Chain rubbing on the front derailleur

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08-30-2008, 12:51 PM
Flur

Chain rubbing on the front derailleur

I dropped off my soon-to-be commuter road bike at my LBS to have my crankset replaced (going from a 52/42 to 50/34) and they also recommended that I swap out the front derailleur, so we did that too. Today I picked it up and rode it home and found that when on the smaller chainring in front if I try to go to the smaller (not even the smallest) chainrings in back, the chain rubs on the front derailleur. The same thing happens when on the larger chainring in front and the larger chainrings in back.

I called the shop and the guy told me that I'll have to adjust the front derailleur when I'm in those gears. Is this really normal to have to do this? I didn't have to before with my old setup, and I don't have to on my newer bike.
08-30-2008, 01:05 PM
Becky

Compact cranks are wonderful, but a little more sensitive to cross chaining than your old 42/52. It's not uncommon to have some chain rub when in the 34 and the smallest cogs in the back. That 16-tooth difference (50-34) is a lot and, as a a result, the chain can drag across the outer plate or even the teeth of the 50.

You can tweak the FD a little, but you'll never be able to use the 34-11 or even the 34-12, nor should you because you're pretty cross-chained at that point. If your left shifter has a trim stop (half-click), that will help with the rubbing too.

Hope this helps!
08-30-2008, 03:41 PM
Flur

I've only got 7 cogs in the back.... if I can't use 2 of them, that seems like an issue that the LBS should have told me about. :(
08-30-2008, 03:57 PM
Flur

After fiddling with it a bit more it seems like the derailleur itself might be hitting the larger chainring just slightly... Maybe the guy that put it on put it too low?? I'm going to bring it back to the shop when my regular guy is there on Tuesday and have him take a look.
08-30-2008, 04:09 PM
Triskeliongirl

I can use the 50/11 or 34/11 combination on mine, actually all combinations with no problems and no trimming (50/34 in front and 11/34 in back). But I put a triple mountain FD on even though its only a double.
08-30-2008, 05:31 PM
Becky

Quote:

Originally Posted by Triskeliongirl

I can use the 50/11 or 34/11 combination on mine, actually all combinations with no problems and no trimming (50/34 in front and 11/34 in back). But I put a triple mountain FD on even though its only a double.

I would venture to say that the triple is what's making the difference. It probably has a wider swing. Even so, I'd be reluctant to use the 34/11 or 12 just because of the extreme chain angle. 50/11 is no problem.

I can use my 34/13 for short periods of time, but not my 34/12 (running a 12-25). It took me a lot of fussing with cable tension to get the trim click in the right place to get the maximum number of usable gears.
08-30-2008, 06:12 PM
Triskeliongirl

I do avoid the most extreme positions because it reduces chain life, but my point is that by using a triple FD I can use any combo I want without rubbing and trimming, which is very nice.
08-31-2008, 04:08 AM
OakLeaf

Quote:

Originally Posted by Flur

I've only got 7 cogs in the back.... if I can't use 2 of them, that seems like an issue that the LBS should have told me about. :(

Unless you've got a corncob in back, those aren't "usable" gears anyhow (i.e. they almost certainly overlap with gears you can get on the other chainring). You're really not supposed to be able to use every gear combination, because of cross-chaining.
08-31-2008, 12:14 PM
Flur

Quote:

Originally Posted by OakLeaf

Unless you've got a corncob in back, those aren't "usable" gears anyhow (i.e. they almost certainly overlap with gears you can get on the other chainring). You're really not supposed to be able to use every gear combination, because of cross-chaining.

Yeah, I've found out this is not even the real issue. The issue is that the derailleur is mounted too low and is hitting the highest teeth on the larger chainring when moving from the larger chainring to the smaller one and vice versa. With the 1/2 stop trim (as he recommended) to get to the MIDDLE gears in the back (I can only use the first 3 without the trim), the derailleur actually hits the chainring teeth consistently through the cycle. I'm a little PO'd b/c this means the guy that did it didn't even run through all the gears and recheck his work before he sent me home on it. He would have HEARD it when he went from one chainring to the other in front - its really loud. And it's pretty easy to see, after looking at my other bike, that it's too low. It's going back to the shop on Tuesday, when my regular guy is there, to be fixed.