Originally Posted by 'nother
Thanks, Sheldon.
I have a question regarding short-cage vs long-cage. My understanding (from the inimitable first poster to respond to this thread) is that the cage length has nothing to do with cog capacity. That is to say: a short-cage derailer will not generally handle a 32 or 34T cog . . . however, not necessarily because of the cage length, but because of the length of the parallelogram/arms. But neither will any old long-cage derailer simply by nature of it being long cage. While I don't think what you have contradicts this, it might be nice to see this issue explained further and clarified.
Specific case of why I bring it up: A while ago I was shopping for a new RD and a guy at a LBS sold me a long-cage Shimano 105, telling me "yeah, it's long-cage, it should work" for my 32T cog. It didn't. Even with an extra-long B-screw adjusted to the maximum, I was unable to adjust it to prevent the jockey pulley from hitting the largest cog, not even close. I exchanged it for a "mountain" derailer and had it working within about 30 seconds. The derailers had the exact same cage length, but different parallelogram size.
Thanks. Now back to carpet-smoking.
First, your undertaning is somewhat flawed or poorly understood.And that is not because it was poorly articulated on my part. There are short cage mtb RD that do shift big cogs. Also, there are old RD that shift big cogs because old freewheels with 33 and 34 cogs were fairly common, and there were even old 38 tooth freewheels... Secondly, that cage length business is the common misconception you get about long cage RDs, and it comes not just form LBS guys that should know better.... Duno how many times I have said it, but cage length is about wrap capacity, not big cog, which is in the parallelogram design. All one has to do is look at current shimano shimano road RD specs. The big cog capability is the same for both long and short cage.That's cuz the parallelograms are identical. The 27 tooth spec is conservative tho and most will shift a 30 and sometimes, depending on the application, one can get away with a 32 with them.If it was me and I wanted to use a 32 or bigger cog, I'd just go with a mtb RD, unless I had a road type just lying around and didn't mind spending some time fiddling with it. Campy spec does not tend to be as conservative and one can get away with less with them...A Shimano mtb RD is different in that in order to clear the bigger cogs involved, it tracks a steeper angle as it traverses the cassette, and that applies wether it is a long or short cage model.