Join Date: Apr 2009
Location: New Rochelle, NY
Bikes: too many bikes from 1967 10s (5x2)Frejus to a Sumitomo Ti/Chorus aluminum 10s (10x2), plus one non-susp mtn bike I use as my commuter
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I've not made a study of the cage lengths of various FDs, but if you have short chainstays and the tire overlaps the FD cage with only narrow clearance when on the granny, your options may be limited.
The inner position of the cage is based on the chainline offset of the granny, the outer clearance of the tire, by it's width. A shorter cage might reduce the overlap from the side of the tire to the crown giving you more room.
Otherwise your two choices boil down to a longer spindle to move the chain line out a bit if that's practical, or resigning yourself to slightly narrower tires. You might also buy a few millimeters by fudging the alignment of the FD slightly and bringing the heel out a bit, but any more than a tiny bit will probably hurt performance.
OTOH if you rarely use the granny, you can resign yourself to keeping the wheels aligned and leave things as they are.
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