Shimano 105 compatible 13- or 14- cassettes I've just purchased a bike with a 105 group-set, 50 + 34 chainrings and a 11-34 cassette. I was expecting to be able to replace the cassette with a closer ratio one, with a slightly lower top gear: maybe 13- or 14-. However the shop I purchased it from said the 105 rear mech wouldn't accommodate cassettes with a top ring smaller than 11 or 12. Is this accurate? Surely if the derailleur can handle the combination of 50T chainring and a 13T or 14T sprocket two or three gears down from top, it can handle it 50T combined with 13T or 14T as top gear as well? Wouldn't a 14T-32T cassette require less forward _and_ backward take-up from the derailleur, compared to the 11-34 cassette that's currently on the bike?

Modern rear derailleurs do in fact have a min and max smallest sprocket specification as well as largest min and max largest sprocket specification. You can find the specs here.

The min and max size of the sprockets is not just affected by the chain slack the derailleur can take up, it's affected by the angle of the line the cage moves along relative to the center plane of the bike to maintain separation from the cassette. This is different for short and long cage versions of derailleurs - a long cage derailleur moves at a different angle than a short cage to track a wider range cassette.

The current RD-7000 SS (short cage) and GS (long cage) min and max sprocket specs are:


SS GS
Low Max 30T 34T
sprocket Min 25T 30T
Top Max 14T 12T
sprocket Min 11T 11T


So if you have a short cage version you can use a 14 tooth smallest sprocket.