Chain rubbing against front derailer



Cusp

New Member
Sep 17, 2006
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Hello,

I just got a Shimano compact crank installed on my bike. I notice there is chain rubbing against the front derailer when in gears:

34x12,13,14 (3 smallers cogs)

-AND

50x12

Should I ride back to the shop and have them adjust the derailer a little better?

Thanks.
 

kdelong

Well-Known Member
Dec 14, 2006
3,477
134
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The reason there is chain rub on this is because you are using a small chainring/small cog combination. By doing this, you are getting too much chain deflection and putting undo stress on it. in this situation you will get chain rub on your front derailer. Without knowing more about your drivetrain, I can't say for sure but I'll bet that if you check you'll discover that you are overlapping and could get similar gearing by going up on the larger chain ring and up one or two larger cogs on your rear cassette. That being said, I would be a little concerned about the 14 tooth cog as it seems that you should be able to use it, except maybe not on a 10 speed cassette. Haven't used one yet so I don't know bout that.
 

Cusp

New Member
Sep 17, 2006
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kdelong said:
The reason there is chain rub on this is because you are using a small chainring/small cog combination. By doing this, you are getting too much chain deflection and putting undo stress on it. in this situation you will get chain rub on your front derailer. Without knowing more about your drivetrain, I can't say for sure but I'll bet that if you check you'll discover that you are overlapping and could get similar gearing by going up on the larger chain ring and up one or two larger cogs on your rear cassette. That being said, I would be a little concerned about the 14 tooth cog as it seems that you should be able to use it, except maybe not on a 10 speed cassette. Haven't used one yet so I don't know bout that.

The smallxsmall combo .... yes I know there will be chain rub. Had it before I swapped cranks. BUT never had chain rub on smallx13 or smallx14. AND I NEVER NEVER had chain rub on bigx12.

Anyway - I called the shop and will ride over there tomorrow to have them take a look.

Thanks!
 

Yojimbo_

Well-Known Member
Apr 17, 2005
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Hope your bike shops are better than mine. I have the same problem (i.e., rubbing on gear combos where it shouldn't be), and I've taken it to 3 shops (took it back to one of them for another adjustment), and they are all hopeless.

I've since tried to fix it myself by researching the adjustments, and I can't do it either.

Check that your chainrings aren't bent - I think that's part of my problem.
 

gclark8

Member
Apr 13, 2004
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Cusp said:
Hello,

I just got a Shimano compact crank installed on my bike. I notice there is chain rubbing against the front derailer when in gears: 34x12,13,14 (3 smallers cogs) -AND 50x12

Should I ride back to the shop and have them adjust the derailer a little better?
Yes, check the derailleur height, but it will only partially help the problem.

Check, is the chain rubbing on the BIG RING when in the 34 and the 12, 13, 14 on the rear?

This is one of the problems with changing to a compact crank and not adjusting the chainline. I have several customers where the problem was fixed by the manufacturer fitting spacers in the drive side betwen the bottom bracket and the outboard bearing. You may need a 45mm chain line to fix the problem.
 

Cusp

New Member
Sep 17, 2006
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gclark8 said:
Yes, check the derailleur height, but it will only partially help the problem.

Check, is the chain rubbing on the BIG RING when in the 34 and the 12, 13, 14 on the rear?

This is one of the problems with changing to a compact crank and not adjusting the chainline. I have several customers where the problem was fixed by the manufacturer fitting spacers in the drive side betwen the bottom bracket and the outboard bearing. You may need a 45mm chain line to fix the problem.

Ok - got the derailer working nicely now.

BTW - bike shop gave me a nice tip about the FD. Said clicking the shifter halfway will slightly move the front derailer away from the frame; like 2mm. A nice option if u have some slight rubbing when on the small cogs in the rear.
 

JTE83

Member
Jan 28, 2004
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Avoid FD rub because it will lead to framd FD bracket failure, and this happened to me !