Compact dual-level load limiting seat belt retractor

A seat belt retractor having first and second serially arranged torsion bars and a switching mechanism provides two levels of load limiting. A bridge bolt rotationally fixed to the inboard ends of the torsion bars is threaded into a locking base, and the switching mechanism prevents bridge bolt translation relative to the locking base in a default condition to limit seat belt load with the first torsion bar. Activation of the switching mechanism permits limited translation of the bridge bolt relative to the locking base to limit seat belt load with the second torsion bar. Additional translation of the bridge bolt relative to the locking base is prevented to re-establish load limiting with the first torsion bar. The switching mechanism includes a set of detent wedges, a retaining band and an electrically activated cutting mechanism, all disposed within the retractor frame.