ACTUATION OF REAR STIRRUP and RIM BRAKES
Bicycles in Asia and Africa use the old English 'stirrup' brake. The stirrup is doubly supported as it pulls radially against the truest surface of the 'Westwood' rim to be very rugged, yet as cheap and light as the less strong cantilever rim brakes. The rod pull from the handlebar roller lever to the front stirrup is direct and strong to match. However, a rear brake is necessary to counter buckling on front braking in a turn, and especially the inadequate friction of a single brake on a wet rim. Here the rod linkage from the handlebars is fairly tortuous and sluggish, and it ties the second and usually weaker hand to the handlebars, precluding carrying anything in either hand, and strictly even signalling.
Such disadvantages are overcome at some expense in European and American 1-3 speeds by an internal 'coaster' brake in the rear hub actuated by reversal of the chain motion by backpedalling. This 1898 invention has proved so strong, weatherproof and reliable that a front brake is usually dispensed with altogether (illegal in the U.K.) Foot-controlled braking further allows people with partial hand disabilities or weaknesses to cycle in safety. One slight danger has always been that the cranks could be vertical at their 'dead centre' when an emergency arises and then immediate reflexive backpedalling gives much less braking force than normal. Coasterbrakes have now been built into heavy multispped planetary gearhubs, but for the most common light and simple derailleur gearing as well as the original stirrup there is no backpedal brake made.
So for cheap but effective braking of all but fixed wheel bikes, a simple backpedal brake actuation was developed. Just as the closeness of the hands to the front rim is exploited in the retained front stirrup, the pedals are conveniently close to the rear rim, and furthermore tap the most powerful muscles of the body. This improved actuation involves only two moving parts to be much easier and cheaper to manufacture, as well as much more powerful. Should backpedalling begin at dead centre, engagement is delayed until the pedals become sufficiently horizontal for good torque application, an improvement on the far more complicated and altogether less powerful coaster hub brakes.
The only penalty of this great simplicity is the minor inconvenience of having manually to reverse the pawl before walking the bike backwards any distance; and then to reset it before pedalling is again possible. Also the rear wheel may lock if the chain is too tight and the brake grossly over-applied to skid, meaning inconvenience in beginning to roll again. The chain slack in derailleur gear prevents any such lockup when the bottom jockey wheel is replaced by a freewheel (accepting that the bike cannot be rolled backwards as was always impossible anyways if the derailleur was not in gear.)
Technical Description. Mark V
All that is needed to engage the brake as the pedals are backed and then to uncouple the brake and pedals on resumption of powering is a ratchet, and the chainwheel with its teeth exposed at its rear edge provides a ready-made ratchet wheel. Engagement at dead centre is prevented by simply removing 3 or 4 teeth in two diametrically opposed sectors of the chainwheel. It still pulls the chain quite adequately as already proven bv the one piece double chainwheels stamped bv Shimano from sheet metal.
A roller from a scrap piece of chain on the engagement stud rotates with contact for minimal wear. The ratchet is made silent by sandwiching the (raised) rim of the chainwheel with a leaf of 1/2” wide steel banding strap, whose pressure is adjusted from the stud. The pawl pivot lies inside of the chainwheel for minimum clearance problems between the teeth and the right chainstay.
The pivot is on a folded sheet metal angle swinging on the crank pin pulling through a lateral pin the stirrup rod forward as the angle rotates locked to the reversed chainwheel The natural spring release of the stirrup should be sufficient to return the arm after braking. For a rear rimbrakes the sheetmetal pulls a spoke which is nicropressed to a flexble 7x19 brake wire turning through a pulley hanging from the seatpost bolt.
To sustain backing up in removing a bike from a storage alley, the pawl needs to be reversible to let the chainwheel rotate backwards, but not forward (to frustrate any resumption of pedalling until the pawl is reset). The simplest way to do this is to form the pawl disengagement stops by punching a long strip out of the ratchet arm. Then these stops can be elastically depressed to pass the pawl over them to reverse its setting. The prototype pictured used a half round stop with multiple centerholes tomatch multiple pawl pivot holes for 44,46,&48 tooth chainwheel sizes.
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Fwd pedalling ratchet position from below and behind. Right: walking (backwards)