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Rear Derailleur Issue/Question

528 views 8 replies 5 participants last post by  FBinNY 
#1 ·
I have an older XTR derailleur on my bike (M952), and when I switched it to another frame (from a Superlight to a Blur LT) it began acting strange. When you turn the cranks backwards, the bike changes gears. Then when you turn the cranks forward it immediately shifts back to the correct gear. Besides that, it shifts perfectly, and I fell like I've adjusted everything correctly. This is not my first time setting up and adjusting derailleurs. I'm not sure what has gone wrong, but I was hoping someone could help me out.
 
#3 ·
grandegrande said:
I have an older XTR derailleur on my bike (M952), and when I switched it to another frame (from a Superlight to a Blur LT) it began acting strange. When you turn the cranks backwards, the bike changes gears. Then when you turn the cranks forward it immediately shifts back to the correct gear. Besides that, it shifts perfectly, and I fell like I've adjusted everything correctly. This is not my first time setting up and adjusting derailleurs. I'm not sure what has gone wrong, but I was hoping someone could help me out.
Your problem has nothing to do with the derailleur.

Auto shifting when backpedaling is a function of chainline. With no guidance in that direction bad chainline will cause the chain to shift as it seeks a better line. There is no real fix, except to avoid back pedaling when using crossed or badly aligned gear combos.

BTW- evidence that the RD is OK is that it instantly establishes the correct gear when you pedal forward.
 
#5 ·
Not necessarily a design flaw, or a frame problem. There are various reasons that cranks are farther outboard today. One is the needed extra clearance for external BBs. Also some frames have wider chainstays to clear fatter tires, again pushing the cranks out.

Look at your bike, and if there's room between the chainrings and chainstays, you might be able to improve chainline with a crank or BB replacement, but I wouldn't spend money just for that. (I'm old school - if it ain't broke don't fix it)
 
#9 · (Edited)
ver-T said:
Couldn't you just add/subtract/change BB spacers to adjust the chainline?
Yes. But onlyIf there's room to spare between the chainrings and chainstay, and if the crank and BB use spacers. I had assumed that if there were more inboard spacing possible wih the existing hardware it would have been done already.

There are also other things that might help marginally, like replacing the chain with one having more bellmouth on the inner plates, but given that everything works OK except when backpedalling in various mis-alligned gear combiantions, the easiest fix is not to backpedal in those combinations.
 
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