Question: Much has been discussed regarding the compatibility of Shimano 7900/7800 systems but what about the new Ultegra 6700 with the old 6600? I recently sent my crankset to Shimano for warranty repair and Shimano was kind enough to replace the unit with a new 6750 crankset complete with BB and chain. How well will it work with my Ultegra 6600 drivetrain and would I benefit from replacing the cassette with a 6700 series to make full use of the newer asymmetrical chain?
Wayne Stetina, who is responsible for R&D at Shimano American, and who has a long racing pedigree, had 7800 cassettes on all of his 7900 and Di2 test bikes the last 2 years with the 7900 chain, including when I rode his bike set up with Di2 in Boulder City during the 2008 Interbike. So, similarly, there is no compelling reason to upgrade to a 6700 cassette before your 6600 cassette wears out.
Stetina does warn that the 6750 crank and chainrings with your 6600 shifting system will require very precise front derailleur and cable tension adjustment to prevent chain rub in top gear because the throw of the shifter/front derailleur combination isn’t quite as far as the slightly wider 6700; it’s actually closer to old 9-speed front chainring spacing width. So the potential problem with that front combination is that, if the front derailleur isn’t perfectly aligned, either the front derailleur won’t want to stay all the way to the outer limit stop, or else it will be rubbing in the trim position in the big/big combination, or in the small/big combination. Wayne has found that it seems to help if the front derailleur is very slightly toed out, so it clears more easily in the large rear cog on both rings, yet still can be tight enough cable tension to hold the front derailleur all the way to the outer limit stop.