A seat belt retractor inertial locking sensor for a seat belt retractor for locking the retractor through engagement of a spool lock in response to inertial forces acting on the retractor, comprising: a housing forming a ball nest surface; a ball mass positioned on the nest surface and being displaceable on the nest surface from a normal position to a displaced position in response to inertial forces acting on the ball mass; and a locking lever mounted for pivoting motion about a pivot axis, the locking lever having an engagement feature for engaging with the retractor spool lock, the pivot axis positioned relative to the center of gravity of the locking lever such that the locking lever is urged to pivot under the influence of gravity toward engagement of the locking lever with the ball mass with the retractor engagement feature out of engagement with the spool lock, wherein, upon certain inertial loads acting on the ball mass, the ball mass moves on the nest surface from the normal position to the displaced position and forces the locking lever to move to a position of engagement with the spool lock; the locking lever forming a contact area with the ball mass in the normal condition along or adjacent to a vertical diametric plane of the ball mass wherein the contact area is the sole contact between the ball mass and the locking lever when the ball mass is in the normal condition, the locking lever further forming a circular rim which contacts the ball mass when the ball mass is in the displaced position to urge the locking lever to the position of engagement, characterized in that the nest surface further forms a central aperture, the nest surface defines a plurality of radially projecting ribs, the nest surface further defines a plurality of V-shaped slots, and the V-shaped slots define a plurality of pointed extensions positioned between the ribs. a housing forming a ball nest surface; a ball mass positioned on the nest surface and being displaceable on the nest surface from a normal position to a displaced position in response to inertial forces acting on the ball mass; and a locking lever mounted for pivoting motion about a pivot axis, the locking lever having an engagement feature for engaging with the retractor spool lock, the pivot axis positioned relative to the center of gravity of the locking lever such that the locking lever is urged to pivot under the influence of gravity toward engagement of the locking lever with the ball mass with the retractor engagement feature out of engagement with the spool lock, wherein, upon certain inertial loads acting on the ball mass, the ball mass moves on the nest surface from the normal position to the displaced position and forces the locking lever to move to a position of engagement with the spool lock; the locking lever forming a contact area with the ball mass in the normal condition along or adjacent to a vertical diametric plane of the ball mass wherein the contact area is the sole contact between the ball mass and the locking lever when the ball mass is in the normal condition, the locking lever further forming a circular rim which contacts the ball mass when the ball mass is in the displaced position to urge the locking lever to the position of engagement, characterized in that the nest surface further forms a central aperture, the nest surface defines a plurality of radially projecting ribs, the nest surface further defines a plurality of V-shaped slots, and the V-shaped slots define a plurality of pointed extensions positioned between the ribs.