Abstract: A modulator for supporting arms of a rear bicycle carrier comprises a support frame including a central post having two ends with which first and second supporting arms are connected; a ratchet holder integrally formed with or separately disposed on the connecting position of the central post, and including a plurality of inner teeth arranged around the internal periphery thereof; a connecting shaft inserted through the ratchet holder, on the two sides of an intermediate seat of the connecting shaft being mounted two ratchet members for engaging with the inner teeth of the ratchet holder; a control unit fixed on a predetermined position of the rear bicycle carrier for being rotated clockwise or counterclockwise to disengage at least one ratchet member from the inner teeth of the ratchet holder such that the first and second supporting arms can be adjusted toward a suitable angle relative to the central post.