An easily mountable disc brake rotor assembly that is lightweight is provided. The bicycle disc brake rotor assembly includes a rotor and an adaptor and is mounted to the front hub of a bicycle. A caliper mounted to the bicycle's suspension fork grips the rotor. The rotor is a ring-shaped plate, which has brake surfaces that are gripped by the caliper. The adapter is mounted to the front hub and has a deformation that secures the rotor to the hub. External and internal splines on the adapter prevent the rotor from rotating and transmit a braking force exerted on the rotor to the hub, respectively.