Using Shimano FDs With 42-Tooth Large CWs

For some years now Shimano front derailers for “compact” drives have specified a minimum 12 -tooth difference between the large and middle chainwheels and 22-tooth maximum difference from the large to small. So, most compact drives are 22/32/44 chainwheel sets. But, some people, like me, are understandably cranky about having to replace their 22/32/42 CW sets just to use a more recent model of Shimano derailer.

Perhaps this is well known (though I have never seen it mentioned in instructions and guides for setting up FDs), but there is a logical and easy way to get good performance with 22/32/44 FDs for a 22/32/42 set: use 4 to 5 mm for the clearance of the outer cage plate over the large CW. That is, simply clamp the FD at a level proper for a 22/32/44 set.

If, instead, you follow the set-up instructions and establish an approximate 1 mm cage clearance over a 42-tooth wheel, the FD cage level will be grossly low. The main result is that a 32-tooth middle CW (only a 10 tooth drop from 42) is virtually certain to rub against the inner cage plate of the FD when the chain is shifted to the large CW (in my case, the chain also strongly tended to jam for the shift to my 42-tooth CW). The resulting CW rubbing sound might be mistaken for chain rub, but close inspection will show the derailleur cage to be pulsating sideways a bit. (Note: remember that we are talking about the more recent models of Shimano FDs – other FDs might not have this problem).

The usual 1 mm clearance for the outer cage plate over the large CW is critical only for avoiding chain rub on the bottom of the cage for some gear settings – that is, the cage “height” (distance between the upper and lower ends of the cage (as clamped to the bike) is fairly tight; so the derailleur must be clamped to barely clear a 44-tooth CW on a 22/32/44 set. But, when using a 42-tooth large CW instead of the spec’d 44 tooth, the cage has an excess of close to 4 mm in height, because the radius of a 44-tooth CW is close to 4 mm larger than for a 42-tooth CW. So, using a 4 to 5 mm cage clearance over a 42-tooth CW in a 22/32/42 set should be OK.

The above modified set-up procedure worked quite well, in my case, anyway. This is predictable, since the procedure establishes a fully proper (that is, as per FD design) level of the derailleur cage for shifts for the inner CW’s of 22 and 32 -- and at least close to proper for shifts for the 42-tooth CW.

Hope the above helps. Of course, it won’t cure other FD problems that can arise.