Front Derailleur trouble

LizardEye

Well-Known Member
I've just fitted two front derailleurs, a Sora to my road bike and a Deore to my mountain bike, and I've
had the same issue with both.

When I adjust the height according to the indicator tab, ie 1-2 mm above the chainring teeth, when I
then shift to the largest chain ring, the middle ring catches on the inside edge of the cage.

And not just slight rubbing, the chain cannot physically move as the teeh are wedged against the bottom
edge of the cage. Only by adjusting the height by at least 4-5 mm will the crank turn.

It seems as though either the cage is not wide enough, or the inside plate of the cage extends too far
down. Of course the third optiono is that I'm doing something wrong but I can't see what.

Any clues?
 

Alun

Guru
Location
Liverpool
I've just fitted two front derailleurs, a Sora to my road bike and a Deore to my mountain bike, and I've
had the same issue with both.

When I adjust the height according to the indicator tab, ie 1-2 mm above the chainring teeth, when I
then shift to the largest chain ring, the middle ring catches on the inside edge of the cage.

And not just slight rubbing, the chain cannot physically move as the teeh are wedged against the bottom
edge of the cage. Only by adjusting the height by at least 4-5 mm will the crank turn.

It seems as though either the cage is not wide enough, or the inside plate of the cage extends too far
down. Of course the third optiono is that I'm doing something wrong but I can't see what.

Any clues?
Two derailleurs on 2 bikes suggest operator issues, is the derailleur moving too far out (away from the frame). Try tightening the outer limit screw, so the chain 'just' climbs onto the big ring but can run freely.
 

arallsopp

Post of The Year 2009 winner
Location
Bromley, Kent
Yep. What Alun said. If the cage is hitting the MIDDLE ring, there's something wrong. Take a pic?
Might be double/triple issue, orientation, or alignment issue, but a pic would probably solve it.
 
OP
L

LizardEye

Well-Known Member
You're right, having the same problem twice suggests I'm the cause rather than the components, but having followed
the instructions to thw letter, I still can't work it out.

I'll ty to post a photo later.
 

MJN

New Member
Location
Bristol
Hi LizardEye,

What lateral position is the derailleur in when you're measuring it's height above the outer chainring? It should be positioned with the outer cage directly above (and parallel[1] to) the outer chainring. That is, it should be extended out either by hand or with a cable attached (i.e. shift up at the levers and pull on the exposed section on the downtube to move the derailleur out further if necessary until you've tensioned/indexed the cable later).

Specifically, it should not be in its default next-to-frame (above the inner chainring) position or else it'll end up too low thus cause too tight a chain angle and/or fouling of the chainrings when it shifts. It sounds like this might be what's happening here?

Shimano used to (they may still do) provide a little block to insert into the mechanism to keep the derailleur extended (i.e. to counter the return spring) in order to ease this bit of the measurement because you then didn't have to wrestle against the spring yourself and could concentrate solely on the height before tightening up the clamp. You'd then remove the block and the derailleur would spring back to the frame.

You haven't got Biopace (elliptical) chainrings have you? If so, these require this setting to be made with the chainrings rotated such that its maximum diameter is beneath the derailleur cage.

Mathew

[1] The parrallel position is important to ensure good shifting performance once set up.
 
OP
L

LizardEye

Well-Known Member
Thanks for the reply MJN.

The derailleur is in the extended position using the 'alignment block' and is directly over the outer chainring when I set the height.

After setting the low limit screw, I tension the cable to pull the derailleur into the high position and before the cage is above the
large chain ring, the inside plate catches the edge of the middle chain ring, or in the case of the Deore on my mountain bike, actually
pushes against the side of the chainring so it can't physically move.

It's as though the cage is narrower than the distance between the large and middle chain rings and so the only way to set it is to make
sure the bottom of the inner plate is higher than the top of the middle chainring, which put the outer plate way higher than 1-2 mm
above the large chainring.

A photo would explain it better but I can't get one that shows what's happening.

Anyway it sems to shift ok as it is so I might just leave well alone.
 

MJN

New Member
Location
Bristol
Thanks for the reply MJN.

The derailleur is in the extended position using the 'alignment block' and is directly over the outer chainring when I set the height.
It sounds like you're doing it right. However, with the derailleur in this position what happens when you then remove the alignment block? Doesn't then the inner plate interfere with the middle chainring?

After setting the low limit screw, I tension the cable to pull the derailleur into the high position and before the cage is above the
large chain ring, the inside plate catches the edge of the middle chain ring, or in the case of the Deore on my mountain bike, actually
pushes against the side of the chainring so it can't physically move.
It's this bit that still makes me wonder if you're setting the height whilst the derailleur is above the large chainring (don't take offence at my doubts; just keen to help you out!).

A photo would explain it better but I can't get one that shows what's happening.
A photo of the derailleur in the outer position above the chainring might help.

Anyway it sems to shift ok as it is so I might just leave well alone.
There is always that option. Proper shift operation is not necessarilly a binary thing - it can be fairly tolerant of variations in position without apparent ill effect.

Mathew
 

QuinnDexter

New Member
General front cage travel should not include physical contact with any chainring. If it does this means it's too low or you have a derailleur that wont work on a triple? What make is your front changer?
 
OP
L

LizardEye

Well-Known Member
Ha ha, I realised what I've been doing wrong.

When it says to set the clearence for the high position so the chain clears the outer
cage plate by 1mm, I've been setting it to clear the INNER cage plate. Dumbass!

So the cage was 3-4 mm too far over, hence catching on the middle ring.

So Alun you were right about operator issues, the issue being an inability to read.

Thanks all for taking the time to help out a fool.
 
Top Bottom