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Chains rubs on front derailleur. Bad chain Line?
#1
Ok, So my chain is rubbing on my f derail. and not matter how much I tweak, it still rubs. I was reading somewhere that this could be caused by too wide of a BB causing a "bad chainline." The shell measures 73mm, will a 68mm fit in there. Or is this even a possibility of problems?

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#2
No! You don't put a 68 in a 73mm BB shell.
If you've not changed anything on the BB / chainset / cranks then it's probably just in need of adjustment.
If the BB is worn though the crank will "wobble" which can cause the chainrings to catch.
See Alex's how to adjust front derailleur:
http://bicycletutor.com/adjust-front-derailer/

[font=Trebuchet MS]Ride hard or ride home alone![/font]
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#3
Nothing has been changed and I am pretty sure the BB isnt worn out. I was juast reading that whenever they assembled the bike or whoever decided what components were going to be fitted, that they may have used a cheap, too wide of a BB for my crankset causing the "bad chainline." So i.e the BB isnt matched with the craqnkset?

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#4
Well you could put a 68 in a 73 and visa versa but it's not done. (except by a crook)
It is feasable that the axle is to long or short (that's the other dimension and is the length of the axle from tip to tip).
Has the derailleur always rubbed?
You could read up on Sheldon Brown,s explanation:
http://www.sheldonbrown.com/chainline.html

[font=Trebuchet MS]Ride hard or ride home alone![/font]
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#5
Do you mean the rear wheel axle? Yes its always rubbed since day 1 and was purchased brand new. Its not one of the best of bikes so thats why when I read that about the cheaper BB not being matched w/ the crankset it started making me wonder if my bike is a victim of this.

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#6
No, the chainset axle.
The axle in the BB that the crankarms are on.
If this is to long then the chainrings will stick out to far.
If you put the chain on the middle chainring (if you have a triple chainset?) and the middle sprocket at the back. Then the chain should be more or less straight.
If the BB axle was to short then it's possible that the inner chainring bolts would catch the chainstay.

[font=Trebuchet MS]Ride hard or ride home alone![/font]
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#7
jstan49
Does the front derailleur catch in all gear combinations? If not at which extremes (front chainset and rear cassette) does it catch…?

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#8
It usually does it on the big cog/ small ring thru 4th cog/ small ring. Big cog/middle ring thru 3rd cog/middle ring, it rubs on the inside of the cage. 6th, 7th, and 8th cog/ middle ring it rubs on the outside of the cage, as well as my big ring/high gear cogs.
I re- read this and confused myself, but this is the best i could explain it.

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#9
Jstan,
I’m not sure what to say… The problem you describe sounds like the front derailleur is not properly angled with the seat post. If you twist the front derailleur (about the seat post) do things change?
Indeed, the symptoms you describe are understandable and follow logically, however I’m at loss to describe why they’re occurring… Concentrate on trying to adjust the front while using middle chainring and all sprockets on the rear. Once you have those sorted the rest will follow …
Sorry, I know that’s no much help (oops)… Maybe take it to a bike shop for advice…

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#10
Hey anything helps. I will try adjusting the position of the derailleur. I would try adjusting the limit screws but I think the derail is broken anyhow. It stills shifts fine, but the limit screws seem to be stuck and whenever I turn them, the derail doesnt move, at all. I think I'am going to break down and buy a new one and see if that helps maybe with a wider cage.

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#11
If you do go down the route of new then try undoing the screws on your existing one with pliers first.
The screws can seize. ( My latest bike, someone had used threadloc on them.)
But if you really study the derailleur you should be able to see where the "arm" touches on the screw ends. (you might have to look from underneath?)
You say you might try a "wider" derailleur.
Does that mean you are trying to use a standard double one instead of a triple?
Can you tell us what shifters and derailleur you have?

[font=Trebuchet MS]Ride hard or ride home alone![/font]
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#12
I can see where the end of screws touch the arm, and when they contact the "arm" thats when the screws sieze up. I readjusted the position of the derail, so now its parrallel with the chainrings, and adjusted the barrel tensioner, now it stopped rubbing on my small ring/thru 4th cog. Havent tried the other rings yet but will let you know.
The derailleur is a Shimano c050
Shifters are Shimano EF-50
Is there a standard for how wide f derailleur cages are?

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#13
Not sure about "standard" for derailleur widths, but certainly triples are wider than doubles.
While adjusting the position of the derailleur make sure that, when on the big ring, there is only 2mm gap between the cage arc and the teeth. (You may have to alter the cable length.)
P.S. As you have discovered - the deralleur arm, when resting on a screw, makes it hard to turn.
Lift the arm off the screw to adjust it.
Sounds like you are getting somewhere near.

[font=Trebuchet MS]Ride hard or ride home alone![/font]
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#14
jstan49,
When I’ve got a seized front derailleur I take it off the bike as soak it in petrol (gasoline), giving it a brush with an old toothbrush from time to time. I don’t but you ought wear skin protection (rubber gloves), when in contact with the petrol.
After a while the derailleur comes out like new, simply re-oil, re-grease and re-fit… Job’s a good un and saves having to fork out on a new one…

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