Gearing Question

[Guest]

Hi Alex, cool stuff. quick question:
I have a road bike with Shimano 105 double chainring cranks 53/39 and Sram 9speed 12-13-14-15-17-19-21-23-26. I'm thinking of doing a hilly century and some suggested I change the small chainring 39 to a smaller 30t (available for triple cranks) and then replace the 12t, 14t & 26t cogs of the rear cassette with 25t, 27t & 30t... is that really practical? or do-able at all?
Thanks!
Jason

[Alex Ramon]

Hi Jason,
Thanks for your comments and great question. I must admit that one of my weaknesses is calculating gear ratios, and I don't want to give you weak advice, so I have to suggest that you post your question in the following forums:<br />
http://www.bikeforums.net/forumdisplay.php?f=10
It's the best place I know of to get technical advice from the many participating bicycle experts.

[Guest]

Alex, thanks for getting back to me, and fast!
Well, let me re-phrase my question, forget about gear ratios, that's hard for any expert to answer because it depends on how strong the rider is, the steepness/grade, head wind, distance rode, etc... Let's just assume that I'm trying to experiment with certain range of chainrings and cassettes, my question for you is:
Is it mechanically possible to modify my current setup of (shimano105 double chainring cranks 53/39 and sram 9speed 12-13-14-15-17-19-21-23-26) to:
1)53/30; 13-15-17-19-21-23-25-27-30<br />
- or -<br />
2)55/30; 13-14-15-16-17-19-21-24-28
I don't wanna buy the parts and then find out later that, the front chainring range of 55t to 30t doesn't fit in the front derailer - too large of a jump? Having a 30t at the rear cassette would require a change of the rear derailer to something that has a longer cage, so that the pulley have enough clearance for the 30t?
I've also heard something about BCD sizing of the chainring/cranks that might make my suggested modification impossible? i thought you might have a better idea of what BCD means and how it'd affect the modifications i had in mind. given these limitations, can you suggest something that'd work and yet closest to what i have in mind?
Thanks,<br />
J

[Alex Ramon]

Sheldon posted a whole archive of helpful information on gearing here:
http://www.sheldonbrown.com/gears/
Hope that helps!