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Old 05-21-11, 09:08 PM   #1
4567890
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Front derailleur help please

Hey guys,

My chain currently rubs the front derailleur when Im on the largest ring/smallest cog (isnt a problem with limiting screws). When I tighten up the cable, the rub goes away but then it will rub when Im on the small ring (I have a double crankset) and largest cog. It seems to rub either the highest gear or the smallest gear. I did notice the derailleur can be pushed farther inward (about 1mm) and rid small ring rub, so could this mean the spring in the derailleur is worn out and not producing enough tension to pull the derailleur inward when the cable tension is loose enough?

Thanks a lot in advance!
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Old 05-21-11, 09:26 PM   #2
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Set the cable length (tension) based on moving the cage to the correct position for good shifting and proper trim on the larger chainring. It should just clear the chain coming from the outermost rear sprocket.

Then shift to the inner, and test for clearance with the low gear sprocket. There should be a bit of cable slack with the FD leaning on the inner limit. From your description there might be dirt or other issues keeping it from moving in all the way. You can try plucking the bare wire away from the frame at the down tube. If that helps the FD move in more, than your issue could be cable friction in the housing.
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Old 05-21-11, 09:34 PM   #3
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If it's not resting on the inner limit screw in the small ring, that's because of your cable tension. I guarantee it's not the derailleur's return spring, or the cable would be slack.

I've come across your problem before, and when it's become apparent that no amount of adjustment will solve it, I just tweaked the derailleur cage with an adjustable spanner. You have to be careful though, cause the steel used there is often very brittle. If you opt to tweak the cage, figure out how to do it with the most minimal amount of bending, in order not to mess too much with the cage's (supposedly) highly engineered geometry - it's easy to mess up shifting if you just go the hack.
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Old 05-21-11, 11:05 PM   #4
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Thanks for the quick replies guys! I havent had a chance to work on the bike yet, but I'll definitely give your suggestions a shot.

Kimmo- I hope I dont have to resort to that, but how exactly do you tweak the cage? Do you widen the cage or bend the cage away/towards the frame?

Thanks.
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Old 05-21-11, 11:31 PM   #5
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Just look at where the chain rubs on the cage, and considering its shape, decide which side you can add clearance to with the least amount of bending. Perhaps it's just a corner of the cage causing the problem on one side? But if it's rubbing along a flat area, there's not much you can do.

And if you have to bend it further than like 1.5mm to get clearance, that might make it reluctant to shift.

This isn't something you want to try unless you've exhausted other options. Ensure the outer cage plate is parallel with your rings first.

Last edited by Kimmo; 05-21-11 at 11:34 PM.
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Old 05-22-11, 01:25 PM   #6
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Make sure it is parallel to the rings.
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Old 07-22-11, 01:00 AM   #7
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Quote:
Originally Posted by davidad View Post
Make sure it is parallel to the rings.
Thanks! I readjusted the derailleur so that its aligned straight, that fixed it.
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