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 first incarnation of this exquisite design. In this version chain tension is provided by a clock spring at the b-pivot and the pulley cage is a simple and understandable shape.Both the spring and the pulley cage are rather reminiscent of the Simplex Champion du Monde - the best selling derailleur of the pre-war years.
If you click the 'photos' tab and scroll to the bottom of the images, you can see some pictures of the relevant gear lever.
Browse associated documents.