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Constant force retractors are a new type of seat belt retractors which, during a crash, can generate nearly constant belt restraint forces on occupants required by advanced restraint systems. There are several types of designs to provide the constant belt force by retractors. The torsion bar is one of them. In this type of design, a metal bar is designed to take the belt load in the torsional mode then to yield at a preset load level and keep the belt load at that level for a certain amount of belt pay-out. This paper first presents the theoretical design of the torsion bar. It applies material plasticity to derive all the equations for the design of torsion bars with circular cross sections. These formulas include the relations between required constant force and belt pay-out, diameter and length of the bar, material properties, and torque to rotation properties. This paper also includes formulas to guide the designs based on prototype test data.