I just ordered a Motobecan Fantom Cross Pro. It comes with a TruVativ GXP External Outboard crank with 48-38 chainrings. I would like to make it a compact 50-34. Can I just change the chainrings? It comes with a Shimano Ultegra 6600 front deraileur. Will I need to get a compact deraileur? I'm using this bike as my bad weather road bike more or less. Will use it on some rails to trails, and possibly a little bit of single track. I live in an extremely hilly area, and I have a compact on my road bike. I'd like to have the same gearing on this bike, but don't really want to buy an new compact crank unless I have to. Thanks for any advice....

What about changing the rear cassette to get the gear inches that you're looking for instead of messing with the front?

The cassette is already a 12-27, don't think i can go any less than that with an ultegra RD. I'm looking for the 34-27 gear I have on my road bike...

What about an 11-23?

looking @ the picture on bikesdirect, looks like that crank is 130 BCD. that means that 38 is the smallest inner chainring that you can run. You can still slap a 50 on there if'n you wanted.

You wouldn't need a different front derailleur, just need to adjust it.

As far as your options, you might consider looking @ the IRD wide-range 10-speed cassettes - IIRC, they have 11-32 which would get you real close (lower actually) to your 34-27. I'm pretty sure folks have used them w/o issue w/ ultegra, def. need to adjust chain length though.

Thanks. I'll look into the IRC cassettes if I find the gearing is too high.

Shimano road rear derailleurs are designed to take rear cogs no larger than 27 teeth. You may or may not be able to make one work with a wide range cassette by monkeying with the B-tension (http://sheldonbrown.com/derailer-adjustment.html#btension) screw. If you want to run a 12-32 cassette you should probably get a MTB rear derailleur.

After thinking about this more, I don't want to change the cassette anyway. I want to be able to swap wheels with my road bike easily if I have a problem with a wheel. Sounds like I'm probably gonna have to get a new compact crank for the bike.

After thinking about this more, I don't want to change the cassette anyway. I want to be able to swap wheels with my road bike easily if I have a problem with a wheel. Sounds like I'm probably gonna have to get a new compact crank for the bike.

Good point.