Bicycle Mechanics - Front Derailleur Knocks Off Chain: Limit screws no help

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abstractform20
01-19-10, 10:04 PM
on the last step of building my first bike, and all i need to do is adjust the derailleurs, cables, etc. so shifting works...

-the FD is aligned properly
-the FD is not too high or too low on the large chainring (sitting at 2mm)
-the limit screw is set to push out the FD as far as possible
--> the FD still rubs and pushes the chain onto the smallest chainring...

what am i supposed to do here?

fetad
01-19-10, 10:14 PM
Sounds like you need to increase the cable tension.
abstractform20
01-19-10, 10:25 PM
will do

abstractform20
01-19-10, 10:42 PM
didnt work....is there a chance my crankset/bb are not screwed in tight enough? im so lost right now...
kludgefudge
01-19-10, 10:57 PM
For starters, you need to disconnect the cable and pull the cage out by hand, to ensure the FD is actually capable of performing the task. double check that you are adjusting the limit screw correctly. I say this because you don't set the limit screw to "push" the derailleur out. For "as far as possible", it might as well not even be there. provided that your derailleur is capable of going where you want it to with no cable, ensure that your shifter is pulling the derailleur to its limit.

For better answers from more qualified people, specify the brand/model of your frame, derailleur, shifters, bottom bracket and crankset, and its approximate vintage. some stuff just won't work together.
kludgefudge
01-19-10, 11:12 PM
didnt work....is there a chance my crankset/bb are not screwed in tight enough? im so lost right now...


Yes there is. If you can move your Spindle farther inward, do so. crank bolt needs to be TIGHT. also assuming square taper cranks, you may have mismatched tapers or somthing and therefore you won't be able to get your crank on as far as its supposed to go. check the net to try and find out if they are either ISO or JIS standard.
In my experience, if the crank is screwed on "tight", the FD should at least not be pushing the chain off the big ring. rubbing a bit and not being able to shift (under load) from the small ring would be the worst I would expect. I've had it happen.

Have you tried adjusting the derailleur alignment "wrong"? If it can be twisted to the point that it is no longer acting on the chain when fully extended it was never adjusted "right" in the first place