TBR Technical Corner: Disc Thickness Variation Measurement under Operational Cold and Hot Brake Judder Conditions (Part 4 out of 4)

Source: Applus IDIADA

Article by: Bernat Ferrer, Project Manager, Braking Systems in Applus IDIADA

Part one can be read by clicking on this sentence.

Part two can be read by clicking on this sentence.

Part three can be read by clicking on this sentence.

ADELANTO, Calif. – This study reveals an advanced methodology and testing capability, rarely performed in the industry: the measurement of the brake disc thickness variation (DTV) on-vehicle, during real operational conditions, and its correlation with the brake judder subjective evaluation, is a clear improvement in the brake NVH field.

The brake-judder investigation has been introduced in part one, followed by the main results (parts two and three). This fourth and last part of the article presents the most important conclusions.


After a complete overview of the measurements taken on the brake disc during different test conditions, the outputs obtained and the analysis carried out led to some interesting conclusions. As this study considers one single brake set evaluated following a specific brake test procedure, the points presented are focused on this precise activity. Because of this specificity, they cannot be considered as absolute statements considering all the brake disc components varieties, but they will help to understand many common situations appearing in the brake automotive market.

The main points can be sorted according to the sections followed in this study:

  • DTV analysis on the different phases of a single brake application: the results showed a notable evolution of the rotor’s thickness variation for each specific phase related with the braking actuation. Three consecutive stages can be highlighted:
    • Braking phase: while the brakes are depressed (contact between pads and disc, with pressure applied) the friction dissipates part of its energy through the disc. The results clearly show this component is affected by this, both in temperature (usual increase during a brake application) and shape (by increasing the thickness variation). This variation also depends on the initial rotor contour, which it evolves from.
    • After braking, still while heating: the immediate phase after the brake application, following the release of the pressure in the system. At this stage, depending on the characteristics of the disc and its thermal capacity, the temperature still increases. This fact causes the DTV to stabilize or even increase, although there is no more pressure or braking actuation on the friction parts. This phase is considerably short with respect to the other two and may not influence the previous braking period, but it is still important as it is generating a thermo-mechanical effect on the rotor (which may affect the following stages).
    • Cooling phase: after reaching the maximum peak, the temperature starts to decrease. At that moment the thickness variation also descends, recovering little by little the initial shape of the disc. The relationship between temperature and DTV shows a hysteric loop, but more significant is the almost complete DTV and shape recovery once the temperature cools down to the starting point. This fact demonstrates why the DTV measurements done in quasi-static conditions (in the workshop with the vehicle lifted on the elevator) usually match before and after a test, providing very little information about the transient behaviour occurred during the testing itself. This can lead to confusing or wrong conclusions when trying to determine the real sources of the occurrences during the testing phase.
  • DTV analysis on a repetitive and hot brake cycle: the results in this section cover a wider range of points and interactions to be analysed. The following areas of analysis were studied:  
    • Thermal interaction: the results on this test show a similar effect to the one described. The difference comes with the cooling phase, which is too short between each brake snub of the hot cycle to allow the temperature (and so the DTV) to recover its initial condition. Therefore, there is a general and constant trend of growth affecting the temperature and DTV, reaching considerably high values in both items.
    • Subjective evaluation: the final goal of all this exercise is to understand the final vehicle users (driver and occupants) perception and the source of the vibrations appearing during the brake actuations. The results in this area show a clear relation between this parameter and the DTV evolution, with low ratings for high values of disc thickness variation. This statement is commonly correlated on pre-treated discs with a certain DTV initially settled or with previously generated DTV during off-braking exercises, but it is almost never assessed on hot brake cycles. The results reveal that this relation also appears throughout this transient deformation, and the link between the rotor’s shape and the driver’s subjective perception is still valid.
    • Objective vibration: similar to the subjective assessment (brake judder), the objective characterization of the vibration excitation also matched the brake DTV input. The results reflect a reaction of the accelerometer sensors placed on the vehicle, coinciding with the peaks of amplitude on the rotor. This demonstrates again (objectively) the real impact of the disc shape on the overall vehicle response.
    • Orders and shape interaction: a final step of relation was shown with the disc contour and order breakdown. This item provides important information on the disc thermo-mechanical characteristics and its behaviour under operational conditions. The DTV order breakdown and its evolution during the complete test helps to objectivise this fact, and offers the possibility of involving the responsibility of the component design team, taking part in the evaluation of the results and re-adjustment of their strategy if necessary.

This study presents important and revealing information about brake disc performance, acting as a main character of the brake judder test and source of its vibration excitation also during hot brake cycles. The valuable information obtained during this exercise highlights the importance of investing in this measurement, which not only has to be consciously well assembled but also accurately analysed. All in all, it becomes a strong tool for real and operational measurements, effective for problem solving activities and useful for validating component design developments.

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, www.applusidiada.com, is located in California and Michigan, with further presence in 25 other countries, primarily in Europe and Asia.

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