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My mother is trying to get back on her bike after years of knee problems + surgery, which so far has gone ok, but she has had some trouble with climbing hills. She's thinking that she should try to get some lower gearing on her bicycle. She currently has a "road triple" with a long cage derailer and a (relatively small, in my book) 11-26 tooth rear cassette.

I figured that with a long cage derailer, she should have plenty of flexibility in downgearing her cassette (with a new chain, of course). She took it to her local shop though, and they insisted that she couldn't downgear any more than to a 28 tooth cassette. When I called to ask about it, they said that even if she replaces her derailer, "with a triple it just won't work". I wasn't able to get more details, just a repeat response, even after I asked if the issue had anything to do with taking up chain slack or chain length on the large front + large back combo.

Every source I've found about the issue doesn't mention anything very triple dependent. Sheldon's site seems to think that shops are generally overly conservative on their downgearing recommendations, but that page seems to expect a bike with a double chainring.

What are the usual limiting factors on a downgearing conversion? What specific concerns should we have with a triple in front? How can I figure out how large a cassette I can get away with? And what ill effects can she expect if she violates the advice of the shop and puts a larger cassette on?

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    It can be done in all cases, but the best path depends on what components the bike has and how low she wants. Mostly we need to know what the right shifter is and whether it's a 30-39-50 or 30-42-52. – Nathan Knutson Jul 25 '17 at 0:11
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    Comment cos not answer - your mother may suffer relapse on healing if she pushes too hard. You might consider an ebike to help her with those harder bits. Its not cheating - its protecting the healing knees etc. – Criggie Jul 25 '17 at 0:21
  • @Criggie Telling that to my mom will be a challenge. She's been doing ok with the current setup, and it's been a year since the surgery, but want's to do some real mountain climbing/loaded touring. (Well, actually, just mountain climbing with the rest of the family sherpaing her stuff...) – T.C. Proctor Jul 25 '17 at 0:56
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    Mothers! Where would we be without them? – Criggie Jul 25 '17 at 1:31
  • Comment that applies to a few answers: Sora 9 speed definitely comes in 11-32 and works with a triple because that's what I've got (30/39/50) on my tourer. There's a (very) long cage Sora derailleur to go with it. There's a compatible 11-34 in existence too. Check out typical tourer gearing and see if the bike shop will replicate that - if not, why not. – Chris H Jul 25 '17 at 5:52
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The triple is irrelevant. So long as the derailleur has enough capacity(which is determined by the size of the chainrings and cogs in the back; this is basically controlling how much chain length derailleur can keep tensioned) and largest cog ratings (for clearance ratings), you can put a bigger cassette.

There may be other ratings on the data sheet -- minimum cog rating, and maximum front chainring difference. These aren't really important. Exceeding the maximum front chainring difference may increase the chance of dropping the chain since the derailleur will need to take up some amount from a front shift.

The capacity is generally conservative -- if you don't use combinations like small-small, you can exceed whats written on the derailleur. Obviously, the derailleur needs to be matched for compatibility to the shifter.

That being said, you may also want to switch to smaller chainrings depending on the size of your chainrings.

Another item might be switching the type of bike your mom is riding -- if she has a regular road bike, a more relaxed geometry like a dutch bike may be more comfortable and ridable. Spending some time working out bike fit may also pay off, even with the existing bike.

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    Just for the sake of completeness, your answer should probably include what "capacity", "largest cog rating" and "clearance rating" mean. – T.C. Proctor Jul 25 '17 at 0:34
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I have a road triple with a 28 tooth maximum. The main limiting factor is where the top jockey wheel sits when over the center-most cog.

On my folding bike I overdid it with a 34 tooth gear, and the top jockey wheel does not clear the big cog. Upshot is that it works fine, with the jockey wheel pressing slightly into the edge of the cog under spring pressure.

Downside is when I shift up (to a smaller cog) its totally possible for the jockey wheel to be hooked on the wrong side of the cog, and you're stuck in the lowest gear until manual intervention.

The fix is to increase the B screw tension, to hold the derailleur down further, but that will reduce the tooth engagement on the smaller cogs, increasing wear.

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    You say that increasing B screw tension will increase wear. I've never heard anything about that; I was under the impression that changing the B screw mostly changed how quickly shifting happens. Do you have any sources mentioning for the increased wear? – T.C. Proctor Jul 25 '17 at 19:45
  • @T.C.Proctor As you increase B screw tension, the derailleur is held out from cogs a bit more, so in the smaller cogs it decreases the total tooth/chain engagement. Fewer teeth engaged with chain increases pressure on the ones that are meshed so its similar to riding on an elongated chain, more pressure on fewer teeth, and the cassette/chain wears faster because of that. If the rear derailleur could follow a curved path along the block, and a curved path forwards to increase chain engagement then that would be better for wear. – Criggie Jul 25 '17 at 20:35
  • That makes perfect sense, but it would be nice to have a source. Even if this is something that does happen, I don't know if the difference is significant enough to matter. My brief scan of google search results doesn't turn up anything, and Sheldon doesn't mention it, so it's raising my skepticism flag. – T.C. Proctor Jul 25 '17 at 21:41
  • Yeah I can't find one either. If you have a badly worn cassette, skipping can be exacerbated by tightening the B screw, and reduced by backing it out. So evidence is based on experiment and personal experience. – Criggie Jul 25 '17 at 22:39
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Shimano road derailleurs are rate at 28 max tooth capacity (even the long cage ones), hence the answer you got. However all is not lost. refer Sheldon brown for how they can be made to work on larger cassettes. SRAM road derailleur work with 32 tooth, so you could switch to that.

A smaller chain ring might be the way to go. Going to compact will get you down to 28 tooth.

Work out what you are prepared to spend, and work back from there. A 32 cassette will probably be able to work well enough, especially if she is careful on the final shift onto the 32. If that failed, you could go to SRAM (Derailleur, shifter and cable) if its in your budget. Another option could be a right of return on a 32, and return it an change chain rings, and maybe go up to a 28 tooth cassette.

A good bike shop would have had a discussion aimed at finding a solution for you, not left you hanging wondering. A great bike shop would have worked towards a solution within you budget.

  • You can also go to mountain. In shimano, <=10 road and <=9 mountain are interchangable. – Batman Jul 25 '17 at 1:16
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    My Shimano 105GS 10 speed is rated for 30T with a triple (and 32T with a front double, so I suspect 32T would work fine). The new Ultegra RD-GS goes to 34T. I couldn't find the previous 11speed Ultegra but my wife is using the GS with 11-32 cassette and I think that is within the specs. – Ross Millikan Jul 25 '17 at 3:17
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This article talks about total derailleur capacity and gives a triple chainring example https://learn.performancebike.com/bikes/advice/buyers-guides/bike-parts-and-components/understanding-rear-bike-derailleurs.

As far as I know, there is nothing intrinsic about a triple that prevents fitting a larger sprocket cassette. If the derailleur has the required capacity (and accommodates the largest sprocket in the cassette), then it will work.

  • Derailleurs also have a maximum tooth size. – mattnz Jul 25 '17 at 0:25
  • @ mattnz - yep, that what I meant by '(and accommodates the largest sprocket in the cassette)' – Argenti Apparatus Jul 25 '17 at 0:57

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