Road Cycling - chainring compatibility

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kato
06-29-02, 11:00 AM
I just bought a new bike and thought it had a 39T small chainring instead of the 42T that it really has (Ultegra SG). I want to put a 39T on since I'm always climbing hills but when I've searched for a 39 it says match it to a 52 or 53B and my large ring is stamped "A-53" (the small ring is stamped "A-42").

Does anyone know if a standard Ultegra SG 39T ring (130mm spacing) will fit with the "A-53" ring?

Also, for climbing, I'm thinking of getting a 12-27 cassette. Is the gear ratio difference between a 42T and 39T more or less than the difference between a 23 and 27 (in the rear)? It could be that I'll be fine with the 23 in the rear if the 39T ring makes a significant difference.

Thanks for any help.

:confused:

SteveE
06-29-02, 04:07 PM
Check out the following URL: http://home.i1.net/~dwolfe/gerz/index.html

A 42F/27R gives you a gear of 42 inches. A 39F/23R is 45.8 inches based on 700C wheels.
pokey
06-29-02, 09:16 PM
Shimano says that for best shifting performance the letters should match.In actual practice it does not seem to matter. There is no 'A' type in a 39.

kato
06-30-02, 09:53 AM
Thanks for the input, I'm not sure why I didn't think of comparing gear inches ... not all there yesterday. The gear-inch link is great, and sounds like the chainrings work together fine, I think I'll try a 39T with a 12-25.

Thanks again!

patrick
:beer:
D*Alex
07-01-02, 06:55 PM
FWIW, gear inches have absolutely nothing to do with the size of the wheel/tyre on the bike. Meter developement, however, does.
John E
07-01-02, 07:56 PM
Originally posted by D*Alex
FWIW, gear inches have absolutely nothing to do with the size of the wheel/tyre on the bike. Meter developement, however, does.

Huh????!

Gear inches = wheel_diameter * chainring_teeth / cog_teeth

48/13 is 96 gear-inches with 26" wheels, 100 with 27"