HOLLOWcage® for Shimano 9100 / 8000. Our revolutionary mono-plate design is the quietest, best shifting, aerodynamic derailleur cage design created to date. It is a dawn of new era for rear derailleurs. HOLLOWcage ® has a carbon-polymer mono-plate construction with bespoke, oversized, full ceramic bearings and the most advanced silencing pulleys ever created.