Originally Posted by
SaiKaiTai
Gotcha... which makes sense since the indexing would have to match up with the different cog spacings.
As for wanting to use alpine gearing, no, not quite... I actually would like to be able to use the 12-30 I have on my LeMond on a new, Ultegra equipped bike I'm thinking of buying; just 2 or 3 extra teeth on the cog but they matter.
You may get away with using the 12-30T cassette on the new bike, but there's a good chance the Ultegra or any other Shimano road rear derailleur won't be able to handle the 30T cog (i.e. the upper pulley will bump into it when in that gear). You may have to get a MTB rear derailleur. This chart is Shimano spec and should be used as a general guideline only (it's possible to work outside it):
Shimano Rear Derailleur Compatibility Chart:
Road SS ("short" cage)............Double chainring....27T or smaller cassette
Road GS ("medium"/"long").....Triple chainring.......27T or smaller cassette
MTB GS ("short"/"medium").....Double chainring.....28T or larger cassette
MTB SGS ("long" cage)............Triple chainring........28T or larger cassette
Models are "upwardly" compatible (that is, a given model will also work with the setups listed above it).