As one of the brake-induced noises and vibrations, hot judder is forced vibration, which is caused by unevenness of the brake disc due to the thermal mechanical interactions in wheel brakes. Brake disc unevenness is normally described and evaluated as the Disc Thickness Variation (DTV) and the disc’ Lateral Run-Out (LRO). DTV and LRO gener-ate Brake Pressure Variation (BPV) and Brake Torque Variation (BTV) in wheel brakes, which are transmitted to the driver and perceived by the driver as the brake pedal pulsa-tion, the steering wheel oscillation, the car body vibrations, and low-frequency drone noises inside a vehicle.

Hot judder is characterized by hot spots on the disc surfaces. The frequency of hot judder is dependent on the wheel rotational speed, showing order behaviors. The num-ber of hot spots generally corresponds to the dominant order of hot judder. In the last decades, most of the hot judder tests have been carried out with brake dynamometers, and high numbers (typically around 10) of hot spots were found in the majority of the tests. The generation and development mechanisms of the high dominant order have been almost the exclusive focus of current hot judder researches. However, the influ-ences of the vibrations and noises (with higher frequencies compared with the low-order cold judder) caused by high-order hot judder on the driver’s subjective perception have been still not clarified. That is to say, it is still unknown in which form and under which conditions, the high-order hot judder can be transmitted to and perceived by the driver, and thus causing customer complaints.

A top-down approach is used in order to investigate the influences of high-order judder on driver’s perception with respect to two aspects: the incidence of high-order hot jud-der in vehicle tests and the drivers’ perception of high-order hot judder. The first aspect is mainly investigated by studying the transferability of dynamometer tests to vehicle tests and by identifying the incidence of high-order hot judder in production brakes. Specifically, identical brakes from one front brake and one rear brake are separately tested with a brake dynamometer and through vehicle tests by means of road tests and chassis dynamometer tests, and all brakes of four production passengers are identified with accelerometers attached on the brake caliper and the caliper bracket.

The perception of high-order hot judder is chiefly studied by investigating its transfer behaviors. Global transfer functions from BPV/BTV to the selected driver interface quantities (brake pedal pulsation, steering wheel oscillation, seat track vibrations, and vehicle interior drone noise) are defined, which establish the links between the hot judder intensity in the wheel brake and the intensity at the driver interface. In order to identify the transfer functions with a high signal-noise-ratio and better reproducibility, a novel testing method is adopted: vehicle tests with brake discs that are artificially modi-fied with the desired surface shapes simulating the high-order DTV/LRO. Altogether three vehicles with seven different modified discs are tested. Two critical levels of drone noise (60 and 80 dB(A)) are selected according to the general vehicle total noise level and the human’s perception characteristics of sound. The perception threshold values of the driver interface vibrations are obtained through regression analysis between their subjective ratings and objective measurements. Based on the critical drone noise levels and the threshold values of vibrations, as well as the global transfer functions, threshold values of BPV and BTV for perceiving the high-order judder are computed.

Concerning the incidence of high-order judder, both the dominant order and the thermal increases of BTV and BPV for the dominant order showed great discrepancies in differ-ent test types (e.g. brake dynamometer or vehicle, drag braking application with con-stant velocity and constant pressure/torque or stop braking application with decreasing velocity and constant deceleration/pressure). Hot judder behaviors in the brake dyna-mometer test were not transferable to the vehicle tests. Besides, no evident high-order hot judder has been identified in all the brakes of the four passenger cars. Generally, hot judder seems to be more likely to be excited at the brake dynamometer than in the vehi-cles. Therefore, brake dynamometer test is still appropriate for detecting hot judder in the early phase of brake development, and thus preventing its occurrence in the vehicle.

Regarding the driver’s perception, the drone noise is the most probable reason leading to customer complaints, since higher than 100 Hz less than 10 Nm BTV is required to perceive the drone noise and 50 Nm BTV can already result in unacceptable drone noise. The driver is less sensitive to the vibrations caused by high-order judder. Roughly at least 20 Nm BTV or 2.5 bar BPV is needed for perceiving the vibrations, and the perception is most possibly due to the steering wheel oscillation or the vertical vehicle vibration. Moreover, some resonances in the transfer paths play a significant role in the high-order judder transmission. Although the investigations in this work are limited to three vehicles, the practical significance of high-order hot judder on the driver’s percep-tion is revealed for the first time. With these results, the impact of the measured BTV and BPV of hot judder, e.g. in the brake dynamometer tests, can be assessed.

Combining these results, the most effective and efficient way to mitigate the high-order hot judder would be reducing its occurrences in the wheel brakes and diminishing the prominent resonances in the transfer paths.