I am riding a 11-25 9 speed and want to swap to 11-30.
I have a 105 derailleur and a triple front crank. Will the rear 105 take the 30t? From what I have read, I think that since I have a tripe up front, the rear 105 should be either a medium or long cage?
Road RDs of this era only came in short (SS) and medium (GS) cage. You presumably have the GS now, which generally is what goes along with a triple.
If it's RD-5500 or RD-5501, regardless of cage length, its specified max large cog is 27t. Sometimes by either maxing out the B-tension screw or swapping in a longer one (M4x0.7) you can coax them to go somewhat larger. 28 usually isn't a problem. 30 might be. It depends on the exact shape of the dropout. If it doesn't work, the penalty is the large cog grinding to a greater or lesser degree against the the pulley when in the lowest gear.
Usually there's no way of telling for sure how well it will work until you put it all together and try it.
The other issue is that depending on what's up front you'll probably be exceeding the total capacity, which basically means you'll have to live with slack in the small/small combinations once you've put a chain on that's long enough to safely accommodate the new large/large combination. This entails some risk of sudden derailment and all accompanying dangers to life and equipment if you accidentally shift into the slack combinations.
Remember that Shimano compatibility are based their expectation that you may be cross-chaining a drive-train as shown in the illustration below. Their charts ALWAYS allow a drive-chain to cross-chain as they do not know the experience of the rider that the bike is being sold to at the point of manufacture and design.
This illustration above shows improper shifting and no experienced rider ever does this.
This illustration above shows proper shifting on a triple . You can imagine from this that even on a double crank that proper shifting would never be in a cross-chain situation.
That being said, I have fond that you can almost always increase 2-3 teeth on a low cassette gear. As Nathan said, you may be able to squeeze and 11-30 on with a b-screw adjustment to maximum. Being a 105 you may be able to get 11-32 as the lower derailleur models like Tiagra and Sora do not have as well designed parallelogram as the upper levels. For instance, the Sora line from 6-8 years ago had a problem doing more than 2 additional teeth comfortably but since models changed last year with the addition to 12 speed, Sora is now a redesigned Tiagra derailleur that in most situations could comfortably increase a cassette by 3 teeth. So not only the model of derailleur but also the years or age of it makes a difference as well. Derailleur hanger position/style to dropout position plays a roll and and sometimes 1-2mm can make all the difference in whether it works or doesn't.
As for capacity, you could solve some of this by eliminating the smallest front chainring or adjusting the front derailleur stops from allowing it to drop down to it. Triples have been deprecated for a reason and even with an older 8 or 9 speed, a double allows enough combination to get up the steepest of hills unless a person is very out of shape and in that case it would only take riding on flatter roads for 2-4 months to build up the strength. Pro racing setup did not have triples even for mountain stages and these triple cranks were primarily for touring setups where one would have panniers and cargo going through mountain passes.
All of this information is not only taken from my own experience, but others and many quotes on the web to back up the notion that.
You would do well to google Sheldon Brown as this guy has a knack for modifying and customizing bicycle equipment, namely drive-trains apart from the documented, working methods and standard and getting them to work smoothly. He has actually talked about this very subject and has many a plethora of online documentation and guides.
Shimano 9 speed road and mountain parts are all compatible. Therefore you can add a 9-speed mountain long cage derailleur to use an 11-30 cogset on the rear along with your triple front. I do so on my tandem, and know many others who do the same. It falls within Shimano's guidelines for compatability. Not necessary or desirable to turn the B-screw all the way out.
Shimano 9 speed mountain derailleur is compatible the shimano 10 speed shifters - but not officially. Reference: http://www.velonews.com/2011/01/bikes-and-tech/technical-faq/technical-faq-with-lennard-zinn-brake-questions-compatibility-and-new-years-feedback_155209 I use this setup on a touring-cross bike: triple crankset 50-30, 11-32 10 speed cogset, 9-speed MTB derailleur. It works well but margin of error is smaller than "normal", resulting in more frequent tuning and replacement of parts as they wear out (chain wear, derailleur play). No problem with slack. Not necessary or desirable to turn the B-screw all the way out.
It should be noted that long MTB derailleurs will be slightly heavier, and also the pulley wheels have enclosed bearings, while road pulley wheels don't. Enclosed bearings means a fraction of a watt of power is lost to resistance. Not a big deal for touring, randonneuring.
