Downgearing with a road triple

Question

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?

Answer 1

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.

Answer 2

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.

Answer 3

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.

Answer 4

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.