11T Bicycle Rear Derailleur Bearing Pulley 4mm 5mm 6mm MTB Bike Bearing Guide Roller Aluminium Alloy Bike Derailleur Accessories. Style: 11T. Lightweight Efficiency: The CNC fully hollowed-out process of thi s bicycle rear derailleur bearing pulley significantly reduces the weight of the derailleur wheels, boosting cycling efficiency while maintaining structural integrity.