SRM 175mm 110BCD with 52/36Rotor Aero chainrings. This has been one of the rare cycling items I bought that always worked and was worth the money. I am no longer racing so I do not need it anymore. The battery was replaced a half a season before I quit using it. The chainrings were installed by SRM so the calibration is spot on, they also replaced a reed switch. The chainrings have plenty of life left in them. Compatibility: Bottom bracket BSA, ITA, BB86, BB30, BB30A, PF30, BBRight, BB386 Evo