TBR Technical Corner: Disc Thickness Variation Measurement under Operational Cold and Hot Brake Judder Conditions (Part 1 of 4)

Source: Applus IDIADA

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


The brake judder phenomenon is one of the main concerns in the automotive industry related to vehicle comfort and market complaints. Many factors contribute to the appearance of judder, the most common one being the brake-disc-thickness variation or uneven-rotor-shape deformation. However, this fact is usually difficult to illustrate, as the deformation takes place during the transient phase of braking and temperature evolution; the disc recovers its initial thickness once it cools down, and the measurement does not add valuable information about the specific braking period or cooling transition. Therefore, a study of the real disc behaviour during these phases is necessary to understand if any unexpected shape is appearing, which might be the final root cause of the excitation of judder vibration.

To understand better how brake disc shape and geometry can affect overall vehicle judder vibration is clearly reflected in the definition of “brake judder:” braking-induced, forced vibration whose frequency is directly proportional to the revolution speed of the wheel.

Vibration, in the 10-50 Hz or higher range, depends on the rotor rotational speed (and hence, on the vehicle speed). Indeed, the upper limit of the frequency is determined by the maximum vehicle speed, the wheel radius and the judder order: for instance, a passenger car braking from 180 km/h can have a 20th-order vibration frequency of 500 Hz, whereas the 1st-order vibration is only 25 Hz.

Related with the driver perception and what is received from the vehicle, the judder appears as vibration in the steering wheel, the brake pedal and the floor pan. In this study, all these locations were considered when referring to “Judder” vibration.

Figure 1: Points of driver evaluation during vehicle judder vibration.

When looking for the root causes of this vibration, the importance of the disc shape, and more specifically its surface layout, comes at the forefront. It directly affects the vibration characteristics (both amplitude and frequency), because of the friction and the materials involved in the braking action.

The temperature also becomes a key factor, presenting different scenarios of thermal deformation and therefore vibration. The hot brake judder test, covering consecutive snubs and heating up the brakes, is one of the clear examples where this transient deformation of the brake parts directly influences the behaviour of the complete vehicle.

In these hot conditions, the rotor thickness variation evolves in an uneven way. The rotor recovers its shape again once the temperature cools down, getting the same disc measurements as the ones obtained before the test, making it very difficult to understand the real causes intervening in this transitory period. That is why a real-time measurement during that stage is necessary to reveal the real disc behaviour causing the vibration.


In order to build a trustable and robust measurement methodology, the necessary tools and equipment is needed. The instrumentation used for this activity corresponds to the common capacitive sensors, same variant as the ones employed for the quasi-static measurements (with the vehicle on the lift), but with higher range. This allows them to be placed slightly further from the rotor surface, avoiding possible interferences during the driving manoeuvres and road irregularities.

Figure 2: Capacitive sensors placement with respect to the brake disc surfaces.

As shown in Figure 2 above, the distances d1 and d2 represent the free gap available for the brake disc run-out oscillation. During the tests described in this study, 1mm distance at d1 and d2 were placed to avoid any interference between components, without any real contact happening at any moment of the test.

On the other hand, the bracket is a second key factor in the robust measurement. In order to fit with the dimensions of vehicle knuckle and hub, a specific holding bracket needs to be designed and built. In addition to this, the bracket has to be designed with enough stiffness to transmit the signal without modifying or amplify the vibrations coming from the vehicle. This verification is made through a modal analysis of the bracket itself, making sure that the relevant natural modes are above the frequencies of the current judder and road ones.

Another step performed to validate the complete process is the comparison measurement check with the standard sensors and quasi-static data. That is, to compare the original shape of the disc obtained on the bench (or with the vehicle static), with the one generated in operational, rolling on the road and at different vehicle speeds. This validation is not only representative of the correct sensors signal reading, but also reflects the appropriate mounting and stiffness of the bracket, even at very high speeds.

Figure 3: Comparison between the usual sensors mounted for quasi-static measurements and operational dynamic ones at different vehicle speeds. Example of a disc sample with 20µm of initial DTV

About Applus IDIADA

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.

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

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