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I'm trying to get my front derailleur to stop rubbing in the high gears, smallest sprockets. To do this I am attempting to follow this guide by shimano:

http://bike.shimano.com/media/techdocs/content/cycle/SI/SORA/SI_5GR0A_001/SI_5GR0A_001_En_v1_m56577569830614926.pdf

It says that when in the largest chainring it should be possible to adjust the derailleur so that it is 0-0.5mm aways from the largest chainring on the side furthest away from the bike. This however is not possible and seems to have been set by the lbs simply by putting the cable at very high tension to push it over so it doesn't rub. I believe this may be slipping or stretching and causing the rub to return.

What's the correct way to do this? Is this normal or is the derailleur being dysfunctional?

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1 Answer 1

up vote 5 down vote accepted

Wow what a horrible guide.

Tightening the gear cable while in the highest front gear is very tough and will only hurt your fingers. I recommend putting the front dérailleur into the lowest gear and tightening it that way. This is much easier!

As for your specific problem, in all my experience you can never not get 'rubbing' (if by rubbing you mean the chain running through the dérailleur over hang and brushing it). However, you will notice that the brushing varies depending on whether the back gears are high or low. Since this varies the angle at which the chain 'points' to the back gears.

So the solution: Since you will always have brushing (a tiny bit at least), you just seek to minimise it. Put the back gears in the most used gear before tuning the front dérailleur. For example on my touring bike I'm usually in my third highest gear at the back, unless going up hills or firing down them. So I tighten my front dérailleur cable while the back gear is as such, this means I only get front gear rubbing when the back gear is perhaps in the very low gears, thus causing the chain to 'go backwards' at a different angle.

If you want to spend a ridiculous ton of money on a fancy dérailleur you can mitigate this, but there's really no need. For mountain bikes the most common gear back gear is the lower one. If you're not sure which you've used most, clean the cassette of a bike you've used well, and look at the wear on each cog.

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This is very helpful. I was going mad trying to eliminate all the rubbing I now have it down to the lowest gear for both front and back and that seems minimal. Is it bad to run a bike, just for a little bit, in a gear that rubs? I imagine this can be bad for the chain more than the derailleur. I might need this combination for hill climbs when touring. –  user1170304 Dec 5 '12 at 16:02
    
Short answer: No, it's fine for it to rub. Keep your chain clean and lubricated and it will last well (I get about 10,000 km before changing). You can get further if in a dry climate where caring for it is easier (up to 16,000 km). If you're particularly worried about the chain rubbing you can get more expensive chains which use different (tougher) materials for the 'wall links' - but I guarantee the chain will need have stretched out (from everyday use) before the side links wear down. The front dérailleur won't suffer. –  Sam Dec 5 '12 at 16:45

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