Or as low as 
Shimano GRX RX815 Di2 Rear Derailleur – 11 Speed
With an integrated chain stabilizer mechanism, the Shimano GRX derailleur provides a quieter experience and shifting security and precision on mixed terrain. For rides on smooth surfaces, the derailleur features and on/off switch for the chain tensioner. The RX815 DI2 version offers the digital precision of Shimano electronic shifting and synchronized shifting. And is compatible with road cassettes with a 34T low sprocket max.
Features
- Silent and uninterrupted rear shifting on rough terrain
- Adjustable chain stabilizer prevents chain slapping and drops
- Shimano Shadow RD+
- Smooth and reliable mixed surface shifting
- Switch stabilizer ON/OFF to match the terrain
- Gravel-tuned clutch tension
- Compatible with current road Dual Control Levers
- Maximum front difference: 17T
- Top sprocket: Max. 11T/Min. 11T
- Low sprocket: Max. 34T/Min. 30T
- Total capacity: 38T
