Abstract: An inertia responsive mechanism employs a freely mounted inertia mass in combination with a seat belt retractor mechanism wherein the inertia mass is sensitive to vehicle deceleration or to strap acceleration, or to both. Disclosed are a variety of arrangements for supporting an inertia mass in a manner to permit free movement of the mass including balls and recesses, conical post ends in holes, frusto-conical surfaces on tube ends and frusto-conical surfaces in combination with balls in grooves. Anti-lock mechanisms prevent inertial locking while the strap is retracting. In a compact inertia reel arrangement, a reel ratchet wheel directly supports the inertia mass. Also disclosed are mechanisms for manually resetting inertia reels.