Rotor Q-Rings SRAM MTB (Outer 120mm BCD): Outer Q-Rings designed to fit SRAM XX, X0, X9 MTB chainsets. Rotor Q-Rings help overall endurance & performance by reducing the dead spots while riding. The oval shape allows for your legs to create a smoother & more effective pedaling stroke. How It Works: Rotor Q-Rings allow for easier moving of the legs through dead spots creating the feel of a smaller circular chainring & enabling your legs to remain in the power stroke for a longer periods of time. Q-Rings change the equivalent tooth size by decreasing it before the dead-spots & increasing it when the rider is at the pedal down stroke Utilising the OCP (Optimum Chainring Position) enables you to customize (fine tune) the position of the chainring for your personal riding habits & riding position on the bike. This ring is designed to fit Sram XX, XO, X9 MTB chainsets with 120mm outer ring BCD CNC machined 7075 T6 aluminium Black anodised 10 speed compatible 4 bolt 120mm BCD 40T is designed to run with the matching 27T inner ring 42T is designed to run with the matching 28T inner ring