decisivemoment

I'm looking at a new 105 rear derailleur and notice the Shimano specs for the medium cage specific a minimum cassette of 11-28 and a maximum of 11-32 -- and they do specify those ranges and not just the individual sprockets. Does this mean there's too much slack chain to run a 12-25 or 11-25 and you'd have to go for a short cage for that? With the short cage topping out at 28T (or maybe 30T at a pinch), that seems a little inconvenient as I'd kind of like to be able to run 12-25 most of the time but something like an 11-32 for a vacation to a hilly area. The concept of maximum capacity makes perfect sense but is there a minimum capacity below which you cannot go if you don't want the derailleur cage to come right back up into itself?

FBinNY 

IMO there's no such thing as a minimum capacity for a derailleur. After all. every cassette includes smaller spockets, so it it'll work with the rated 28t, it'll also work with the 25, or even the 12.

I suspect that minimum capacity specs. aren't intended to be a minimum as much as the smallest size at which they're necessary. So, if your cassette tops out at 26t, you don't NEED to use the medium cage, but that doesn't mean you can't use it.

Feaduin

You can run a more closely spaced cassette if you want. What the minimum means is that is the range of cassettes that the derailleur is going to shift optimally on. Going to a more closely spaced cassette will mean that the jockey wheel will not be able to maintain consistent spacing off the cassette cogs. That being said, I doubt that the difference is going to be that noticeable.

beanbag

The two numbers you need to look at are "Total capacity" = 37T and "maximum sprocket" = 32T.

Total capacity = (big+big combo) minus (small+small combo).

For example, if you have 50/34F and 11/32 R, then the capacity = (50+32)-(34+11)=82-45=37

This combo will just barely work, namely the big-big will be very tight and the small-small will be very loose.
If you swap the cassette to 11-25, then the small-small will be equally loose and the big-big will be a little looser than before.

Just keep in mind that you'll be keeping the same chain when swapping cassettes back and forth, so don't accidentally make your chain too short for the 11-32.

raria

This

Bike Gremlin

No, there is no minimal capacity. With enough max. capacity and optimal chain length, the RD cage will remain within it's working "sweet spot". However, getting a long cage RD where a short cage one (with smaller capacity) would serve fine does have a (really minor) flaw: you get some needless extra weigth and perhaps arguably slightly (if measurable at all) slower shifting.

decisivemoment

Eventually ended up setting it up with the 5800 long cage and the 11-28 cassette. Seems to work very well though it doesn't look quite right on the bike.....and I inadvertently set the chain too short for a bigger cassette so if I do add something for a mountain goat before the current chain wears out I'll just have to get a new chain early. Losing the #16 sprocket feels strange but it has been an excuse to use the 17 and practice faster cadence.