Abstract: An improved differential drive mechanism includes a clutch which is engageable to retard relative rotation between driving and driven members. A clutch actuator assembly includes a pair of flyweights which are rotated about a central axis of the actuator assembly at a speed which varies as a function of the speed of relative rotation between the driving and driven members. Upon the occurrence of a predetermined rate of relative rotation between the driving and driven members, one of the flyweights moves outwardly from a retracted position to an extended position in which it engages a stop element and initiates actuation of the clutch. In order to promote engagement of a stop surface on the flyweight with the stop element and locking up of the differential upon movement of the flyweight to the extended position, the distance which the stop surface projects outwardly is maximized for a given extent of angular movement of the flyweight between the retracted and extended positions.