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As I see, all bicycles sold with coaster brake never feature external speed gears with sprockets and derailleur. Bicycles that feature such a gear mechanism offer rim or disk brakes, operated by hand. Why is it so? Is coaster brake technically incompatible with derailleur?

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    Does this answer your question? Why there are no or few bicycles with noiseless wheel rotation mechanism and external speed gears?
    – RLH
    Commented Aug 26, 2022 at 13:36
  • (My comment above was automatically generated by Stack Exchange when I marked this question as a duplicate. For context, the linked question from the same asker originally had the same title as this question now has [“Why there are no or few bicycles with coaster brake and external speed gears?”] and asked a combination of that question and a question about the appeal of noisy vs noiseless hubs. I and another poster answered the question in the original title, then the OP changed the title and stated that they had meant to ask the other question…
    – RLH
    Commented Aug 26, 2022 at 14:49
  • … It was suggested that they restore their original question as it had upvoted answers and ask the hub-noise question separately; instead, they have reposted the question originally asked and answered elsewhere.)
    – RLH
    Commented Aug 26, 2022 at 14:50
  • How can a single speed bicycle have the speed gears?
    – nightrider
    Commented Aug 27, 2022 at 7:58
  • 1
    I have made an effort to edit. I hope the meaning was not lost. At least matches the answer well now.
    – nightrider
    Commented Aug 27, 2022 at 14:03

2 Answers 2

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As already answered the previous question, coaster brake is incompatible with derailleur.

Coaster brake requires significant tension on the lower part of the chain when it is applied. Derailleur on the other hand requires extra length of chain to allow switching between different sized cogs and a spring-loaded chain tensioner to take up the slack. The tensioner works only if the lower part of the chain is not under tension from braking.

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It is. A derailleur requires chain tensioner which makes it impossible to apply tension backwards, or at least it would have a huge free travel for the lower chain run to become tight again. The amount of that free travel would depend on the gear you are using, resulting in unpredictable free travel.

For example consider Bromptons. Even the 1-speed units have a chain tensioner due to the folding mechanism. You won't see coaster brakes in Bromptons.

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