This invention relates to devices for limiting the belt forces which are generated in safety belts of vehicles and, more particularly, to such force-limiting devices having a band with a slot and a slot-widening member which forces the sides of the slot apart in response to stress applied to the safety belt. 
A belt force-limiting device of this design is known, for example, from German Offenlegungsschrift No.  1,506,641, in particular the device shown in FIGS.  7 and 8 thereof.  In this known device, a bolt-shaped slot-widening member fixed on the vehicle body is engaged in a longitudinal slot provided on a safety belt fitting element.  In case a stress exceeding a defined belt force is applied as a result of a vehicle accident, displacement of the fitting element relative to the slot-widening body causes energy absorption as the longitudinal slot is widened.  However, it has been found that in designs of this kind, the belt force level required for slot widening varies quite considerably over the displacement path, especially if the longitudinal slot is relatively long in order to ensure a predetermined, defined forward displacement path.  This results from the fact that in the end regions of the longitudinal slot, relatively large forces are required in order to widen the slot, whereas substantially smaller belt forces are required to widen the middle region of the slot because of the relatively soft side portions of the component containing the slot. 
However, for belt force-limiting devices of this kind, it is desirable that the belt force required for deformation of the slot over the entire forward displacement path should be approximately constant or increase only slightly.