Bicycle Mechanics - front derailleur problem
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03-27-11, 02:50 PM
posted this in the road forum as well. hoping someone either here or there can help.
halfway through my ride yesterday, i got stuck in the big chainring. shifting still moved the FD, just not enough to push the chain down. i worked on it for a while today with some help. messed around with limit screws, cable tension, etc. it's not stuck in the big ring any more, but...
if i adjust it so that it shifts UP ok, if i push the shifter all the way, it sometimes throws the chain off the outside of the big ring.
if i adjust it so that it doesn't drop the chain, it won't shift up at all.
FD is shimano 105 5600 braze-on. shifters are shimano 105 5600 (for double or triple). crankset is fsa gossamer pro compact BB30. bike is relatively new (bought it new in august 2010), so it has less than 1400 miles on it, 400 of which are on the trainer.
thoughts? does this seem like a problem with shifter, derailleur, or cables? or maybe even crankset/chainrings?
If this were a newly set up FD, I'd say your cable is too tight. But cables don't magically get tighter, so we have to consider other possibilities.
I'd start by looking to see if something is binding the wire or mechanism anywhere.
For the moment forget the shifters and see if the derailleur works when you shift by drawing the wire away from the frame like a bow string. If necessary use this method to reset the inner and outer limits. Also check to see if the cage is at the right height (1/16" above) and parallel to the outer chainring.
If the FD doesn't work in "manual overide" than you have a FD problem, possibly something binding the pantograph, or a broken return spring.
If it works by the bowstring method, you know the FD itself is OK and something is binding the wire between the frame and shifter. A common cause is the housing extruding partly through the fulcrum (frame stop) or ferrule causing it to neck down and pinch the wire.
03-27-11, 07:15 PM
i'm not a trained mechanic, but based on what i've read, the derailleur position (height and parallel) looks ok to me.
i did use the "bowstring" method to set the limits. still had the same problem - if i adjust it so that it shifts up, it throws the chain, and if i correct that, it won't shift up. it's like there's no middle ground for some reason. i had the same chain-throwing problem about a week ago, but i reset the limits and adjusted everything and was able to get it to work fine. don't know what's different now.
might just end up taking it in to the shop, but i like learning to do this stuff myself, so i at least wanted to try.
Just to check, is the FD model one correctly matched to compact cranks, and the right width, ie. 9s or 10s.
I 'm noodling with why you can't get upshifting without over-shifting. The only thing I can come up with sight unseen is that the cage is too wide. But that would have been true from the beginning. I'd continue shifting by the bowstring method and observe carefully as it shifts and see if you can see why it's slow or overshoots.
03-28-11, 09:47 PM
FD is, according to manufacturer's website, "105 braze-on front." don't know what it's specific to...i can check the actual FD tomorrow for a model number or something but the bike isn't with me at the moment.
went over it today with a friend who's a pretty good mechanic. he adjusted derailleur positioning, limit screws, cable tension, etc., and couldn't fix it. think i'm just gonna take it to my LBS tomorrow and see if they can figure it out. if it's an FD problem, maybe i can just get a new one under warranty, since i got the bike new less than a year ago.
thanks for the help, though.
03-29-11, 09:35 PM
I've seen this problem a countless number of times with 105 triple front shifters set up to run on double chain rings. The cable tension is correct, but the outer limit screw is a little loose. Everything is working fine until the rider slams into the big ring a little harder than usual. The cable stretches (or the housing compresses) just enough to let the shifter click into position for the non-existent third ring, and now the cable is too tight against the ratchet to allow a downshift. Then the rider forces the downshift and breaks the ratchet inside the lever. This will also happen to a cable that has stretched over time. The rider shifts again and gets that third position.
The first thing to do is dump some cable tension, by either loosening the barrel adjuster, loosening the limit screw, or unfastening the cable from the derailleur. Then, with a loose cable, use the bowstring method to reset the outer limit screw. With Shimano 10-speed systems the cage needs to just barely clear the chain. Finally, readjust cable tension so the cage hits the the limit screw simultaneously with the lever clicking into the second ring position. Fine tune for trim and you are done. Recheck cable tension periodically.
03-30-11, 10:40 AM
oldbobcat - you might be right about how it got stuck in the big ring, but that problem was fixed pretty quickly and the lever seems to be fine. we did that and reset the limit screw with the cage just barely clearing the chain in the big ring and it wouldn't shift up at all, and if we moved it out more, it threw the chain off.
as for that chain-dropping/adjustment problem, took the bike to my LBS yesterday. guy looked at it for about 5 seconds and told me the FD cage was bent. he took some pliers to it and now it shifts WAY better than it ever has since i bought it new last August. makes me think it was already bent when i got it, because it never quite shifted up the way i expected it to (i had to basically stop pedaling), but it does now.
so problem solved. thanks for the suggestions though.
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