Originally Posted by Al1943
This question has been asked about a jillion times. Suggest you search the archives for a variety of answers. If you want the bike to be exactly like an original double you'll need to replace the crankset, bottom bracket, and both derailleurs. And the chain will need to be re-sized. Or you can run with the triple derailleurs. Or you can remove the granny chainring and adjust the front derailleur low limit screw.
Replacing the derailers would be throwing money away. That's a silly suggestion.
The chain does
not need to be re-sized. See:
http://sheldonbrown.com/derailer-adjustment
Removing the 30 tooth ring (I refuse to call a ring that big a "granny") would work, but the chainline would be poor as a double with the original bottom bracket.
Assuming this is the 105 Octalink triple, replacing the 118 mm ("triple") bottom bracket with a 109 mm ("double") bottom bracket would, in fact fix the chainline. Sorry I didn't think to suggest this. Only issue is that the little bumps that support the 30 tooth chainring
might hit the chainstay with the shorter bottom bracket, depending on the details of your fram. Most likely this won't be an issue.
Sheldon "Save Your Money For Something Useful Or Fun" Brown