Belt retractor with pretensioner

The inertia reel (10) is fitted with a tensioning device (12) working with a piston moving in a cylinder (22). The piston is provided with a section of a toothed rack engaging with a pinion (28) when the drum (18) is rotated. At least one segment of the section can be separated from the piston after the tensioning device (12) is activated preventing the pinion from interfering with the piston and facilitating the freewheeling of the reel without a clutch required between pinion (28) and reel or an air valve integrated in the piston or the cylinder.