Source: Applus IDIADA
Article by: Bernat Ferrer, Project Manager, Braking Systems in Applus IDIADA
Brake judder appearance is a big concern for many OEMs related with vehicle comfort. The characterization of this event, when caused by disc deformations, is a key factor when trying to identify possible countermeasures.
In the first part of this article, the problem was introduced and the testing methodology, described. This second part, therefore, already deals with some of the main results.
Results under an operational braking application:
After obtaining a correct validation of the measuring process, the results of this study will show the real behaviour of the disc under the different braking scenarios; not only at a cold brake temperature condition but also under more severe heating cycles where the thermal stress can cause different deformations on the component.
The very first result is the obtaining of the evolution of the disc thickness variation during a brake event; that is, to see how it is evolving in the different phases where the disc receives physical and thermal stress:
- Braking event: friction parts in contact with pressure acting on both sides of the disc by means of the pads.
- Immediate post-braking phase: without brake pressure applied, but still in the transient period where the temperature of the rotor keeps increasing, due to its thermal capacity.
- Cooling phase: decrease of temperature after reaching the peak obtained in the previous phase. The disc dissipates its thermal energy through the ambient air and by the ventilation obtained while rolling the vehicle.
This exercise is also valid for real judder application tests, such as the cold brake judder, where the initial temperature of the brakes is controlled under a maximum target before each application and a period for cooling between each snub is necessary. With these conditions, the disc under study already experienced a clear fluctuation in its shape and thickness variation, being affected by the temperature rise and pressure applied:
The figure above illustrates the behaviour of the disc thickness variation during a braking cycle; the rotor contour changes increasing the DTV up to a 46 percent of its initial shape during the brake application (phase #1). In the next stage (phase #2), even though the friction parts are not compressed any more (as no brake pressure is appearing), the disc is still warm and heating up, while the DTV keeps almost stable. In the last part of the cycle (phase #3), the temperature starts cooling down, and the rotor recovering smoothly its initial shape again.
The relation between these two parameters, DTV and disc temperature, is clearly seen in the graph below. The evolution reproduces a quite constant increase and decrease of the thickness variation during the heating and cooling phases, with some hysteresis appearing in the component deformation and therefore in the DTV values. However, its starting and final values are very similar once the temperature recovers its initial state.
This behaviour is key to understanding the phenomenon affecting next hot judder evaluation test. The usual test sequence is to measure the rotor’s DTV before and after the test performance. This locates the scenario in the very left hand part of the graph of Figure 5 above, providing absolutely no information about the real situation happening during the next phases. As a consequence, sometimes it is difficult to understand vehicle’s change of behaviour during the heating cycle periods (hot judder), without any objective justification or demonstration. In the following steps of this study, a focus on the disc shape and its deformation will be done during a hot brake judder test, emphasizing the influence of this item on the final vibration detected on the vehicle and perceived by the driver.
Results under an operational hot brake judder test (thermal effect):
The temperature profile takes additional importance when considering a hot brake judder evaluation (in this case, additionally, at high speed). This test consists of simulating a real situation such as driving at fast speeds on highways, with slow and continuous brake applications to control the traffic fluctuations, increasing the temperature through the repetitive applications. This process is sometimes translated to a vibration phenomenon appearing on some parts of the vehicle, usually on the brake pedal, steering wheel or complete vehicle body. This activity is then reproduced in the proving ground, by frequently repeated cycles of constant deceleration or pressure snubs at high speed, evaluating the vibration felt and its evolution during the whole test. The vibration, usually stays hidden when performing the very first brake applications (while the brakes are still cold), and appears at a certain phase of the test when the brakes get warm.
Figure 6 shows the test cycle example of a hot brake judder test evaluation, with constant brake applications of 2 m/s2 between speeds of 150 km/h and 80 km/h. The time between each snub is too short and makes it not possible to cool the brakes down again at the same initial value, so the temperature profile keeps increasing up to considerable high levels.
Looking now at the disc mechanical deformation behaviour, its evolution also seems to follow the same criteria with respect to the DTV, with an increasing trend during the thermal phase and a shape recovering again after the cool down:
The graph above reflects a similar behaviour as the one showed on the cold brake analysis, with a direct reaction up and down of the DTV response when each brake application is released. However, in this case the time between each snub is too low to allow its shape recovery, so the initial DTV is higher than the previous at every start of braking. This situation puts the disc (and therefore the whole vehicle) in a complete new condition at the beginning of every single brake snub, making possible the appearance of unexpected or unknown excitations during the hot judder test cycle.
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With over 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.
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