I posted this in the Mechanics Forum, but I thought I might get more response here. My apologies for it not really being C&V.

I am about to install an Ultra Drive 10-speed 12-30 rear cassette onto one of my bikes and I am contemplating the use of a Campy 10-speed medium cage rear derailleur. This would be the derailleur that came out when Campy came out with the first 10-speed 13-29 cassettes. The documentation on this derailleur indicates the max cog is 29, but I'm thinking that may be because Campy's max cog at the time was 29. In any case, I am interested in knowing if anyone out there has used this derailleur with a 12-30 and if it handles that cassette acceptably. Or must I go with a long cage dereailleur. This bike will be for mountain riding so I am sure the large cog will be used occassionally. Would the concern with this setup be adequate chain wrap or would the concern be pulley clearance from the cassette or are both of these things issues? Thanks very much for any assistance!

A lot depends on your chain length and the sizes of your chainrings. You can probably push it to 30 under the right conditions -- try it and see if you can make the jockey wheel avoid the largest cog. Another option is to swap individual cogs, as I often do to get the ratios I want, and make your own 12-29.

Would that be as simple as taking a 29 off a 13-29 and replacing the 30 with it, since it's the last cog?

It's impossible to say, as a lot has to do with your particular set-up. The most important factor is the length of the derailleur hanger. They vary and depending on what you have you may get more or less than 30T. I've seen derailleur capacity affected by up to 4T depending on the hanger. The other factor is the length of the axle slot in the dropout and where you position your wheel. A long slot and running the wheel at the back will often allow you to run extra teeth. The final factor is proper adjustment of the B-screw which affects the clearance to the large cog.

I have the new 12-30 paired with a Centaur 50/34 and Centaur med cage RD. The dropout is a Campy 1010, I believe.

Zero issues.

That's good to know. I believe a standard 53-39 would pose less of a challenge to the medium cage rear derailleur with the 12-30 cassette than your 50-34 both in terms of chain wrap and pulley positioning. Does that make sense or not?

I have a short cage Veloce derailleur, the 13-29 10 speed cassette and a 50/34 crank. It's outside the official range, but it works great. The small-small combination has the two pulleys folded way up, but the chain still has a little clearance there. So I would think a medium cage should work on your 12-30.

First, Campy's recommendation before the 12/30 was to use the medium cage derailleur with a compact and the 13/29, and to use the long-cage for a triple with 13-29. I have a 13/39 with triple on one bike, and the long cage. If I am not careful with chain length (number of links), I can get interference between the jockey wheel and the large rear sprocket. I would take Campy's recommendations as they state them, unless I have time and $$ for a lot of experimenting. Shimano is said to have margin in their specs, but I'm not sure if it's true for Campy.

Second, the 12-30 has a spread of 18 teeth, where the 13-29 has a spread of 16 teeth and a larger large sprocket. That might have been enough for Campy to alter the rear mech geometry, unless the cassette is intended for compacts.

I really don't have experience with making a 12-30 work. But I do have a lot of Campy experience, and I would go with their guidelines.

The OP's proposed setup will exceed rec's by one tooth.

I've worked on 10sp bikes where a 13-29t exceeded the short-cage Record derailer's rec's by fully 3 teeth, with but a trace of pulley contact, so would expect a med-cage to handle 12-30t without complaint.

I don't see much variance in dropout hanger length on contemporary frames, almost none, but the use of horizontal dropouts will allow adjustment for better or for worse. I don't expect such good shifting with the axle slid back in horizontal dropouts.

And hopefully the OP won't be using the near-entire range of the cassette while in the small ring, that's just wrong imo (crossing from small-to-small), but many of today's gearing setups encourage this if the rider isn't fast enough to fully use the big ring. At least this can be the issue that the big 12-30t cassette (and a compact crank) somewhat solves imo.

I can recommend the OP's proposed fitment, even with a compact, if the rider isn't wanting to run too far into the small-small range and respects the limits of minimum required chain length (+2 links) when setting it up.
The other methods of chain-length sizing are not safe when using sprocket sizes outside of the rear derailer's prescribed complete gruppo.

It only takes a 1/3" to make a 2T difference. Most manufacturers offered at least two different lengths and we don't know the dropouts or era of the OP's frame.

Thanks very much for all the knowledgeable responses. The bike is being built up and in spite of Road Fan's very accurate rendering of Campy's chronology and specifications with respect to the release of the medium cage RD, I am going to give the medium cage RD a shot with the 12-30 cassette. I won't come anywhere near the small small combo that dddd mentions so that should not pose a problem. And I am hoping I don't have to fudge desired wheel placement in the dropout as T-Mar suggested.

I also wanted to mention that I posted this same thread on the Mechanics forum (after all I think this qualifies as mechanics) and I received a few responses but nowhere near the detailed and thoughtful analysis that I received here. Many thanks!

Keep us posted as to how it works out.