Help me out - Front derailleur is driving me crazy

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jockster
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Joined: Fri Apr 13, 2012 7:25 pm

by jockster

It appears that no matter what I do, I can't get my Shimano front derailleur adjusted properly despite having done this plenty of times in the past with good results.

When I shift to the inner chainring & most inward cassette sprocket, the chain rubs against the inside of the derailleur arm, without cable tension and inner limit fully unscrewed. It is like the front derailleur is too narrow.

I've tried to set it higher than needed & adjusted yaw angle to the extent that the braze-on attachment allows, but then I instead get "opposite" scraping when riding the large chainring and small sprocket. What can I do here?

eric
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by eric

Have you measured the chain line? http://sheldonbrown.com/chainline.html

Which crank, bb, derailleur, chain, cassette?

How long is the frame's chainstays? Short frames have larger chain angles.

by Weenie


jockster
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Joined: Fri Apr 13, 2012 7:25 pm

by jockster

eric wrote:Have you measured the chain line? http://sheldonbrown.com/chainline.html" onclick="window.open(this.href);return false;

Which crank, bb, derailleur, chain, cassette?

How long is the frame's chainstays? Short frames have larger chain angles.



Frame is a 54cm Cervelo with 399 mm chainstays. Chain line is about 44mm (centre of frame to centre of big chainring)
Crank: Rotor 3D
Front derailleur: 105 5700
Chain: DA 7900
Cassette: Ultegra 6700 12-23 - with one spacer between hub and largest sprocket
BB: DA 9000 & 105 5700 - no difference
Chainrings: 5700 & Rotor QXL - no difference

I just purchased a bag of chainring spacers that perhaps would allow me to angle the FD inwards to clear the inner chainring whilst spacing the large chainring outwards. That could maybe be a start?

mattr
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by mattr

Chainline sounds a bit narrow?
Thought shimano double was 43,5, 44mm to the outer will give you *about* a 40 mm chainline, might be too narrow for the 105 front mech. Might need to grind a bit off the front mech stop or body (or, if its still resting on the screw, take a couple of mm off the end.)
Twisting the front mech to clear will just give you a rub somewhere else, raising it will give you poor shifting. So those are two options i'd not bother pursuing too far.

An other alternative (that i've done with some MTB mechs, on botched together systems) is to locally tweak the inner front mech cage, just to give yourself an extra mm or two.

jockster
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by jockster

mattr wrote:Chainline sounds a bit narrow?
Thought shimano double was 43,5, 44mm to the outer will give you *about* a 40 mm chainline, might be too narrow for the 105 front mech. Might need to grind a bit off the front mech stop or body (or, if its still resting on the screw, take a couple of mm off the end.)
Twisting the front mech to clear will just give you a rub somewhere else, raising it will give you poor shifting. So those are two options i'd not bother pursuing too far.

An other alternative (that i've done with some MTB mechs, on botched together systems) is to locally tweak the inner front mech cage, just to give yourself an extra mm or two.


I looked it up - my front derailleur has a specified chainline by Shimano of 43,5mm and the crankset specified at 44,5, so this might very well be the cause of the issue.
Last edited by jockster on Sun Oct 06, 2013 9:29 pm, edited 2 times in total.

afalts
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by afalts

Yep seen this before, especially with same setup as you (Rotor, Cervelo and Shimano).

Look for what's blocking the FD from moving more (not always the limit) and use a dremel.

mattr
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by mattr

Could be it then.
I used to work on a Metron TT bike years ago that we had masses of issues with, as the shape of the frame meant that the front mech wouldn't even shift onto the small ring most of the time. Too much monocoque in the way!
IIRC we ended up filing a couple of mm of metal off the back of the mech to allow it to swing further over.

jockster
Posts: 87
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by jockster

What do you guys think about using a triple front derailleur instead?
Triple FD's from Shimano are built for 45mm chainline which is only .5mm away from my cranksets chainline.

mattr
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by mattr

Chainline is in the middle of the two rings, (or middle ring of three). Not on the CL of the outer.

So a triple mech won't help much, you need to find out what is stopping it moving inboard, is it the body of the mech, the linkages, the stops? (the frame?)
Then dremel a bit off. So it can swing in another couple of mm.

Or get the chainline right.

User Name
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by User Name

is it a BB30 or similar?
If not, and this may sound like butchery in this forum, but what about shimming out the right cup with a 1mm shim?
I've done this on one old-ish frame, because the small chainring rubbed on the stay

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mythical
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by mythical

jockster wrote:It is like the front derailleur is too narrow.
Bend the cage to be wider? I recommend to do this carefully though, should you go that route. I've done it before and it always worked.
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Mr.Gib
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by Mr.Gib

Heard about this before with some cervelo's.

Ironic perhaps but the 10 speed Sram Red Yaw FD would probably solve the problem. It's the only FD that I have ever used that had no rubbing in any gear combinations.

(This assumes you are using compatible shifters.)
wheelsONfire wrote: When we ride disc brakes the whole deal of braking is just like a leaving a fart. It happens and then it's over. Nothing planned and nothing to get nervous for.

eflayer
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by eflayer

not sure why you are having this problem. but what about the idea of a small spacer between the bb shell and the screwed in bearing? that would move the crankset outward just a bit and keep the chain from rubbing on the inside of the derailleur blade. not sure how this would affect shifting, but would definitely move the crankset outward...just a tad. spacers are made that slide on over the bearing threads and right up against the inner face of the bearing.

jockster
Posts: 87
Joined: Fri Apr 13, 2012 7:25 pm

by jockster

eflayer wrote:not sure why you are having this problem. but what about the idea of a small spacer between the bb shell and the screwed in bearing? that would move the crankset outward just a bit and keep the chain from rubbing on the inside of the derailleur blade. not sure how this would affect shifting, but would definitely move the crankset outward...just a tad. spacers are made that slide on over the bearing threads and right up against the inner face of the bearing.


I do have a few unused spacers laying around from MTB Hollowtech bottom brackets that I could use on the driveside, but wouldn't this cause issues with engagement of the left crankarm on my Rotor crankset?

I hate to loose a crankarm mid-ride.

jockster
Posts: 87
Joined: Fri Apr 13, 2012 7:25 pm

by jockster

I lent a dremel today and grinded off the stops on the derailleur. Had to add longer limit screws afterwards and don't think I can remove more material from it. I tried bending the cage a bit too.

From having chain rub in the four largest cassette cogs, I only now have it when riding on the largest cog and small chainring!

I have however contacted Rotor and will ask what their tolerance on bottom bracket widths are and what they say regarding adding a small spacer, 0.5mm or so, to move the crank more outwards.

by Weenie


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