Rear derailleur getting sucked in, chain skipping and wrapping around the chainrings
08-31-14, 02:47 PM
#1
Junior Member
Thread Starter
Join Date: Apr 2005
Posts: 97
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 28 Post(s)
Likes: 0
Liked 4 Times
in
1 Post
Rear derailleur getting sucked in, chain skipping and wrapping around the chainrings
I have 2 bikes, one mountain, one touring. Both have Deore LX long cage rear derailleurs and both have the same problem. The rear derailleur "jumps" forward sometimes, causing slack in the chain and resulting in the chain skipping over the gears. Sometimes when shifting between small and medium chainrings, the derailleur gets pulled all the way forward and the chain wraps around the chainring and gets jammed.
My mountain bike has a pretty old drivetrain, but the one on my touring bike only has about 200 miles on it. Whats goin on?
My mountain bike has a pretty old drivetrain, but the one on my touring bike only has about 200 miles on it. Whats goin on?
08-31-14, 03:48 PM
#2
Senior Member
Join Date: Apr 2009
Location: New Rochelle, NY
Posts: 38,716
Bikes: too many bikes from 1967 10s (5x2)Frejus to a Sumitomo Ti/Chorus aluminum 10s (10x2), plus one non-susp mtn bike I use as my commuter
Mentioned: 140 Post(s)
Tagged: 1 Thread(s)
Quoted: 5787 Post(s)
Liked 2,580 Times
in
1,430 Posts
Two possibilities approximate your description, though not exactly.
The first and most likely with an old drive train that didn't always do it. Is chain suck. Here the RD isn't magically jumping forward creating slack, Instead the chain isn't disengaging from the bottom of the chain ring and getting carried up the back side. This pulls the RD forward as you describe. If you're lucky the chain finally pops free, if you're not it gets jammed up against the bottom of the chaining, jamming the crank until you backpedal a short arc to free it. The best evidence of a chain suck problem other than feeling it happening is a beat up right chain stay underside from the chain jamming it against it so often.
The other, in your case less likely, possibility is that the RD is a bit high, and you can get three point contact with the chain sandwiched between the sprocket and upper pulley. I said it wasn't likely for you because this would happen from the outset when you first changed a cassette or readjusted the system.
BTW- it could be different causes of the two bikes.
If you diagnose the bikes and chain suck seems to be the issue, search for posts about chain suck and how to fix, or come back here for more help.
The first and most likely with an old drive train that didn't always do it. Is chain suck. Here the RD isn't magically jumping forward creating slack, Instead the chain isn't disengaging from the bottom of the chain ring and getting carried up the back side. This pulls the RD forward as you describe. If you're lucky the chain finally pops free, if you're not it gets jammed up against the bottom of the chaining, jamming the crank until you backpedal a short arc to free it. The best evidence of a chain suck problem other than feeling it happening is a beat up right chain stay underside from the chain jamming it against it so often.
The other, in your case less likely, possibility is that the RD is a bit high, and you can get three point contact with the chain sandwiched between the sprocket and upper pulley. I said it wasn't likely for you because this would happen from the outset when you first changed a cassette or readjusted the system.
BTW- it could be different causes of the two bikes.
If you diagnose the bikes and chain suck seems to be the issue, search for posts about chain suck and how to fix, or come back here for more help.
__________________
FB
Chain-L site
An ounce of diagnosis is worth a pound of cure.
Just because I'm tired of arguing, doesn't mean you're right.
“One accurate measurement is worth a thousand expert opinions” - Adm Grace Murray Hopper - USN
WARNING, I'm from New York. Thin skinned people should maintain safe distance.
FB
Chain-L site
An ounce of diagnosis is worth a pound of cure.
Just because I'm tired of arguing, doesn't mean you're right.
“One accurate measurement is worth a thousand expert opinions” - Adm Grace Murray Hopper - USN
WARNING, I'm from New York. Thin skinned people should maintain safe distance.
08-31-14, 04:06 PM
#3
Senior Member
I put a new aluminium large chainring on a bike and developed the exact symptoms. On the stand I narrowed down the problem tooth on the chainring. I whittled-filled on it a bit and marked it with a felt tip pen to see where it was rubbing and gave up.
A new ring fixed it.
A new ring fixed it.
