New answers tagged drivetrain
To specifically answer your question, you will have to change the rear mech (derailleur). If for nothing other than 11sp chain will not fit onto the jockey wheels correctly. You could change the jockey wheels, but they are thinner on 11sp vs 9sp. To take this a step further, you will need to replace all of the groupset components, including the brakes as ...
There are numerous top end bike companies selling their bikes with part 105 11sp builds . You need obviously the 5800 shifters, front/rear mech and chain. But you typically see money being saved by using non 105 brakes and chainset.
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