Bicycle Mechanics - Chain scrubing agaist derailleur

Hi,

Chain is touching the inner wall (closer to the bike wheel) of the derailleur whell house i.e. it is not in the center of the wheel house. This is true for all gear settings. On the cassette the chain is centered: this is easily done by adjusting the knob that tightens the wire. How can this be solved?
Of the two derailleur wheels I mean the upper wheel only. The lower one is OK.

Thank you.

By "derailleur wheel house" do you mean the thing that holds the two jockey wheels underneath the sprockets? If you do, it sounds like your derailleur isn't properly adjusted - google "how to adjust a rear derailleur", or put it into YouTube.

How many cogs do you have on the back? I can't tell for sure from your description but I suspect that your derailleur hanger is mis-aligned. That's a common issue. Shift into a gear that makes the arm point straight down and look at your derailleur from the back. If the arm seems to be pointing toward the rear wheel, that's it.

If you have 8 or fewer cogs on the back, you may be able to bend it back straight by hand and eyeball. If you have 9 or more rear cogs, I need to use a gauge to get the alignment accurate enough to index properly.

At the beginning i thought he was asking about the FD.

I can't tell for sure from your description but I suspect that your derailleur hanger is mis-aligned. .... If the arm seems to be pointing toward the rear wheel, that's it.


The hanger may be bent so the lower end of the RD is inboard from the vertical position. That would mean the chain leaves the upper pulley at an angle toward the inside cage plate.

But, it would have to bent at a pretty severe angle for you to see the difference at the top pulley. I suspect that you have a floating upper pulley (moves horizontally on it's shaft) which is standard on Shimano index derailleurs. Some of these are also slightly asymmetric, so what you're observing may SOP for your derailleur. It's possible that you might float the pulley outboard by adjusting the trim inward a bit (tighten cable at the barrel adjuster) but don't do it for this reason. Adjust the trim for best shifting in either direction, and let the chips fall where they may.

BTW- rule number one of bike repair --- don't fix what ain't broke. You didn't say whether your "problem" was affecting shift performance. If it isn't and shifting is good on both directions, your best bet is to leave it alone and stop obsessing about your bike.