Tuesday, November 2, 2010

Seatbelt Load Limiters: A Dangerous Safety Defect

A load limiting device or torsion bar (hereinafter simply referred to as “load limiter”) were introduced originally by some manufacturers in the early eighties in conjunction with vehicles equipped with airbags. The theory or justification behind such devices are that they are intended to reduce belt induced injuries such as rib fractures by allowing forward movement of occupant’s torso when belt loads exceed some threshold. The release of the webbing allows an occupant to move forward until the occupant’s forward motion is finally arrested by the deployment of the frontal airbag. In theory, this design concept may work; if an occupant were to be involved in a 35 mph NCAP full frontal barrier crash test where the torsion bar releases webbing and the airbag deploys arresting the forward movement of the occupant. In virtually all other crash scenarios, i.e. rollover, offset frontals, frontal impacts where the airbag fails to deploy or any impact where airbags do not deploy, this design is extremely dangerous because it introduces slack into the seatbelt system; thereby failing to restrain the occupant and may permit the occupant to be ejected, or slam into the interior of the vehicle despite the fact that the occupant did exactly what they were supposed to in properly wearing their seatbelt.
In any crash scenario where an occupant was belted, but sustained significant injury, one place to look is whether or not the seatbelt retractor was equipped with a load limiting-type device. As mentioned, common scenarios include rollovers and offset frontal collisions where an occupant may miss the airbag because of the PDOF, or the airbag fails to deploy; since load limiters are designed to work in conjunction with an airbag.
[1] One of the original attempts at purported load limiting was sewing the seatbelt over itself or sewing a fold into the seatbelt webbing, more appropriately called rip stitching. The rip stitching would then pull apart when loaded introducing slack into the seatbelt system. Rip stitching, because it was such a dumb idea, for the most part went away and was replaced by a device referred to as a torsion bar built into the seatbelt retractor. The torsion bar is a metal rod that will twist when sufficient force is applied. The torsion bar will twist under load allowing seatbelt webbing to be released from the retractor thereby introducing slack into the system. The subject of this paper and presentation focuses on the deadly, deceitful and dangerous defect-torsion bars.

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