Originally Posted by
Mithrandir
I've read that generally anything below 50rpm for sustained periods of time will eventually hurt your knees.
I know I have to hop off whenever I hit around 50 or my legs will give out. I'm not sure doing a mile at 30rpm is very smart, but I am not an expert either... I just know that I couldn't do it, and that 5-6 rpm extra is probably not significant enough to bother with.
My solution was going with a 26/34 low gear. I know most cyclists succumb to peer pressure and get gearing that is wildly inappropriate for them because they want to pretend they are pros, but at some point we have to realise that only the right tools are going to help us do the job properly. Perhaps you could go with a road triple, those often go down to a 30t granny, though at 3.4mph that should still only get you around 10-11 rpm extra, at around 40-41rpm.
For reference my 26/34 lets me do 3.4mph at 57rpm. I am not at all ashamed of using mountain bike gearing, especially if I'm climbing actual mountains.
Can't comment on the 50rpm bit, but agree that a good low gear is useful when climbing, especially when carrying extra weight. My granny gear is 30/32, a friend of mine has a 28/32 gear. On my MTB I can go down to 22/34 which is insanely low by comparison.
If you swap out your chainrings you may also need a new FD - they tend to have limits relating to the maximum tooth difference between large and small rings, and a minimum tooth difference between large and middle ring (if it's a triple)
My MTB chainrings are 44-32-22, my cross bike has 50-39-30 and my friend's touring bike is 48-38-28. As you say, if you've got the right tools you can do the job. Chris Froome might be able to climb the Pyrenees with a 39/25 gear but I can say with some confidence that if I did manage it (which is doubtful) I'd be desperately slow, to the point anyone timing me would be more likely to use a calendar than a stopwatch.
If your RD will support a larger cassette you can gain a lot by increasing the size of it. While on the face of it you might not like the idea losing your high gears (I instinctively want to keep my 11-tooth sprocket at the back even though I never actually use it) the simple reality is that you probably don't use the big gears all that much. On my MTB I'm often in the largest gears when I'm on the road, on my cross bike I think the only time I've ever used the 11t sprocket is when I was testing the bike after giving it a good clean.
My cross bike seems to give me a wide enough range to do all I need and then some, with gearing that ranges from 50-11 (that I never use) right down to 30-32 (that I have used several times when climbing). Depending how you are at climbing generally, in terms of your own strength and the hills around you, a 48-38-28 paired with an 11-32 cassette will probably help.