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newbie gear question. Double or Triple crank(3 posts)

newbie gear question. Double or Triple crankbigwaves
Feb 2, 2003 6:31 AM
I am building my first bike. I am currently riding a triple crank. For my new bike i would like to go with a double crank. There are a lot of hills in my area that i sometimes use the "granny to get up" Is it posible to get fairly close to the low end of a triple crank with a double by gearing and crank arm length?
stick with the triple...C-40
Feb 2, 2003 7:52 AM
Gearing is simple to calculate and compare. Just take the chainring teeth and divide by the cog teeth. If you multiply this number by 27 you get "gear inches" which is a meaningless number but it's a universal number used for comparison.

For example, if you have a 30 tooth inner chainring on your triple , with a 27 tooth largest cog, the gear ratio is 30/27 = 1.11. 27 x 1.11 = 30 inches. If you were to use a double chainring, the standard today is a 53/39. The only way to get close to the same lowest gear ratio would be to use an MTB cassette with a 34T largest cog. This would give you a nearly identical lowest gear of 31 inches, but the gaps between the cogs are quite large and not great for road use. You will constantly be shifting up and down, only to find that the ratio you need isn't there.

If you stick with a triple you can use a more closely spaced cassette that will be much more pleasant to ride. As you gain experience, you may find that you no longer need the triple. At that point you may want to consider a double.

The other thing to note is how low a cog you use with the triple. If you have a 30/27 but don't use anything lower than a 30/23, then you may want to purchase a 12-23 cassette which has the important 16 tooth cog. 9 speed cassettes with a 25 or 27T largest cogs don't have a 16T.

I'm assuming that you are only considering 9 speed equipment.
re: Well, maybeRusty Coggs
Feb 2, 2003 9:00 AM
Depends on what you are using now. A 30 granny with a 23 big cog gives 35 gear nches.A 39 x29 using a campy 10 double gives 36 gear inches.That's the best you can do with road stuff.A shimano drivetrain using a mtb cassette and RD can give even lower GI,but you might not like the bigger jumps between cogs.So it depends o what GI you need and how you want to get there.