strop

Okay, usually you can get away with this kind of mix and match even though Shimano says they are not compatible (not even B level compatible), and I'm usually the first to push the envelope but I have to say I wouldn't recommend this.

I recently bought a 2010 Specialized Tarmac Pro SL which came with mostly Dura Ace and Ultegra brakes, FD, cassette and chain. The ST-7900 only barely pulls enough cable on shifts to span the the big and small cogs on the FC-7900 39/53. Yes it works but you have to adjust it very carefully so that the plate isn't rubbing on the chain and if you shift up then pedal the chain wont jump, you have to be pushing the lever AND pedaling for the chain to move. Back pedaling has twice caused the chain the jump inwards and end up wrapped around the bottom bracket.

There is also no trim on either big or little ring, despite at least one Shimano document claiming there was trim on the little ring but not the big. Bottom line is I will be replacing with an FD-7900 at the first opportunity.

DMF 

Thanks for the info.

operator

I ride 7900 daily. ST-7900 features trim on the small ring, that is false information.

strop

For a properly set up system with 7900 I believe that is the case although at the moment I have no way of testing this. In order to make the 6700 work you have to cable it so that the trim position selects the big ring. If you want the full swing to lock on the big ring you have to push the lever ridiculously hard and to the point where the lever sticks in the swing position (yes, I've seen it do this while the service manager at the LBS was trying to shift this morning). At the risk of breaking a $300 DA shifter, I'll take the no-trim option. Hopefully this works a lot better with the FD-7900 which I'll get in a few days. I'll post an update when it's installed.

strop

Finally got everything adjusted properly. This was using the FD-7900 but I suspect the FD-6700 could have been made to work better than I had. Yes there is trim on the little ring but no trim on the big ring, although you really don't need either the cage is that wide on the FD-7900.

The key to making it work was putting plenty of slack in the cable before you tighten the cable clamp. Shimano do say this in the manual but it's so counter-intuitive to anyone who has worked on bikes that I suspect like me they figured it couldn't possibly be true and went ahead and pull the cable as tight as possible before clamping (as you would with just about every other brake or derailleur I've ever worked on). If you do that the ST-7900 won't click into position until the derailleur is at the point of pushing the chain off the big ring and onto the ground, and if you set the end stop (outer screw) to the correct position (0-1mm clearance to the outer plate of the cage) then the shifter won't click into position at all (and you will break the shift lever trying).

With everything properly adjusted there is no chain rub on the cage except for the little front/small rear combo and the trim stop takes care of that by moving the cage out a tiny amount. The shifting is smooth and the although the cable is fairly slack in the little ring it's not bad enough to have it touch the rear wheel, at least on my Tarmac Pro SL frame.

operator

So lesson is RTFM.

GG.

HillRider

Which if followed, would reduce the posting traffic on this forum by at least half.

strop

And leave little else but cynical comments by people who know everything. Not my vision of the ideal forum but to each their own.

And no the lesson is not RTFM. Save that for people who actually post without reading the manual. I thought it was obvious from what I posted that I had done the adjustments as outlined in the Shimano published documentation (not to mention the printed copy that comes with the part). The issue was one of how to interpret the instructions as outlined. I admit I'm obviously a complete dunce for not figuring it out straight away but given the number of people who obviously made the same mistake (OP, my LBS mechanic and whoever originally set up my bike) I figured it would be helpful to clarify the interpretation of what it says in the manual.

So you see, the forum really can serve a purpose more useful than deriding others who are clearly not the uber-mechanics you guys are.