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27.-BACK-PEDALLING RIM BRAKE (B.S.A.).
When this type of brake is fitted to the back stays, motion is transmitted from the clutch by means of bell cranks and rods to the top of the diagonal and thence down the back stays.
B.-BRAKES WITH CABLE TRANS- MISSION. 109. The Bowden Brakes. The operation of the Bowden wire may be explained in this way. It would be possible to transmit sufficient pull to actuate a brake on the back wheel by means of an ordinary wire conducted round the angles of the frame. But in that case unless the angles of the frame were furnished with suitable pulleys, the friction and consequent difficulty of using the brake would be very great; the wire would be liable to break at the angles, and would be exposed to rust. The Bowden cable is conducted in a conduit formed of fine wire twisted into a close spiral and protected from the weather by a waterproof composition.
This conduit, with the cable inside it, is flexible enough to be bent round the angles of the frame, yet a short arch of it, such as is found at such bends, is sufficiently rigid to serve without buckling as a fulcrum for the pull of the cable. In order to minimise friction, the conduit is taken round corners in easy curves, and is properly lubricated. Friction and consequent stiff- LEVER.
LEVER. ness in working and possible injury to the cable result when the conduit is taken in sharp curves or not properly lubricated. This result is often CYCLING] found in certain pseudo-Bowden brakes, in which a wire is taken through a metal tube in short, sharp curves. These brakes look neat, but are an intolerable nuisance in working.