Rear derailuer

Question
I have a 21 speed bike and the chain jumps on the rear sprocket in a hihger gear. I would say when the rear derailure is in the upper 3 gears. I replaced the rear derailure and chain. I have adjusted the shift so it shifts ok . but when I put pressure on the peddles is when it jumps.  The reason I replaced the derailure was I thouhg it was week but the bike still does the same since I chnged them.  thanks

Answer
Dan,

I think there are a couple of things this could be, but it is really hard to diagnose without seeing the bike.  Nonetheless, I will explain the possibilities based on the fact that it sounds like the derailleur is out of alignment with the rear gear cluster.

1.  Easier fix.  The wheel could be misaligned in the dropouts (the slots in the frame that the wheel axle goes into.  To fix it, try loosening the quick release and making sure the wheel is centered evenly between the chainstays (the tubes that run back from the base of the seat tube to the dropouts).  Then tighten down the quick release, if that is the problem, the bike should shift properly.

2.  Harder fix.  The derailleur hanger (the part of the frame that the derailleur attaches to) is bent.  The only way to diagnose this is to shift into one of the middle gears.  Hang the bike up so you can look down the chain.  Now look at the orientation of the derailleur pulleys (the little wheels the chain goes around on the derailleur).  Does it look like the pulleys are on the same plane as the gears in the rear cluster?  Next look at the derailleur hanger.  Is it bent at all (even slightly)?  If the pulleys are off and the hanger is off, then the derailleur will shift unevenly back and forth across the gears and cause the problem you are describing.

The fix for this one is to have the bike shop realign or replace your derailleur hanger.  Realignment takes a special tool that costs at least $40 and is somewhat hard to use (I have one and it's a pain).  It is easiest to have it done for you, because if you buy the tool, you won't necessarily use it often.  Plus the bike shop will likely have a replacement hanger on hand if the bike frame is a type that takes a replaceable derailleur hanger.

Have a look and write me back with more details if these haven't helped you figure it out.  What is the history of the bike?  Have the rear gears ever been replaced (a wear issue)?  

Good luck,

Graeme