Join Date: May 2011
Location: Oslo, Norway
Bikes: 1951 Armand Carlsen, 1969 DBS Deluxe, 1949 Diamant, 1978 DBS Winner Tandem, 1955 Herkules... to infinity and beyond!
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According to Shimano specs
, an RD-7900 can accept a rear cog of maximum 28 teeth. Usually you can exceed such limits by one or two teeth. My 105 rear derailleur works fine with a 30 tooth sprocket, even though it's rated for 28.
Another thing to bear in mind is the total capacity, or the maximum chain wrap, of your rear derailleur. The RD-7900 has a total capacity of 33, meaning that with the 16 tooth difference between your smallest and largest chainring (50 minus 34), the difference between the smallest and largest rear cog can only be 17 teeth. Again, you can overstep the specs a bit as long as the RD doesn't double up on itself.
Last edited by VeloBrox; 07-11-12 at 08:00 PM.
Reason: Fixed link.