Forensic Analysis of Seat Belt Retractor Torsion Bars

The analysis of twisted torsion bars can aid the forensic analyst by providing the number of times the retractor spool has turned during the load limiting portion of an accident scenario. By knowing the number of times the retractor spool has turned, one can calculate the amount of webbing that has been added to the system during load limiting. The exact amount of webbing added per turn of the retractor spool will vary depending on a number of factors, such as webbing thickness and amount of webbing on the spool at the start of load limiting. Knowing the amount of webbing added to the system under load can help corroborate other forensic evidence, such as marks left on the webbing by the plastic coating of the D-ring. To develop a method for determining the number of turns of a torsion bar a series of torsion bars were twisted from one quarter turn up to four full turns and analyzed. The angle of the grain lines in the surface of the twisted bars was then used to calculate the amount of twist for each bar. Torsion bars were then twisted statically and dynamically in retractor tests to validate the methodology developed. The analysis in this paper does not evaluate the load required to turn the torsion bar.