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Some shifter designs have become known for causing broken cables, anecedotally because they're causing too much repeat spooling and unspooling over too tight a radius. For example, broken cable heads inside some STI generations is a well-known occurrence.

Other shifters have more cable movement but don't have those issues, a common example being the 1:1 shifters where replacement is done by lifting the cover off the shifter.

How do you design a shifter and stay on the right side of this?

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  • Simple: You notice that the cable is failing and replace it before it breaks. Apr 12 at 21:35
  • @DanielRHicks I have one of the Sora shifters that's known for breaking cables. They normally last 5000km, but one recently only did 3600km, from the same batch - I buy in bulk. You don't get much warning. You tweak the indexing as you sometimes need to, then think a few days later that it's not right. Within 100km of that point it snaps. Last time that happened to me I was 200km from home, exactly at the far point of a 400km ride, and had actually opened up the shifter a few days before to have a look. No visible strands were frayed, and the kink sets in immediately.
    – Chris H
    Apr 13 at 21:23
  • Overall the stress is sufficiently concentrated that your window for noticing is very small
    – Chris H
    Apr 13 at 21:24
  • If I can get a picture I might turn this into an answer, but I'm not sure if it's visible without a complete strip-down. Some designs lay the cable along a nicely radiused plastic carrier then abandon it unsupported to flex over the edge of the plastic. Many drop brake levers do something similar , but the shifter mechanism concentrates the flex in a smaller length of thinner cable, for a much tighter bend, and the wear is far more concentrated.
    – Chris H
    Apr 14 at 11:50
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How do you design a shifter and stay on the right side of this?

You don't.

You design a shifter that with repeated use breaks the cable. Then you expect the user to change the cable whenever it breaks, limping home without gear shifting function if the cable fails during a ride.

David Gordon Wilson has written a text about cable fatigue. According to it, the diameter ratio should be 72 for long life and 42 at a minimum. With 1.2 mm gear cable, it means 50.4 mm diameter or 25.2 mm radius at a minimum. Bicycle shifters have a far tighter cable bending radius.

The reason this works is because shifting is not safety critical in the manner brakes are. Bicycle brakes too have small bending radii in their cable path, but not as tight as shifters. In brakes, some design effort is needed to not immediately fatigue the cable. The brake cables still fail despite this.

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  • Or the user wises up, and carries a spare cable. First time it happened to me I changed it in a bike shop car park with a sandwich in one hand. Second time, in a pub car park in the dark. Last year, however, a strand got caught where I couldn't get it out with roadside tools and the spare was stuck. I was very grateful for my triple chainrings when I had 200km to ride home with a stuck rear shifter. It was slow but not excessively so.
    – Chris H
    Apr 13 at 21:31
  • But where I don't agree so much is in the expectations - cables are wear components, and the old galvanised ones got stiff before they broke. It should be sufficient to replace them regularly, something like the rear shifter cable with every cassette (or every other chain if wear is high) and the front with every chainring. Your 2nd link about brake cables explains why my gear cables so so suddenly - yes, one strand goes first, but the others are just as worn so you get little warning.
    – Chris H
    Apr 13 at 21:36

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