The BSA 4-Star used a unique ‘tube’ design. The cable nipple lives inside the pulley hub and the cable runs up the tube (around the smoothly radiused bend and inside a concealed spring) and out by the adjustable cable outer stop. The cable is clamped at the lever. As the cable is pulled, the pulley hub slides along the tube, moving the chain (you can see this all explained in UK patent # 640,473).
Fiendishly simple and beautifully made, but wasted on a single pulley design that was never going to change gear crisply.
This is the second incarnation of this exquisite design. In this version chain tension is provided by a spring running along the chainstay and attaching at the eyelet close by the adjustable cable stop. The design has also been 'improved' with a bizarrely shaped pulley cage and a large protective extension to the hanger plate. No steps forward, three steps back.
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