Bicycle Mechanics - FD big ring troubles
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09-04-11, 12:31 PM
This is a '79 Fuji Grand Tourer btw. I'm having serious trouble shifting from the small ring to the big ring. I've followed Park's derailleur adjustment guide and I have adjusted gears on other bikes without trouble before this one. The issue is the shift from the small to the big ring is very slow if at all. If I adjust the limit screw out to the point where it throws the chain the shift is still just as bad, it just culminates with the chain being thrown. This FD is different than my other bikes in that when the cable is slack the derailleur shifts to the big ring (and yes the cable is totally slack when shifting to the big ring). There is a wobble in the crank but the shift from the large to the small cog is crisp and precise so i'm doubting that is the issue but maybe i'm mistaken. I've also tried lubing the spring in the FD to no avail. Anyone have any ideas? This is driving me nuts.
09-04-11, 01:05 PM
I suspect you have too much friction in the system. Is there a section of housing on the cable where it leads to the derailleur? Often the dependent loop of this housing collects moisture and eventually rusts, causing increased friction and poor shifting. You can try dripping oil into the housing to loosen things up, but often replacement (with a modern, plastic-lined housing and cable) is the best solution.
09-04-11, 01:46 PM
Is the chainline ok ? If the BB axle is to long ,(moving the chainrings to far from the frame ) then the FD can not reach (make the shift ) . yes check the housing and replace if needed to .
09-04-11, 03:13 PM
Chainline looks fine and the BB crank arms and chainrings are original. I hadn't thought of the cable housing. I've got some extra laying around that i'll try.
09-04-11, 04:48 PM
Anonymoose: Don't confine you inspection to the housing; rusted/corroded or frayed inner wire, especially inside the lower housing run, will cause problems. You mentioned lubing the derailleur spring, how about the pivot points? Pull the exposed inner wire like drawing a bow; does the derailleur move freely by hand and return quickly and smoothly to the limit screw via the spring?
09-04-11, 06:09 PM
I switched out the stem shifters for dt suntour powershifters recently but like a dope did not switch the cable housing (the cables themselves are new) out at the bottom so I will definetly check that when I get home.
This FD is different than my other bikes in that when the cable is slack the derailleur shifts to the big ring (and yes the cable is totally slack when shifting to the big ring).
If I'm reading you correctly, you're FD is reverse sprung, meaning the cable pulls it down to the inner ring, and the spring returns it to the outer. If that's correct, you have very few options. Nothing you do with the cable or housing will help since the cable doesn't control this shift. Focus on making sure the FD can move freely and crisply under the spring, then the alignment will be key.
I've found that often rotating the FD so the cage is slightly (not more than slightly) tail out can help the shift to the outside, though it makes trimming more difficult.
09-04-11, 07:27 PM
You are correct about the reverse sprung fd and I did align it tail out briefly but found it was even more apt to throw the chain. I'll try to fiddle more with the alignment and lube in case it is getting hung up returning to slack.
09-04-11, 08:05 PM
You may be able to prevent the chain-throwing with the limit screw. I almost always adjust FD's with a little tail-out.
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