Winding spring especially for seat belt retractor

In a seat belt retractor comprising a frame, a main shaft rotatable in said frame, a spool mounted on said shaft for rotation therewith, seat belt webbing having one end attached to said spool and adapted to be wound thereon, and a rewind spring motor mounted on said frame for winding said webbing on said spool and permitting said webbing to be extracted from said spool, the improvement comprising:

• said rewind spring motor comprising:
• a drive spool rotatable with said main shaft;
• an idler spool mounted for rotation about a second shaft spaced from said main shaft;
• and
• a rewind spring having a stretched S-shape and disposed between said drive spool and said idler spool;
• said rewind spring having a first end in communication with said drive spool;
• a second end in communication with said idler spool;
• a normal use region extending from said drive spool toward said idler spool and comprising a first portion in communication with said first end and having a first natural radius of curvature, and a second portion in communication with and extending from said first portion toward said idler spool, said second portion having a second natural radius of curvature, said second natural radius of curvature being larger than said first natural radius of curvature, the transition between said first and second portions being defined by an abrupt increase in the natural radius of curvature;
• and a surplus use region extending from said second portion to said second end of said spring.