Shimano 7900 Front Deraileur
 

I have recently aquired a BMC SLC 01 with Shimano 7900 groupo, incuding 50 / 34 Compact and 12 / 27 rear. Shimano, and many others claim that in the large chainring (50) one can use the complete cassette without chain rub on FD plates. Even though I know about the dreaded cross chaining, I cannot adjust to get no rubbing in 50 / 27 and 50/24 . I have followed Shmano install / adjust directions exactly, as well as other online sources and rechecked my methology several times.. no luck. Anyone out there with similar experience and / or suggestions? Thanks

It is possible that the chainstays on your frame are so short that the derailler cannot be positioned to allow for noise-free operation in all gears. Possible, though not likely.

There are a number of causes of chain rub on the front derailler. Start with the high stop screw. Make sure that it is set such that you have just enough clearance in the 50/12 combo to not have chain rub. Any excess clearance on that end will reduce your clearance when using the larger cogs. Next, check the angle of the cage. You want the outer cage parallel to the chain when using the 50/12 combo. If the derailler is angled any further out than than it will again reduce your clearance when using the larger cogs. The derailler height might also be a factor depending on the shape of the inner cage. You'll want the derailler positioned as close as possible to the top of the larger chainring in order to maximize clearance (assuming a shaped inner cage. Not being familiar with DA7900, this may not be applicable).

Who claims? If that claim is true why the heck is there trim on the front derailleur? Why does shimano specifically tell you about this in their ST-7900 document?

50x27? Im not a shimano person but the chain will be pretty crossed in any bike i believe. And if you adjust the FD to do not rub in the 50x27 probably it will rub in the 50x12 because the chain cross a little bit at the other side also. I dont know shimano brifters but there is a way to adjust little buy little the FD position using the brifters? With campangolo u can adjust tiny bits but no idea if shimano does that.

ps: why to use 50x27 when you can use maybe 34x19? and the chainline will be almost perfect, and gearing ratio is almost the same. That you can do something doesnt mean it is the best to do specially when the chain will be so crossed. Just something to think off.

I recall reading about DA 7900 when it first came out and it was said that Shimano got rid of the front derailler trim function. Did they add it back in at some point or was that a mistake?

joejack951 - Thanks for the reply. I have checked and brechecked all items you mention. Despite Shimano's "no cage rub"statement, maybe 50 / 27 is just too much crosschain angle. I wonder f a 12 / 25 would achieve Shimano's claim? Thanks

Operator - Thanks for your reply. To answer your questions: (1) Shimano claims. (2)The trim on the FD only applies to small ring (34) but unlike the 7800 series there is no trim on big ring (50). I'm nottoo sure this was a brilliant ïmrovement by Shimano ???

ultraman6970 - Yes, you are 100% correct and I subscribe to the "crosschaining is forbotten" theory, but in an anal engineering mind, why does Shimano make "no rub"claims if in real life it is not attainable. As I said earlier, maybe 50/27 is too much, in which ase the "no rub"claim should be qualified

The size of the largest rear cog should have little to no effect on the derailler rub. The angle of the chain is virtually unchanged when simply changing the size of the largest rear cog.h

Aside from your derailler set up, one other item that could affect your ability to cross chain is your chainline. Are you using a DA 7900 crank? Are you using a non-standard wheelset of some sort that might position the rear cassette further inward or outward?

joejack951 - yes, the whole drive train is DA 7900 with DA 7800 wheels. I did check the chainline and it is within a mm of the 45 mm recomended by Shimano ... weird, eh? I will play around some more but honestly have worked with Shimano 7800 on other bikes and have never had so much trouble setting up / tuning. Maybe I'll just set" up as per 7800'and accept the rub in 50 / 27 as slap on the wist for even thinking about using that combo. Thanks for your suggestions.

SRAM Rival has the same "no trim needed" claims, but I haven't been able to reach that nirvana on my CAAD9, and I've read a number of similar reports. I admit that the chainrub is minimal, and I will use that gear in a pinch.

I think it's a lousy design choice. Even in cases where it's possible to adjust the bike without chain rub, you have to get the cable tension exactly right, have perfectly true chainrings, perfectly stiff BB, etc. Trimming the FD is not such a hardship that I'd give up robustness to not have to do it.

If it makes you feel any better:

from http://www.parktool.com/repair/readhowto.asp?id=75

Kind of a copout IMO, but it's something I tell myself to keep me from endlessly fussing with it.

stedalus - I am comming around to believing you are correct and that I just need to "get over" not being able to achieve the"nirvana"that Shimano , and others apparently, lay claim to. Bottom line...... get out , between windy, spring snow storms and enjoy a great bike and almost "perfect" gruppo. Thank you all kindly for your suggestions and guidance to reality !
Regards, IGD

All -Ah, ha, I've got it !! With advice from Joseph at LBS, I spent some time with chain in 50/12 and made some fussy "microadjustments" to cable tension with barre adjustor and high limit .. very, very "picky" as am I, sorry about that ! Shimano, your 7900 FD adjustment procedures leave a lot to the imagination !! Long live 7800 series