TBR Technical Corner: Influence of Friction Material on Corrosion-Induced Brake Judder during the Removal of Rust (Part 3 out of 3)


Source: Applus IDIADA

This TBR Technical Corner is the final segment in a series of three articles on the impact that different friction materials have on corrosion-induced brake judder —both at system and vehicle levels— during the removal of rust by Narcis Molina, Project Manager, Braking Systems in Applus IDIADA.

The first part of this series can be viewed by clicking HERE.
The second part of this series can be viewed by clicking HERE.

As explained in the first and second parts of the article, this work presents a methodology that incorporates both dynamometer and vehicle tests into the investigation of the impact that different friction materials have on corrosion-induced brake judder —both at system and vehicle levels— during the removal of rust. In the first part, the problem and the methodology were introduced; in the second one, some results were presented.

Figure 13 illustrates the diminution of DTV throughout the cleaning test for set 2 (NAO; dynamometer). Indeed, the post-corrosion condition (black, solid line) covers the complete chart; most of the corroded material is easily consumed during the first stop, experiencing no noticeable change from the third snub onwards.

The abrupt thickness change between 130 and 190 deg. corresponds to the brake pad “footprint;” as seen in  Figure 1d for set 1 (LS; dynamometer), the disc surface covered by the lining material can be easily identified —although partially oxidized due to the use of spacers in the friction interface to simulate the caliper rollback.

This DTV shape leads to a dominant 1st order vibration during the first cleaning snub.

DTV is usually expressed in terms of peak-to-peak value. Resuming the analysis of the vehicle test (sets 3 and 4), Figure 14 suggests that an acceptable correlation can be found between the peak-to-peak DTVs and the subjective assessment. Note that, due to the complexity of measuring DTV during the actual test, very few points have been gathered —which proves to be insufficient to guarantee a proper correlation level.

The intrinsic characteristics of a brake dynamometer allow the DTV to be monitored more frequently —as detailed, again, in Table 9. Therefore, the statistical population can be enhanced by considering the objective data —expressed as BPV and BTV in terms of overall (RMS) level— rather than the vehicle-restricted subjective evaluation. These results are represented in Figure 15, which suggests that both braking pressure and torque fluctuations are proportional to the DTV and, consequently, to the amount of oxidized material.

The generation of corrosion is strongly dependent on the friction film deposited on the disc surface during burnishing and, consequently, is directly related to the lining material composition. This disparity, reflected by the differences found in terms of DTV and the subsequent vibration response between the different sets, can be counteracted thanks to the revealed linear relationship. 

Figure 16 depicts both BPV and BTV, where both fluctuations have been normalized by means of the peak-to-peak DTV values —they are expressed, therefore, in bar/mm and Nm/mm, respectively. Like this, the impact that the DTV level —i.e. the thickness of the oxide layer— has on the vibrational behavior can be neutralized. Sets 3 and 4 (vehicle), despite the reduced statistical population —due to the limited number of DTVs—, exhibit a very similar evolution. Hence, it can be stated that low steel pads are more sensitive to any given corrosion-forced DTV input, thus leading to a larger vibration response.

The rust removal rate can be indirectly estimated by evaluating the evolution of the DTV:

Where DTV0 is the original (pre-test) peak-to-peak DTV value and DTVn corresponds to the peak-to-peak DTV value after the n snub.

This approach assumes that no other geometrical irregularities —such as wear— are introduced throughout the corrosion-cleaning test.

Table 11 summarizes the DTV evolution for all four sets. Note that sets 3 and 4 (vehicle testing) only contain information about the measurements conducted after 10, 20 and 30 snubs. This sequence proves inaccurate, since most of the deposited material is removed over the first stops. However, Table 11 suggests that no friction material stands out over the other, thus exhibiting a similar corrosion-cleaning rate.

Conclusions

The principal conclusions that can be inferred from this study are:

  • Presented dynamometer and vehicle procedures have proved to be appropriate to understanding the impact that corrosion-induced vibrations have, respectively, at system and vehicle levels.
  • Dynamometer- and vehicle-based methodologies have been directly benchmarked. Common trends have been identified throughout the removal of rust, thus confirming the suitability of using a single-end dynamometer in the localized investigation of braking-induced, forced vibrations such as judder.
  • A salt spray chamber has been employed to artificially form the oxide layer under steady-state climate conditions. The aim of using an environmental chamber is not to reproduce an equivalent in-vehicle disc corrosion, but to guarantee a robust and repetitive procedure, thus allowing a quick but trustworthy comparison between the different tested samples.
  • It has been confirmed that the thickness of the oxide layer is primarily affected by the ingredients that make up the friction material, which are deposited on the rotor surface during the process of burnishing.
  • The overall vibration response of the brake assembly —be it in the form of pressure or torque fluctuations, or longitudinal caliper acceleration— has proved to be linearly proportional to the DTV and, consequently, to the thickness of the corroded layer.
  • Both low steel and NAO materials succeed in reducing the corrosion-induced brake judder vibrations during the reduced cleaning test sequence composed of 30 snubs. Furthermore, they both present a similar rust removal rate. Nonetheless, it has been found that the steel-containing friction material is prone to generating larger vibration amplitudes for any given oxide layer thickness.

About Applus IDIADA

With more than 25 years’ experience and 2,450 engineers specializing in vehicle development, Applus IDIADA is a leading engineering company providing design, testing, engineering, and homologation services to the automotive industry worldwide.

Applus IDIADA is located in California and Michigan, with further presence in 25 other countries, mainly in Europe and Asia.

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