On determining the proper size for your big chainring:
The best thing to do is look at the cogs that you use most on your cassette while in your big ring. If you tend to be in the middle cogs most of the time, leave your big chainring as is; if you are on the bottom, smallest cogs most of the time, consider going with a bigger chainring; if you are in your biggest cogs in back, then go with a smaller chainring.
The same principle holds for selecting a small chainring.
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