Put a FSA Compact Crank on the wifes Dual. Sized the chain, but when you goto the small front ring to the 12 or 13 cog on the back the chain rubs the big ring. Is there a fix for this…or is it something you just have to deal with?
I think this is something to be generally avoided.
Cross chaining like that should be avoided. That said, I think the drop from the 50 to 34 is a large one. She may be better off with a 48, or even 46 big ring.
I have fairly short chainstays, so I generally avoid the 3 smallest cogs in back with the 34 chainring. If you hear a slight ringing sound, you know you’re in a cog you shouldn’t use. Switching to a 50/36 or a 48/34 alleviates this somewhat.
-jens
First of all you shouldn’t cross chain that much. That should be avoided.
You don’t mention the size of the bike but if it has shorter chainstays that chain angle will be more extreme and may cause some rubbing. Look at the chainstay length.
If the CS are “normal” length, is the crank on too far? If somehow you overtightened the crank arms that might move the crank just far enough inside to make the chain angle too much to get to the lower gears w/o hitting the BR.
Just some thoughts.
EDIT: I misunderstood the post. I thought that problems was chain too long and rubbing on small cogs. Didn’t read carefully (and reference to chain sizing made me think it was chain length issue). If rubbing on big chainring, then no real solution except avoiding being cross chained. END EDIT
I had the same issue and for a long time just avoided being cross-chained as noted by others above. Then I installed a new chain and made it a link shorter. Now I don’t rub when I’m in 34 and 12, but chain is almost, but not quite, too short in 50/27 combo. (See Park Tool chain sizing guide: http://www.parktool.com/repair/readhowto.asp?id=26 ) However, I’d rather have occasional use of 34/12 and just avoid 50/27.
The best solution is a long cage rear DR – the type used for triple front DRs. Normal DRs are not meant to handle compacts. See article by Slowman on ideal bike specs and scroll down to drivetrain: http://www.slowtwitch.com/cgi-bin/parse.pl?url=http://www.slowtwitch.com/mainheadings/product2006/tribikes/perfectspec.html&text=chain
Or, I can’t imagine he’d mind if I quote him:
While the front derailleur “problem” may be phantom, the rear derailleur issue is real. Standard Shimano derailleurs are built to work best up to 26 total teeth or so, that is, you add the difference between chainrings to the difference between large and small cogs and you get “total teeth” (that said, standard Dura Ace spec allows for 29 total teeth). The system I’ve described has a 16-tooth diifference between the 50 and 34 tooth chainrings, and a 15-tooth difference between the 12 and 27 tooth cogs. This 31 total teeth range requires a longer cage than comes on a standard Shimano road derailleur. I would therefore spec Shimano’s “IRD 7800 GS”, which is its Dura Ace rear derailleur made for triple chainrings. No, I’m not running a triple, it’s that longer cage I’m looking for. (If I wanted to make a down-spec’d, cheaper bike for sale I’d just pick the corresponding longer-cage version of Ultegra or 105).
Put chain in position to check shortness (see parktool) and by hand double up the chain one link to see how stretched out the RD gets. You might be able to drop one link as I did. Indeed, if your wife’s biggest rear cog is 25, you are closer than me to being within limits of standard rear DR ( 50-34 - (25-12) = 29 ).
Had this exact problem on a bike last night. The complicating issue is that the drop from big to small ring is too big. Makeing the big ring smaller or the small ring bigger would most likely improve things.
You may also try a longer bottom bracket (impossible with external bearrings). The longer BB will improve things on the 13 and 13, but the chain may drop off the front chainring to the inside if the BB is too long. Be wary of that.
The last thing to do is check the frame alignment (best to do this at a shop and it takes 5 min) to see if the stays are shifted to one side or another. If they are, then you’ve got a larger issue.
I’ve noticed as a general rule of thumb, the chainstays on bikes with steep seat tube angles tend to be on the short side, which makes it more difficult to get a good chainline in all gears.
If it really annoys you and have exhausted all possibilities, you can just ajust your derailleur limits so that you only have an 8-spd (or 7).