of the slip or about whether symmetrical or non symmetrical slip behavior is present for example a slip indicator of this kind can be defined as follows thus here the slip indicator si is defined as the ratio of the amplitudes of various frequency portions f 1 and f 2 in the acceleration frequency spectrum one of these frequency portions preferably the frequency portion f 2 can correspond to the oscillation frequency f whereas for example the other frequency portion f 1 can correspond to an uneven multiple or to an even numbered multiple of the oscillation frequency f for example one could use three times the oscillation frequency as frequency portion f 1 using this example fig5 shows the progression of amplitude at the oscillation frequency f as a roughly dotted line and the progression of amplitude for three times the oscillation frequency f is shown as a finely dotted line plotted against the extent of capping of the amplitude of the acceleration quantity b here the capped amplitude corresponds to 1 0 times the slip free state in fig2 a whereas the decreasingly capped amplitude represents the state of increasing slip for the slip free state that is an uncapped amplitude with value 1 0 the amplitude at the oscillation frequency f is a maximum thus for example at normed value 1 0 with increasing capping that is with decreasingly capped amplitude of the acceleration quantity b the amplitude at frequency f also decreases in the acceleration frequency spectrum whereas the amplitude at three times the oscillation frequency thus at 30 hz increases accordingly the slip indicator represented by a solid.