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Front Derailleur Crunch
#1
Evening All. Back again for another teaser.

I've been working on a family member's mountain bike, it has Shimano Revo gripshifters and a Tourney drivetrain.

I offered to replace the cables which were frayed and rusty, the rear mech has gone back together and is shifting fine, but the front derailleur - absolute nightmare.

At this point it's worth mentioning, whilst an amateur, I have successfully stripped and rebuilt my own two bikes, one a road bike and a one a hybrid, so I'm fairly confident in the basic set up of derailleurs.

So, the problem is that the front derailleur does not always shift from middle to large chainring. Sometimes it will shift up cleanly and crisply with no fuss, other times, it looks like the chain is slightly twisting and jamming against the outer cage. Drop the shifter back and try again and it will shift perfectly.

I've raised the FD - no change, in fact I would say it made it worse. I've lowered it - no change, maybe worse. The FD sits about 2mm above the large chain ring for best results.

Cable length/tension (depending on how you prefer to refer) makes no noticeable difference. Limit screws set up as per standard guidance and thereafter adjusted either way to try and make a difference - it will either come off the crank side or not change at all, so I revert back to where it 'should' be and I'm back to intermittent success.

I can only assume that if it is changing cleanly 50-75% of the time then the set up is ok as otherwise it wouldn't work at all?

I then do a bit of Googling and consider worn chain, worn chainrings, worn shifters...the can of worms opens.


I've had a similar problem with an 8 year old Bianchi Dama with a Campag set up - no crunching or twisting of the chain, just no shift up from middle to large chain ring except when on the work stand where it works flawlessly. Under load, it won't shift up.

Maybe it's me.
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#2
try rotating the FD slightly (very slightly, like one degree or so) around the seat post.

1 to 2 mm above the large chain ring when the derailleur edge is centered on the chainring is Shimano's normal recommendation.
Nigel
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#3
Don't forget to put the drive side crank arm at 3 o'clock position to take a look at the 1-2 mm measurement
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#4
(06-01-2013, 12:23 AM)nfmisso Wrote:  try rotating the FD slightly (very slightly, like one degree or so) around the seat post.

1 to 2 mm above the large chain ring when the derailleur edge is centered on the chainring is Shimano's normal recommendation.

Thanks nfmisso, I had tried that, both front end and rear rotated, I may have rotated too much I guess, but it didn't help.
(06-01-2013, 01:04 PM)bobtravers Wrote:  Don't forget to put the drive side crank arm at 3 o'clock position to take a look at the 1-2 mm measurement

Hmmm... Didn't do this. Have to say out of the numerous online guides I've read and my bike bible this is never mentioned. What's the theory behind it? Are the chainrings not truly circular?
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#5
Not on every chain ring, older Shimano Biopace, Q-rings, and others are oval instead of round... So if you always take a look at the measure between FD and chainring with a 3 o'clock crank arm position, you'll be setting it right at every time!
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#6
(06-03-2013, 01:45 AM)bobtravers Wrote:  Not on every chain ring, older Shimano Biopace, Q-rings, and others are oval instead of round... So if you always take a look at the measure between FD and chainring with a 3 o'clock crank arm position, you'll be setting it right at every time!

But not ever oval ring is at a maximum at 3 o'clock - works for Biopace, but not all of the others.
Nigel
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