9-speed system to 11-speed drivetrain upgrade? I am currently upgrading my bike from a 9 speed to an 11 speed system.  I am currently using shimano 105 components and will continue to use them within the new system.  Current bike has a shimano 105 rear derailleur but does not specify if it is unique to a 9 speed system.  Will I need to buy a new rear derailleur?
This page has the best answer I've found so far

http://blog.artscyclery.com/science-behind-the-magic/science-behind-the-magic-drivetrain-compatibility/

Reading between the lines, the cable pulls from 9spd to 11spd are probably close enough, but the derailleur ratios are significantly different. 1.4 vs 1.7. This means
that the 11 spd indexing in your new shifters will not work correctly with your old
derailleur.