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Discussion Starter · #1 · (Edited)
I would like to know which gear combinaisons should I avoid on a 10-speed road bike (53x39T, 12-25T). I guess the biggest chainring - biggest rear sprocket and smallest chainring - smallest rear sprocket should be avoided, but what about the biggest chainring - 2nd sprocket? When I compute the gear ratios, it looks like I could use only the 12,13,14, 15, 16, 17 & 19 T sprockets while I am using the biggest chainring and use the smallest chainring with sprockets 15, 16, 17, 19, 21, 23, 25 ...

Rear / Front / Ratio
12 / 53 / 4.42
13 / 53 / 4.08
14 / 53 / 3.79
15 / 53 / 3.53
16 / 53 / 3.31
17 / 53 / 3.12
19 / 53 / 2.79
21 / 53 / 2.52 <= What about this?
23 / 53 / 2.3 <= What about this?
25 / 53 / 2.12
12 / 39 / 3.25
13 / 39 / 3 <= What about this?
14 / 39 / 2.79 (same ratio than 19 / 53)
15 / 39 / 2.6
16 / 39 / 2.44
17 / 39 / 2.29
19 / 39 / 2.05
21 / 39 / 1.86
23 / 39 / 1.7
25 / 39 / 1.56
 

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I try not to cross-chain any more than I have to. I completely avoid the big ring and the big sprocket, and the little ring and the little sprocket. Any other combinations I don't worry about too much. Usually if I need to be on the upper end of the casette I want to be in the small ring anyway.

I have about 7000 miles on my casette, and usually get about 2500-3000 miles out of a chain. Hope that helps.
 

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It all depends on your chainline. One of my bikes allows me to ride a "big-big" (53X23) without any problems or noise. On this bike, I am not able to use a 39X12 or 13.
On a different bike, I can not use the same 53X23, or even the 21, but I can ride the 39X13 without any noise.
 
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