outermost edge of the respective radially outward protrusion 21 and the dale represented by the bottom 11 a of the respective outer recess 11 this effective circumferential length l of each of the outer recesses 11 is of a value preferably within the range of 0 30 to 1 40 times and more preferably within the range of 0 60 to 1 30 times the maximum radial width fm between the outermost peripheral edge 9 a and the innermost peripheral edge 9 b in the illustrated embodiment however the effective circumferential length l is chosen to be 1 0 times the maximum radial width fin i e of a value equal to the maximum radial width fm thus the bottom 11 a of each outer recess 11 is of an arcuate shape occupying a portion of the circle concentric with the axis of rotation of the brake disc 2 and is positioned radially inwardly from the outermost peripheral edge 9 a similarly each inner recess 12 has a depth h defined between the bottom 12 a thereof and the innermost peripheral edge 9 b which depth h is chosen to be preferably within the range of 0 15 to 0 25 times and more preferably within the range of 0 17 to 0 23 times the maximum radial width fm of each braking surface 9 as is the case with the depth e of each outer recesses 11 in the illustrated embodiment however the depth h of each of the inner recesses 12 is chosen to be 0 20 times the maximum radial width fin it is accordingly clear that the bottom 12 a of each of the inner recesses 12 is positioned radially outwardly from the innermost peripheral edge 9 b in the brake disc 2.