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Front derailleur chain rub

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Front derailleur chain rub

Old 06-19-15, 12:13 AM
  #1  
nemeseri
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Front derailleur chain rub

I have chain rub whenever I'm on the large chainring and even in the middle of the cassette. I adjusted the high limit screw and it helped some, but I had to unscrew it almost all way out to have proper chain clearence. I guess that's not the way to go. I have a cable adjuster on my front derailleur cable, but I've been always confused which way I should turn it and why.

Should I increase the cable tension instead of messing with the limit screw?
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Old 06-19-15, 01:12 AM
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your best bet is to start from scratch. There are numerous sites that demonstrate how to set up deraileurs. Shimano.com, parktools.com, sheldonbrown.com.

but to answer your question directly, both of your deraileurs are basically just a spring loaded guide. if you disconnect the cable completely the deraileur will relax. your front der will be against your frame. the 2 limit screws set how far in (close to your frame) and how far out (away from the frame) it is allowed to go. there is no need for your front der to go any further inward than where you small chain ring is.

you can "eyeball" the alignment and while holding the cage tighten the liit screw until you feel the resistance of the spring. when you release the cage it should stay in line with the ring.

then do the same for the large ring, actually you can give it a little extra outward motion.

then you can pull the cable taut and connect it to the der. your shifter will know the spacing between the rings.

start with your barrel adjuster screwed %90 in. as you unscrew it it adds more pull to cable which will move your der away from the frame. This is used for very minor tuning/adjustment

Also I believe the bottom of your front der cage should be 4mm above your large chain ring teeth. rotating the angle of the der can greatly affect its performance, I dont know the sure fire method to set it up but a little trial and error always works for me.

good luck.
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Old 06-19-15, 11:30 PM
  #3  
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For future reference for others. I went to my LBS and asked the mechanic guy for advice. He explained the cable tension so well that I was able to adjust my front derailleur properly. There is no need to adjust the stop screws if you can force the front derailleur into the right position by pushing the lever. You only need to adjust cable tension.

1., Shift down to small chainring
2., Use the additional trim to make sure there is no tension in the cable.
3., Increase the tension by turning the cable adjuster clockwise if you look from above.
4., Switch back to large chainring and check the chain clearance. If it's still rubbing, go back to #1

Sorry for the noob post, but I hope it might help others later.
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Old 06-20-15, 01:11 PM
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You like so many others don't (or didn't) understand limit screws. Just as their name implies they "limit" the ends of the derailleurs travel range, and prevent it from moving beyond the set "limits".

Think of them as you might the barriers along the sides of a bridge. They don't affect driving or lane selection, you do that, as does the cable in your derailleur. The barriers are only there to keep you from driving off the side of the bridge and into the river below.
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