Originally Posted by
ericm979
I think that the 30t cog will not work well on the older (7800/6600 and prior) short cage rear derailleurs that have a nominal 27t max cog. You can push them somewhat, especially if you tune the B screw, but 30t may be too far. It may differ depending on the frame's chain hanger and where it positions the derailleur.
If you have a 50/34 or 52/36 compact then those derailleurs definitely won't be able to handle the chain wrap. You can push it a bit past the specification but that's too far.
I use a 50/34 and 12-30 for extra difficult climbing races. I use a 7603 triple rear derailleur for it.
While the rated max cog on 5600 & 6600 is 27, there is plenty of safety factor built in. I've installed 11-32t cassettes with great results. I do not recommend the 11-32 without qualification, since an unusually short derailleur hanger could cause a problem.
Chain wrap is much the same issue. Most derailleurs will accept more than the rated value. The Specialized Tarmac SL3 Pro uses a 52 & 36 crankset with an 11-28 cassette and this combination exceeds the chain-wrap specification more than a 50 & 34 crank and the 12-30 cassette drivetrain. The chain length just needs to be precise since any extra length will cause a chain to sag with the small-small combination. I would not use a 52 & 36 compact with an 12-30, but a 50 & 34t compact should work as will a 53/52 & 39 standard crankset.
Having said that, installing a Ultegra 6703 "GS" long cage derailleur is an ideal solution for users buying a new rear derailleur and installing an 11-32 or 12-30.
I would hope that Sram or Shimano would create a 1070 or 6700 level 12-29 cassette would also include the 16t and would have the following cogs: 12, 13, 14, 15, 16, 17, 19, 21, 24, 29. This would avoid all the issues and would be lighter than the 12-30.