I think I understand the derailleur mechanism pretty well, yet I still find it surprising how the chain catches on the new cog so reliably. Is there something I'm missing? Is it just with the proper derailleur alignment, the chain will inevitably catch?

  • About a century of practice. A big jump in the technology occurred with the invention of "indexed" shifting, as that involves having "ramps" on the sides of cogs to lift the chain from a small cog to the next larger one. And, properly adjusted, "indexed" shifting eliminates the "searching" that was necessary after an unskilled shift, in order to get things perfectly centered. (A skilled cyclist could hit the center of the adjacent gear on a non-indexed bike nearly as well as on an indexed one.0 Commented Sep 19, 2017 at 0:33

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A great deal of engineering has gone into making shifting smooth and predictable. Up until ~1984, shifting was done by feel and sound: the shift lever moved continuously through its range, and you moved it until it caught the adjacent gear. Different brands had different actions, and you had to develop some technique.

In 1984, Shimano rolled out "indexed shifting" (there had been previous attempts, but this time it really took). Here, the shift lever clicked into specific positions; the spacing between gears on the freewheel was standardized (before, the spacing varied within a freewheel); the shift cable was pre-stretched so the action wouldn't change; and so on. There was a lot of skepticism that indexed shifting would stay reliable in the long run, but it did, and took over.

Since then, there have been additional refinements. Individual gears on a cassette have "ramps" that help lift the chain from smaller gears. Gears are aligned relative to each other to ensure good uptake. Chainrings have pins to help lift as well.

  • Also, once the tensioned chain is over a sprocket, it pretty much has nowhere to go but to drop onto the sprocket teeth. The miracle of modern derailleurs is that it does so smoothly and without skipping. Commented Sep 19, 2017 at 1:55
  • The ramps on the cogs were a part of the indexed shifting scheme, pretty much from the start. Without them it wouldn't work. A big part of the problem is the need, with the older schemes, to "over-shift" when shifting to a larger cog. The ramps largely eliminate this requirement. Commented Sep 19, 2017 at 2:11
  • I actually had the first iteration of Dura Ace SIS. It didn't have ramps, although IIRC it did have twisted gear teeth to try to help catch the chain. As far as I can tell, ramps were a part of gears around 1990.
    – Adam Rice
    Commented Sep 19, 2017 at 16:31

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