Can a buckle designed with a lock for the latch when struck on the face, back, or side, also have this same feature when accelerated along the longitudinal axis? Six seatbelt buckles from various manufacturers were tested to determine their dynamic characteristics in the longitudinal direction along the mounting stalk. Patented designs of the buckles were intended to prevent inertial unlatching of the buckle. Although they may perform well in lateral and vertical directions, when force is applied along the direction parallel to the mounting stalk the buckles could be made to release. If the buckle is mounted in the vehicle with a rigid stalk, could impact pulses be transmitted to the buckle to cause release?

A test apparatus was constructed where the buckle could be mounted with the stalk and webbing. The webbing could be preloaded and the buckle was accelerated by impacting the mounting point at the base of the stalk. This acceleration pulse was studied for the effect of the preload on the webbing and to determine the minimum pulse required to release the buckle.

This study documented the acceleration required to cause a buckle and latch plate to inertially release. The acceleration required causing the unlatching of the buckle increased as the preload of the webbing increased up to 100 pounds. Any preload in excess of 100 pounds prevented inertial unlatching.

It was also found that the threshold of inertial unlatching is not so much a function of the maximum acceleration, as it is the area under the acceleration curve which is the change in velocity.