The Ultegra RX Shadow RD+ rear derailleur is Shimano's first Di2 road rear derailleur with an integrated chain stabilizer mechanism. It allows for a quieter experience and smoother shifting on mixed terrain. The chain tensioner or clutch can be switched off using the ON/OFF button, for rides on smooth terrain.
It is a bit of a hybrid that sits somewhere in between the MTB and Road rear derailleurs - just like the GRX groupset.
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So why does this derailleur exist you ask? It was developed and released a few years before GRX and provided a clutch mechanism for those who wanted to ride gravel on their road bikes.
It has a total capacity of 39T, with a max front difference of 16T. The largest sprocket max is 34T, the smallest low sprocket you can use is 28T.
On the other end of the cassette, the top sprocket max is 12T while the top sprocket min is 11T. Shimano are known to be a bit conservative with their min/max limits, so you can probably run a 36T/38T max and 10T top if you really wanted to.
The derailleur supports direct mount, but you can also mount it on a standard derailleur hanger.
Indexing, trimming, or fine-tuning Di2 rear derailleurs is done through micro-adjustment. Read how to set up your Di2 rear derailleur perfectly and ride without those grinding noises.
Read how to install the RD-RX805 / RD-RX815 / RD-RX817 Di2 rear derailleurs.
Did you know that you can mix Di2 components from different series and categories? Mixing derailleurs is not recommended, but anything else is fair game!
Synchronized Shifting - what is it exactly? What does it do and how can you set it up? This page will guide you through the process of setting up synchro shift.
Shimano front derailleur and rear derailleur specifications... what do they mean, really? This page explains all of them, and why they matter.
