mzeffex 

So I was biking today and everything was working fine, then suddenly I hear that annoying sound of the chain lightly rubbing on the front derailleur. I THINK I have index shifting? I always get confused. It clicks when I shift and I have to adjust it just right.. Anyways, I can't get this chain to stop hitting. I've tried the high/low screws and can't get it to work. Any help? rather not go to bike store as they charge $15 to just fix that..

DannoXYZ 

There's a fine-adjustment called trimming. It's very seldom that you can set up an FD with zero-rubbing as you shift the rear. Usually you can do about 4-5 gears in back. Then trim the front, then do the remaining ones. Then reverse the process when you shift the other way.

There are two critical adjustments on the FD that makes a HUGE difference in how much sweep you can have in the rear:

1. FD height. The lower and closer the FD is to the chainring (1-2mm gap), the larger the angle the chain can bend before touching the sides.

2. cage-rotation. Contrary to what's taught to bike-shop muppets, you DO NOT want the outer-cage parallel with the chainring. Follow the guide on Park's website and set the outer-cage parallel to the chain in the tallest gear (big-ring/smallest cog).

mzeffex 

How do you do this "trimming" you said?

oldbobcat

"Trim" is a half-stop or a light click that moves the cage over enough to stop the chain rub but not enough to move the chain. If your limit screws and cable tension are correct, you should have six distinct positions for the front derailleur cage to accommodate different states of cross-chaining. You can find them by flipping the levers just far enough so they click.

DMF 

Well, I don't want to alter a time-tested procedure, but I find with mine (6503) I have to rotate the FD past that point, with the tail pointing visibly inward, to get full coverage.

You're right, though. Get either one of these critical settings wrong and your final setup will be ... substandard. Change one and you start the whole adjustment procedure over. (Which ain't necessarily a bad thing.)

DannoXYZ 

Interesting... The idea is to have the outline of the cage be outside of the sweep-range of the chain. From above, the path of the chain flares out towards the back from side to side. So the path is like a dove-tail shape.

In the old days where FD had parallel sides, this caused a problem because the back was too narrow. Then Shimano started to flare out the back of the FD-cage to make room for the chain. But if you put the outside parallel to the chainring, it's only flared out on the inside. So by rotating the rear towards the outside, you end up with an equal amount of flare on the inside and outside.

This is of course assuming a straight chainline with your chainrings lined up near the middle of the rear cluster. If you've got chainline that's inwards of centerline of the cogs, you'll want to rotate the FD cage outwards at the rear. More often than not though, the chainline is outside of centre and you end up having to rotate the FD cage inwards at the rear to cover the sweep.

AndrewP

Low price FDs on department store bikes often dont have index shifting, but they make a series of little clicks as you move the twist shifter through its range. If this is the case you need to look down at the chain and see which side of the der cage is rubbing, then move the shifter to move the cage over by a small amount. "Trimming" is a feature on index front shifters that will move the cage by fraction shift.
The H and L screws are to limit the shifter moving too far and throwing the chain off the outside or inside.

DMF 

I'm starting to suspect that something is bent somewhere, but I've been to lazy to follow up on it.

Your prose gives me a decent picture of what the adjustment is trying to accomplish. (A drawing would be useful if you know of one.) Maybe after a Black Russian* I'll go downstairs and contemplate it in light of newfound er ... enlightenment.



*That's with Patron Café. I've had my fill of Kahlua (which admittedly sounds incredible).

mzeffex 

I have a Specialized, not a "department store" bike..

Also, I just went to the bike store and the nice guy there who's done a lot of stuff on my bike did it for free.