The brake mechanism has an oval cuboid block (1) forming the brake ram engaging in a disc (2) having two grooves off-set by 180 degrees, or simply striking against two 180 degree off-set stops (22) on the chain wheel. It is then displaced as far as is necessary through a cable pulley to operate a brake. The brake ram can engage through a guide disc radially or axially in the associated back-pedal disc (2). The mechanism can be built in between the pedal bearing and chain wheel.
