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I am currently running a 10-speed Tiagra compact double 50/34, 12-30 CS-4600 with an unknown type derailleur (labeled "KH", "RD-4601", "JAPAN VIA" ). I am happy with the 12-30 ratio, but would not mind a 32 or 34.

The problem is:

  • 10-speed 12-30 is not available anymore, only 11-32 and 11-34.
  • The derailleur might not take 11-32 if it is a short cage.
  • I don't know which derailleur I have. It is labeled "RD-4601" (no -SS or -GS). The bike catalog says "short cage", but it doesn't look like a short cage to me. The distance between the cogs is about 85mm. It doesn't look like either one in the exploded view in the manual either (the tab to hold the chain near the upper cog is either at 45deg for the -SS or horizontal for the -GS model, but mine is nearly vertical: http://byka.info/system/files/documents/components/Derailleurs/EV-RD-4601-3424A.pdf

derailleur image

What is a good upgrade strategy? I don't want to buy the Ultegra 10 speed cassette, which is expensive and probably won't last as long.

I looked at the 11-32T CS-HG500-10 (4700), but I am not sure it would work with my derailleur. Some posts say it does, but officially it only supports a 30T low sprocket.

Unfortunately the 4600 medium cage derailleur is hard to find and I don't think the 4700 derailleur is compatible with my shifters (different pull ratio).

Would the 105 RD-5701-GS derailleur be compatible with my shifters?

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The Shimano 105 rear derailleur, RD-5701, would be compatible with your current drivetrain. It is a 10 speed derailleur with the typical actuation ratio found on all Shimano 7, 8, and 9 speed rear derailleurs and 10 speed road rear ders with the exception of Tiagra 4700, as you've noted.

The CS-HG500-10 cassette is also compatible with your current set up (it sure appears to be a GS cage in the picture of your derailleur) and if you were to swap in an RD-5701 derailleur.

See Shimano's Compatibility Chart for 10s road rear drivetrains. Note the 11-32t cassettes require the increased capacity a GS cage derailleur brings.

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    The derailleur doesn't have to be 10 speed for it to shift a 10 speed cassette. If it's dialed in properly, any derailleur (with sufficient capacity and proper pull ratio) will work.
    – jayded-bee
    Jun 26, 2023 at 8:30
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    @jayded-bee That's absolutely correct. Any Shimano 7 thru 9 speed, mountain or road, plus 10 speed road derailleurs will work given the cage length affords it enough capacity.
    – Jeff
    Jun 26, 2023 at 14:13
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    @guest As noted in the linked post, using the derailleur out of spec can introduce some issues. Unless you have a thing for keeping your road bike "pure-bred" (we wouldn't blame you -- a lot of cycling is fashion!), you would be better served by a derailleur that you know can handle a 34T cog. Doesn't have to be 9 speed, either, or a discontinued (and pricy!) Deore, like you suggested. A RD-M3020 (Acera) would work just fine.
    – jayded-bee
    Jun 26, 2023 at 21:57
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    @guest Shimano specs regarding capacity and largest cog a derailleur can handle are fairly conservative. It's common in the wild to go over spec limit by 2 or even 4 teeth and still enjoy normal shifting. As you are using a 30t max sprocket now, which is probably spec for the 4600 GS cage, moving to a 32 should be no problem at all. You may have to wind the B-screw of the derailleur in a bit. This will create more room btw the jockey wheel and the cassette that will be taken up by the larger diameter 32t cog.
    – Jeff
    Jun 27, 2023 at 5:25
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    @guest What the compatibility chart I linked to in my answer is saying is that the RD-4601 and RD-5701 with GS cages are spec'd to handle a large sprocket that is between 27 & 32 teeth (inclusive of those numbers).
    – Jeff
    Jun 27, 2023 at 5:30
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Shimano and SRAM 10 speed cassettes are all interchangeable. Brands like Microshift and SunRace also make cassettes that you could use. Campagnolo cassettes have different spacing, so those are off the table.

Shimano's 10, 9, 8, and 7-speed road groupsets all have compatible rear derailleurs, EXCEPT for Tiagra 4700. Your cheapest option (new) would likely be something like a Claris or Sora RD (depending on where you are buying, of course). Just make sure it is not 11 speed or a 4700, and that the RD tooth capacity is large enough for the gears that you want to use.

You could also get away with using a RD from Shimano's MTB range, as those have absolutely huge cog capacities, but I am a road cyclist so I don't have much knowledge about those.

Perhaps, you could also use SRAM (or other manufacturer's) derailleurs, but I don't have the data at hand to tell you what all is compatible.

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These are the components I used:

  • CS-HG50-10, 11-36 (CS-HG-81-10 would be an alternative at 2x the price, CS-HG-500-10 the road version, only up to 34t)
  • Acera RD-M3020 7/8 speed derailleur, as suggested by jayded-bee
  • Chain +2 links (by largest chainring + largest cog method, calculation based on chain stay suggested shorter length)

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