Automotive Seat Belt Torsion Bars – What Are They And What Do They Do? – Part 1

Koning, Richard. (1998). Seat belt retractor having a torsion bar (EP1119473B1). European Patent Office.

Seat belts in cars contain a device called a “seat belt retractor”. This device holds the seat belt webbing and also contains a seat belt locking mechanism that activates during a crash or hard breaking. Many seat belt retractors have an internal part called a “seat belt torsion bar”.

Seat Belt Torsion Bars, What are they?:

Seat Belt Torsion bars are members of the “load limiting” family of devices used in automotive seat belts to manage the crash force experienced by vehicle occupants during crashes, primarily frontal ones.  Load limiting devices operate on the scientific principle of energy management and dissipation. The device itself is used to absorb some of the energy of the crash, thereby theoretically reducing the amount of energy, or load, experienced by the occupant.

Specifically, a seat belt torsion bar is a small, cylindrical bar of relatively soft metal located in the shaft of the spool in the seat belt retractor. The primary purpose of the torsion bar is to reduce the loads applied to the occupant by the shoulder belt portion of the seat belt.  The way a torsion bar operates is that when the seat belt retractor locks up during a crash, the continuing loads on the shoulder belt will cause the torsion bar to twist and rotate and pay out seat belt webbing.  In doing so, the twisting of the bar will use up some of the energy of the crash and lower the loads that the shoulder belt applies on the occupant’s chest.  This load reduction generally causes the head to move farther in the crash than it would had the torsion bar not been present.

The risk of this method of load reduction is the fact that the seat belt webbing spools out of the retractor as the torsion bar twists under loads, thereby “lengthening” the seat belt.   This ultimately increases the amount of forward displacement of the occupant and, if not countered properly, will result in potentially injurious head impacts, injuries to numerous parts of the body and even the potential for occupant ejection from the vehicle.  In an attempt to counter the risks posed by the torsion bar, the automotive industry designs torsion bars to be used in conjunction with the frontal airbag.

Stay tuned for Part 2!