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Big cog hits derailleur pulley

Old 06-22-09, 12:57 AM
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Big cog hits derailleur pulley

Even after completely screwing in the B screw, the biggest cog still lightly rubs the top derailleur pulley.

Shorten the chain?
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Old 06-22-09, 06:05 AM
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Shortening the chain will make it worse. Are you running on the large wheel in the front when this happens? Double or triple chain wheels?

Last edited by thompsonpost; 06-22-09 at 07:15 AM.
 
Old 06-22-09, 07:42 AM
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I have a bike with a 38 tooth in rear and had same problem. I swiveled the whole derailleur hanger down about a 1/2" in the dropout. I also drilled a new hole in frame to attach derailleur to new hole in dropout although that isn't necessary as the axle nuts hold it firmly in place the screw is just for convenience to hold the derailleur when removing wheel.
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Old 06-22-09, 08:38 AM
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MTB cassette and a road derailleur?
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Old 06-22-09, 08:44 AM
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My rear derailleur does this as well, but only on the biggest front chainring. I wanted less chainslap so I shortened the chain by just a smidge. If it's still ridable with the cog touching I would leave it, and if you're ever in a situation where the lowest gear on the biggest front cog is really useful, you can just downshift in front.
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Old 06-22-09, 10:43 AM
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Do you have room to move your axle in the dropouts (e.g. with semi/horizontal dropouts)?
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Old 06-22-09, 10:49 AM
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Originally Posted by Wordbiker View Post
MTB cassette and a road derailleur?
I think Wordbiker has the clue. If you're running a 38 in the rear and two rings in the front, you're stretching things a bit.
 
Old 06-22-09, 10:54 AM
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reverse the direction of the b-tension screw, head facing the frame will give you a bit more room.
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Old 06-22-09, 11:02 AM
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Originally Posted by aeo View Post
reverse the direction of the b-tension screw, head facing the frame will give you a bit more room.
+1!!!
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Old 06-22-09, 12:59 PM
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Doh! Sorry guys, I should've known better than to not post specs:

2008 Cannondale R6, double chainwheel. It's brand new.
Full Tiagara.
Problem occurs when it's on the smallest chainring and largest cog.

There's also another problem: When the bike is on a stand and the pedals are spun on high gears fast (Biggest chainring, 3 smallest cogs), there's a definite low pitched noise and vibration from the cranks, almost sounds like break rub (but it isn't!). Chain isn't rubbing on the FD, and putting my ears to the RD it doesn't sound like it's coming from the RD. I can feel it in the cranks when I spin them. On easier gears (e.g. 5 largest cogs and largest chainring this doesn't happen). Taking the chain off gets rid of the problem, so it's probably NOT the BB or the cranks. WTF is going on?
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Old 06-22-09, 01:27 PM
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not sure this will fix your particular issue, but i also noticed that the pulley was too close to the largest cog and ended up cutting the housing shorter and was surprised to find it resolved that and some other issues.
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Old 06-22-09, 10:49 PM
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It was the clipless pedal, it's old and needs replacement....the frame really amplifies all sounds
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Old 06-22-09, 11:12 PM
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Originally Posted by surfengine View Post
not sure this will fix your particular issue, but i also noticed that the pulley was too close to the largest cog and ended up cutting the housing shorter and was surprised to find it resolved that and some other issues.
Lol

wow
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Old 06-23-09, 05:51 PM
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Originally Posted by dextor View Post
Doh! Sorry guys, I should've known better than to not post specs:

2008 Cannondale R6, double chainwheel. It's brand new.
Full Tiagara.
Problem occurs when it's on the smallest chainring and largest cog.

There's also another problem: When the bike is on a stand and the pedals are spun on high gears fast (Biggest chainring, 3 smallest cogs), there's a definite low pitched noise and vibration from the cranks, almost sounds like break rub (but it isn't!). Chain isn't rubbing on the FD, and putting my ears to the RD it doesn't sound like it's coming from the RD. I can feel it in the cranks when I spin them. On easier gears (e.g. 5 largest cogs and largest chainring this doesn't happen). Taking the chain off gets rid of the problem, so it's probably NOT the BB or the cranks. WTF is going on?
Hi dextor,

Is your derailleur cage vertical when you're in your lowest gear? If it tips rearward like this / shortening your chain "one" (inner and outer) link will cause the upper derailleur pulley to pivot slightly downward and away from your largest rear cog.


Greg
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Old 06-23-09, 06:47 PM
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Originally Posted by oldpedalpusher View Post
Hi dextor,

Is your derailleur cage vertical when you're in your lowest gear? If it tips rearward like this / shortening your chain "one" (inner and outer) link will cause the upper derailleur pulley to pivot slightly downward and away from your largest rear cog.


Greg
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Old 06-23-09, 11:33 PM
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Originally Posted by operator View Post
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Except for old Hurets and some others... a derailleur cage's pivot point is in front of the axis of of the top pulley so that as the lower end of the arm is pulled forward the upper pulley moves downward to clear the larger diameter cogs...

...and conversely as the arm pivots back to wrap more chain, the upper pulley rises for more accurate shifting of the smaller diameter cassette cogs.

Greg
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