Rear Gears need to be compatible with the gear levers you intend to use with them to give you the correct movement per click to suit your sprockets. Genrally speaking you keep to brand although some combinations of makes produce useful results. A good source of information about mix 'n' match is here

The maximum capacity of a gear is in 2 forms: Firstly a maximum difference between the biggest (in number of teeth) combination of ring and cog you want to use and the smallest.

Eg If you ride 34-50 chainrings and have a 12-25 cassette the maximum capacity of you gear must be at least (50+25) - (34+12) = 29

Personally I would never use (or encourage anybody else to use) the bottom rear cog on the outer chainring so without the 50x25 option which would twist sideways excessively, I would do the calculation as:

(50+23) - (34+12) = 27

Assuming the next to bottom cog was a 23 of course.

The other capacity issue is the maximum botom cog the gear can cope with. Modern racing short cage gears usually can cope with a 29 but beyond this size a touring gear will be needed.

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