consequently, the pressure plate seems to be a poor solution to guarantee a uniform pressure distribution on the pad at high frequencies.

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