Self energizing disc brakes

A self-energising disc brake comprising a housing, axially spaced first and second surfaces in said housing, radial pilot lugs in said housing, axially spaced first and second pressure plates centred on said pilot lugs, adjacent faces of said pressure plates, being provided with cooperating oppositely inclined angularly spaced recesses defined between end faces, balls located in complementary pairs of said recesses, said recesses having an angularly spaced relationship relative to each other to form a pitch circle of constant diameter, a first unitary rotatable friction member disposed between said first pressure plate and said first surface, a second unitary rotatable friction member disposed between said second pressure plate and said second surface, each of said friction members comprising a disc, and first and second linigs of friction material for engagement with the adjacent pressure plate and the adjacent surface, respectively, carried by opposite faces of said disc, and means for moving said pressure plates angularly relative to said housing to initiate application of said brake with said pressure plates also moving axially to urge the friction members into engagement with said first and second surfaces, respectively, due to a tendency for said balls to ride up said end faces, said pressure plates thereafter being carried round with said friction member to provide a servo action, wherein a braking face of at least said first lining of friction material of each of said friction members is separated into only two annular rings by only a single annular groove, and said groove is substantially coincident with said pitch circle of constant diameter to increase the degree of cooling in said recesses.