Source: Applus IDIADA
This TBR Technical Corner is the first 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.
ADELANTO, Calif. –Disc corrosion has emerged as an important field of study within the brake community due to the diverse friction materials that are currently used in each market, and their inherent differences in terms of rust cleaning.
This article 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.
Brake judder is a braking-induced, forced vibration whose frequency is directly proportional to the wheel angular velocity and, consequently, to the vehicle speed. Cold judder is commonly thought to result from geometrical deviations such as disc thickness variation (DTV) or disc lateral run-out (LRO), which can be caused by uneven lining wear, discontinuous friction film generation between rotor and lining, imperfect rotor machining or uneven corrosion, among other reasons. Note that there might be strong couplings between the different irregularities; furthermore, these originally-static (i.e. cold) deviations may induce uneven temperature and pressure fields, leading to dynamic variations of the disc geometry (e.g. temporary DTV growth) associated with hot judder. This paper focuses on the braking-induced vibrations that are caused by the oxide layers irregularly generated on the cast iron disc surfaces as a result of corrosion.
Numerous authors have explored the propensity to stiction, i.e. corrosive adhesion, by evaluating the influence of different disc/pad chemical and mechanical properties —such as pad porosity and hydrophilicity, friction material acidity and surface topography, disc microstructure or friction material formulation . Some essays have investigated the effect of corrosion on the tribological performance of the sliding interface by analyzing the disc or the lining material composition. Other papers have studied the wear propensity of previously-corroded discs. However, fewer investigations have addressed the impact of corrosion on vibrational instabilities such as stick-slip or judder, which fundamentally affect the driver’s comfort during braking —although, if judder, can lead to erratic reactions by an inexperienced customer.
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This article is intended to study the effect that different friction materials have on corrosion-induced brake judder as the oxide film is removed during the early cleaning stages.
This work involves both vehicle and dynamometer tests. Two different commercial brake pads are used in the investigation: a low steel (LS) type and a non-asbestos organic (NAO) type, whose steel contents inherently differ; their exact composition, however, cannot be disclosed. Table 1 summarizes the distribution of the material sets.
Table 2 lists the main specifications of the test vehicle, a crossover SUV:
Similar test conditions are reproduced in a single-end brake dynamometer; its configuration is detailed in Table 3:
The vehicle and the corner fixture incorporate the same front brake assembly —note that the rear axle is not investigated—, whose principal specifications are found in Table 4:
Firstly, originally-green brake discs are burnished in order to allow a uniform friction layer (transfer film) deposition on the grey cast iron surface (Figure 1a). Table 5 summarizes the bedding procedure for the vehicle and dynamometer tests:
Disc corrosion can be artificially generated by means of multiple techniques. This study uses an environmental chamber that monitors both the temperature and the relative humidity, thus adhering to the recommendations stipulated in ASTM B117. The specimens are placed in the salt spray chamber (Figure 1b) and then exposed to a continuous, indirect spray of (five percent salt) water solution —also referred to as fog or mist. The chamber climate is maintained under constant steady-state conditions, i.e. at +35 ºC and a controlled relative humidity of 95 percent. The total test duration is 72h. To promote a uniform and even material deposition (Figure 1c), discs are placed at ±30º from the vertical axis throughout 36h, and then symmetrically rotated for the remaining 36h.
Note that spacers are located in the friction interface in order to simulate the caliper rollback —the rear pads (not considered in the present study) would be effectively clamped to recreate the parking condition. Thus, the surface covered by the brake pad is partially oxidized (Figure 1d) —as opposite to other works, where the area of the friction material presents no rust whatsoever. In those cases, the ‘footprint’ of the pad guarantees a prominent torque variation because of the abrupt thickness change and the inherent friction differences between the corrosion-free area covered by the pad and the remaining rusted surface.
As listed in Table 6, the vehicle is instrumented with a complete array of sensors that allow the corrosion-induced vibration to be characterised.
The acquisition system of the brake dynamometer, on the other hand, inherently captures the rotational speed, the hydraulic pressure and the braking torque. A rubbing thermocouple is also installed in order to track the disc temperature.
Table 7 indicates the test sequence that is conducted to remove the oxide layers from the cast iron disc surfaces.
The procedure is limited to 30 snubs given that the current study is aimed at investigating vehicle judder during the early corrosion-cleaning stages, thus setting it apart from other works that examine the long-term impact that the oxide layer has on the vibration response of the braking system.
Regarding the vehicle test, an expert driver subjectively evaluates the rust-induced vibrations —i.e. the steering wheel shake and nibble, the brake pedal pulsation and the floor pan vibration. The subjective grades range from 1 to 10, as per the criteria shown in Table 10.
The thickness of the oxide layer is monitored by using a couple of non-contacting capacitive sensors; they measure the distance between the probe itself and each disc rusted face along the circumferential direction. The subtraction of both signals allows the calculation of the variation in disc thickness, which —as shown in Figure 2— is examined at a single radius, the center of the friction ring. It is important to emphasise that the resulting magnitude contains information about the geometrical irregularities of the disc —due to the cast iron machining, uneven wear, etc.—, as well as the roughness caused by the oxide layer deposition.
During the dynamometer test, static DTV measurements are conducted prior to the cleaning test itself —i.e. after submitting the discs to the corrosive environment—, and then after every snub during the first 5 stops. The gap between measurements is then extended, recording the DTV after 10, 20 and 30 stops. This measurement sequence (Table 9) focuses on the early
stages of the cleaning procedure since most of the deposited material is expected to be removed over the initial snubs. However, for the vehicle test, the DTV sequence needs to be shortened —4 measurements: pre-test, post- 10, 20 and 30 stops— since each DTV check involves pausing the test in order to return the vehicle to the workshop.
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.