through the wear of the brake system this may be attributed to the fiberous structure of the frictional materials as contrasted to the non fiberous structure of metalic particles numerous alternative embodiments regarding the secondary frictional material are available several of these are shown by fig3 6 and 7 11 it has been found that only a small amount of the secondary frictional material need be exposed in order to generate an audible signal thus in fig3 a plurality of alternative segments of secondary frictional material 64 and primary frictional material 62 have been provided to make up the alternative brake disc 60 in fig4 another alternative brake disc 70 is equipped with circular discs of secondary braking material 74 adjacent to and surrounded by filler material 50 below the surface of the primary braking material 22 in either case a continuous flat ring of primary frictional material 22 covers the secondary frictional material as in the preferred embodiment shown in fig2 in fig6 a cross section of secondary frictional material discs of fig4 shows the core plate 18 a circular disc of secondary material 74 on each side of the core plate and then the primary frictional material 22 outermost on each side note that a filler 50 may surround the discs of secondary material this filler may be either the primary frictional material or an unspecified third type of consumable microporous frictional material also a similar filler may be.