Shimano's largest road cassette is 27, Campy is 29. But you can put a mtn bike cassette and derailleur on the back of the road bike to get lower gearing. I did - XTR and the cassette is 34 so I have a 12 - 34. I did it because of a cross country trip but like it so much have kept it on (plus I can't find the 27 - think I loaned it to someone and never got it back). Shifting is a tad clunkier but love the granny when needed!!!!