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According to Shimano’s claimed weights, the R9150 Di2 group for rim brake is 2051 grams, 50 grams lighter than 9070. The R9100 mechanical group for rim brake is 2097 grams, four grams lighter than R9000
For disc brakes, Shimano is claiming a group weight of 2389 grams for the 9170 group, and 2445 grams for the 9120 mechanical shifting group.
http://www.bicycling.com/bikes-gear/road-bikes/first-look-shimano-dura-ace-r9100
By erm, 800 grams?