Downgearing with a road triple 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?
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.