Originally Posted by
dabac
8 of 9 on a 7 is a direct swap, the bike won't "see" any difference in the way the rear wheel and the sprocket cluster sits in the frame.
The RD doesn't care much about the numbers of speeds it's moving over. Unless you're too far off with the chain width( but running a narrower chain in a wider RD is usually OK) or the amount of chain wrap required the old RD should do fine. The exceptions to the rule are the RDs with an oddball amount of travel vs cable pull. (SRAM, some Dura-Ace being the most common examples)
If you're worried about chainline, keep in mind that you're takling about a bike with derailers here. It HAS to be tolerant of chainline error to work at all.
Thanks for explaining this. And now for the wildcard. I'd really like to run a compact crank up front, 36-52 is my preference, or the more common 34-50. My cassette would be the 12-27. I've been reading a fair bit about chain wrap and the published Shimano specs. I must admit I'd prefer to run the short cage RD-6500 even though the total chainwrap would be 31t which I believe exceeds Shimano's published specs. In practice, since I meet Sheldon Brown's definition of a competent cyclist ;-), I would never be running small chainring to smallest cog.
If performance will suffer with short cage 6500 under the above scenario, I'm happy to go to long cage 6500. I don't have the parts yet, just trying to get organized.
thanks again