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I need suggestions from experience or a table of compatibilities in order to find a compatible rear derailleur for folding bicycles (20" and 18" wheel)

I am aware that many the 28T max cog in the specs of many derailleurs is too conservative and they can works perfectly with 30 or 32T, specifically being 1x7 or 1x8 speeds

Notes:

I've already tested a Tourney RD-FT30(SS) with a link extender, a it reached the 34T but the derailleur position and tension was excessive.

Bike1 specs:

  • Wheel 20"
  • freewheel of 7 speed and 34T max sprocket,
  • 1x7, shifter from "microSHIFT" brand (7v, index, comp. Shimano Mountain)

Bike2 specs:

  • Wheel 18"
  • freewheel (SHIMANO MF-TZ30-CP) of 6 speed and 34T max sprocket,
  • 1x6 shifter from "sunrace" (6v, index)

Derailleur acronyms:

  • SS: “Short Cage”
  • GS: “Medium Cage”
  • SGS: “Long Cage”
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  • One additional problem is that a 20" wheel isn't much larger than a long derailleur cage, and it can pick up a lot of dirt from the road and is exposed to damage. A link extender (hanger adapter) would make it even lower too. Just something related to think about.
    – Criggie ♦
    Aug 9, 2022 at 11:11
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    Yes, the wheel size is imposing the cage length requirements, and specifically when wheel is 18" :o
    – Daniel Perez
    Mar 16 at 7:24
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    Id doesn't answer your question, but SRAM X5 short cage will shift an 11-40 fine and I have succeeded with an 11-42 as well. It is jideal for a 20" wheel, and cheap.
    – Noise
    Mar 16 at 7:51
  • thanks for this interesting info, i've never used SRAM but I assume it is not compatible with the Shimano world (chain, cog)
    – Daniel Perez
    Mar 16 at 8:29
  • The chains and cassette sprockets are the same as/interchangeable with Shimano but the derailleur and shifter must be matching SRAM parts and obviously the number of sprockets and type of chain must be the correct speed. In this case, 10speed.
    – Noise
    Mar 16 at 19:44

1 Answer 1

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RD-TY200-SS is a current low-end one that does it. That's probably the practical answer.

RD-M952-GS is old, high-end, and somewhat sought-after, but it does meet what you're looking for. There may have been a few other similar era Shimano GS mountain models.

I'm not a master of mixing and matching Shimano rear derailleur uppers and lowers, but I've seen people do it and that could also be a practical solution for you. The idea basically is to find a mountain upper (the mounting bolt, cable attachment, and parallelogram part) that gives the 34t clearance, and then a short cage road lower that interfaces correctly with the cage pivot.

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    Officially max cog for most shimano derailleurs is 28T and a tourney short cage without parallelogram movement will have even more problems. In fact from my experience they tourney (SS) requires a extender link adapter to reach the 34T but it position and cable tension is so forced that it break with the use.
    – Daniel Perez
    Aug 9, 2022 at 8:09

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