Time for a comparison between the new 12 speed Dura-Ace Di2 rear derailleur and the previous 11 speed Dura-Ace Di2 rear derailleur. On first glance, they look pretty much similar, but there are definitely some differences that are not obvious at first sight.
From the above picture, the overall structure looks similar. Both still use the same adapter design to attach to the frame derailleur hanger, and the motor unit location looks similar.
Outer link design is different, as the new design language looks more angular compared to the rounded design of the previous version. Colour wise, the new derailleur uses a uniform black anodizing, instead of the grey to black gradient on the previous version.
Inner link seems to be made of fibre reinforced plastic, either with carbon or glass fibres. End of the rivets are covered with a black sticker for better appearance.
Motor unit location is quite similar, tucked away behind the rear derailleur and under the cassette. Limit screws are located more or less at the same positions.
Saver unit design looks similar as well.
Inner plates are made of carbon fibre, although the new 12 speed cage is quite a lot longer since it has to accommodate 11-34T cassettes as well.
Cage length of new 12 speed Dura-Ace rear derailleur R9250 is about 93 mm.
Cage length of old 11 speed Dura-Ace rear derailleur R9150 is about 72 mm.
Another view showing the differences in cage length. There is a stopper plate on the left that is molded into the outer plate, as compared to a separate stopper pin seen on the right.
Some differences in the outer plate construction as well. New plate on the left has the plate stopper molded directly into the carbon plate, as compared to a separate stopper pin as seen on the right side.
Here is the biggest difference. New derailleur on the left has a charging port, hidden under a cover. It also houses the wireless units and other electronic stuff for wireless communication with the 12 speed Di2 shifters.
Weight of previous R9150 rear derailleur is just 197 grams, very lightweight.
New R9250 rear derailleur is slightly heavier at 216 grams, but packs a lot more electronics into it. It also has a longer cage to cater to larger cassettes.
Overall, the biggest changes would be the longer cage for larger chain capacity, and the integration of electronics to allow wireless communication with the shifters, plus charging as well.
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