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Mr. John Smith

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Brake judder is currently a hot topic for brake testing and investigation. In order to have a robust approach to judder investigation and problem solving both subjective and objective evaluations are needed. In this paper, a correlation study between subjective and objective results during a judder event is presented. A DTV sensitivity test is performed in order to have the biggest range of subjective ratings.

The vibration, at certain vehicle points, is first pre-processed using a series of human perception filters (defined in ISO2631-1 [1] and ISO5349-1 [2], depending on the part of the body excited). Next the impulsive root mean square (IRMS) of the vibration perceived by the driver is calculated for different window sizes. The judder phenomenon is finally quantified using different methods such as maximum peak or integration. The objective results are then compared with the subjective ratings, obtaining the coefficients of determination (R2) values.

In order to compare the subjective rating with the objective measurements a single value of acceleration must be calculated. This can be done in different ways. Four different calculation methods are used and presented in this study. The correlation between subjective rating and objective results are calculated using the coefficient of determination (R2). The correlation is generally very satisfactory for all the methods and window lengths. Three methods with higher correlations are identified. The window length of 1s shows the best correlations in general.

One of the reasons for the good correlation is the fact that the range of subjective ratings is high in DTV sensitivity tests. Subjective ratings only have a resolution of about 0.5 and range from 1 to 10. Trying to correlate tests with a limited range of subjective results might give worse correlations.

Judder subjective evaluation is very much dependent on skilled driver ratings. This study allows us to have a powerful tool for correlating the subjective marks with objective defined parameters, using human filters in order to improve the correlation. This allows us to increase the confidence in subjective ratings and setting up objective targets based on driver perceptions in the vehicle development phase.

Correlation between subjective and objective results reaches around 70% for the steering wheel and body vibration, and around 60% for the brake pedal vibration. One of the reasons for the good correlation is the fact that the range of subjective ratings is high in DTV sensitivity tests. Another reason for the high correlation values is that the human filters are similar to passband filters centered between 6Hz and 20Hz. Those are the usual frequencies of the first and second order rotation of the wheels.

TECHNICAL PAPER Judder is a phenomenon that normally occurs when braking at high speeds with low decelerations. It is felt by drivers as a vibration on the body, brake pedal and steering wheel and as a low frequency noise. It is normally caused by disc thickness variation (DTV).

Judder can be differentiated between cold and hot judder depending on the origin of the mentioned disc thickness variation (DTV) [3] [4]. Cold judder can appear when there is DTV at ambient temperature. DTV generation can occur, for example, when driving for long distances on motorways without braking. When performing a DTV sensitivity test, discs with different DTV are artificially created and subjective judder tests are performed. An array of accelerometers is located on the steering wheel, seat rail and brake pedal in order to objectively analyse the judder perceived by the driver. The DTV sensitivity test allows testing the same vehicle with different levels of judder, from good to low ratings.

Judder is subjectively rated in three different categories: steering wheel, brake pedal and body. The driver gives a rate from 1 to 10 for each category and snub. Score 10 means that the driver does not feel any judder at all; rate 1 is the lowest.

The human sensitivity to vibration depends on the frequency of the excitation and the part of the body feeling it. ISO2631-1 and ISO5349-1 describe the filters that are used in this study to weight the measured acceleration so that the human sensitivity of the affected body part is taken into account in this analysis. The impulsive root mean square of the filtered signal is then calculated. The IRMS gives the energy of the vibration felt during a short time window. Different window sizes are explored in order to find the best correlation between objective and subjective results. The window sizes used are based on the experience of driv