The Hollow - Tech II 12-speed Ultegra chainset with integrated power meter offers an ideal balance of light weight and stiffness, making it the perfect choice for both enthusiast and competitive riders - Accurately calibrated strain gauges are integrated inside the crank arm, this provides a stable measurement system through rotation from both sides for extremely precise data+/-2% accuracy of strain gauge measured power - Bluetooth and ANT+ with wireless firmware update and data acquisition system - Easily rechargeable integrated battery with fully waterproof reliable electric systemHollowGlide hollow outer chainring gives vast improvements in rigidity by virtually removing outer chainring flex and hugely improves shift performance, even under high load - The chainring teeth have been developed to work in conjunction with 12-speed HG chains for increased contact to provide greater efficiency and improved power transfer - Industry leading Hollow - Tech II cold forged 4 arm hollow cranks have been evolved for maximum rigidity while reducing weight - Wide gear pitch spacing increases clearance between chain and leading teeth to provide compatibility to frames with O.L.D. 135 / 142 mm - A wide range of chainring options and crank arm lengths are available to meet the needs of all riders and racing disciplinesHollowTech II integrated bottom bracket uses a wider bearing placement and larger axle diameter for increased stiffness and efficiencyHollowTech II integrated bottom bracket seal design has minimal bearing interface ensuring smooth rolling and increased durability(sold separately)