as will be described further hereafter the tunnel wall 66 has a drive shaft aperture 82 through which the drive shaft 28 passes the aperture 82 being large enough to receive a sleeve 98 which extends axially from brake disc mounting hub 84 which will be described further hereafter positioned concentrically with the center of drive shaft aperture 82 and the axis of drive shaft 28 is an aperture 86 which passes through mounting member 70 so as to accommodate the sleeve 98 of hub 84 as best shown in fig4 and 13 a rigid circular brake disc 88 having a central axis 89 and obverse and reverse flat faces 90 and 92 respectively includes a central hub 84 and is rigidly attached to the hub 84 by any known means a plurality of rivots 94 being preferred radially extending channels 96 are cut into the disc 88 to eliminate undesirable heat distortion of the disc during braking the hub 84 has an outwardly extending sleeve 98 which extends axially from the disc face 90 in the direction of the tunnel 66 and is coaxial with the shaft 28 the apertures 86 and 82 of the mounting member 70 and the tunnel respectively are of sufficient diameter to permit easy passage of the sleeve 98 through the apertures and into the tunnel interior so as to minimize the amount of usable axial space occupied by the brake 10 the hub 84 is provided with an aperture 100.