Automotive Seat Belt Torsion Bars – What Are The Design Issues? – Part 2

In Automotive Seat Belt Torsion Bars – Design Issues Part 1, the design philosophy behind the use of load limiting Automotive Seat Belt Torsion Bars and their risks and purported benefit to occupants through reduction of shoulder belt loads traded off against the control of head motion.

The window of purported seat belt torsion bar benefit occurs when the occupant is perfectly aligned and is moving into a deploying airbag.  However, it is clear that problems may arise if the torsion bar is allowed to deploy during non-frontal crashes where the frontal airbag does not normally deploy, or the occupant bypasses the airbag due to the direction of the crash forces.

This has been shown to occur in crashes such as oblique impacts, side impact and rollover crashes. In these crashes the restraining seat belt forces can be high enough to cause the torsion bar to twist and the webbing to spool out. But, the direction of the crash forces is such that the frontal airbag does not deploy, therefore not providing the necessary control of head motion. Also, the additional webbing introduced into the seat belt will cause the seat belt to be loose. A loose seat belt can result in either partial or total ejection of the occupant from the vehicle.  This may also occur in a multiple impact event where the first impact results in the torsion bar introducing additional webbing that degrades the occupants’ crash protection in subsequent impacts or rollovers.

When additional webbing is introduced into a seat belt by any means, including a torsion bar, a host of severe injuries may be created that otherwise would not have existed. Effective countermeasures must be incorporated into the design of the torsion bar to prevent or at least minimize these unwanted responses. The most effective design countermeasures are: (1) have a high enough threshold such that the risk of deployment during non-frontal crashes is minimized; (2) incorporate a physical stop to the twisting of the torsion bar, so that only a limited amount of webbing is spooled out during the deployment process; and (3) Incorporate a locking latch-plate into the seat belt system to prevent webbing from being pulled into the lap belt. Currently these countermeasures are being utilized in newer vehicle’s seat belts, but older vehicles may not have them.