tangerineowl

Wondering if there's something specific about the shape/length of the inner? cage plate which determines how successfully it will catch and move a chain from a 24t to a 40t ring ?


I was previously running a 26/40 but am trying to get a 24t to work (12/27 cassette; I really like the cog spread) using the same 10sp Red front derailleur.


Outer plate is currently parallel with large ring, and about 1mm or so above closest point to large ring.


Chain drops down fine onto the small ring, using the shifter.


When i shift up, one of the inner plates on the chain catches/sits on one of the four pins on the inside face of the large (ramped and pinned) ring (which I've read this is ideal) but the chain doesn't (most of the times) lift up that little bit higher and move over onto the large ring teeth. Both rings are clocked in the correct position.


With the front derailleur shifted in its outward position I can manually by hand place the chain on the large ring without any cage rubbing in the majority of the cassette spread (which from what I understand is how things should be mostly).


So I'm wondering if perhaps there's something about the shape/length of the inner? cage which is causing this? or maybe the cage requires a slight toe out (or toe in?) to put a little more leverage on the chain when its moving that last bit from the pin and up onto the teeth ?

Andrew R Stewart 

Everyone talks about cage height above the large ring. Few mention cage height above the middle (triple) or inner Double) ring. It's also important and a factor to pushing the chain against the next larger ring. 16T difference between the rings is a fairly large one. Remember that 16T out of 40 is a much larger percentage of difference then 16 out of 50. Andy

HillRider

Can you loosen the high limit screw to let the front derailleur move further outward and more over the big chainring? Just be careful not to overdo it and have the chain spill to the outside.

WizardOfBoz

Are the large and small chainring designed to be used together? That is, are the pins and ramps and such designed for this large/small tooth combo?

Is the derailleur up as per spec? It sounds like the front spacing is ok (Shimano recommends 1-3mm IIRC, I take your word for it that 1mm is correct).

Besides the position above the chainring (and it being parallel or angled per the mfrs instructions), the two things that affect the shift seem to me to be limit screw position and cable tension. It may be useful to check that your limits are set up as per spec (just because they worked with the "easier" chainring combo doesn't guarantee this). The high limit screw could be tweaked but when this is done its important to ensure that the cable is taut so as to actually move the FD cage to the limit. I'd try to set limits up as per mfrs specs first, and check out messing with cable tension a bit.

Do you have a barrel adjuster on the FD? Or an inline one? On my Shimano brifters, I can push the handle in to shift to the larger ring, but also I can push it a little further to give it a little oomph. I don't have to as the system works as advertised, but if I needed to I could put the handle to move the FD up a little. It would go back down to the detent position when I released the handle but (if you could do this) it might be enough to put the chain onto the large chainring.

Setting the high limit screw to allow a little more travel (adjusting cable tension to exploit this) might help, but then the chain might rub when the larger rear cogs are used in the rear. Of course one shouldn't cross chain but the FD cage in a position too far outboard could limit even normal chainline transmission.