The Effect of Swedish Tethers on the Performance of Rear Facing Child Restraints in Frontal Crashes

Rear Facing child restraints (RFCR) have various component designs which can couple the restraint to the vehicle. Swedish tethers, which link the upper portion of the child restraint to the vehicle floor, prevent rearward rotation in rear impacts and during rebound in frontal crashes. They also simplify installation of restraints by allowing better control of the installation angle and removing the need of spacer devices. The objective of this study was to test the effect of Swedish tethers on RFCR in frontal crashes. The tethers reduced forward excursion and rotation, and had a positive but minor effect on injury values. The more secure attachment to the vehicle caused by the Swedish tether could also be beneficial in other crash types. INTRODUCTION The vehicle belt, lower LATCH belt, or ISOFix anchors serve as the primary components which attach Rear Facing Child Restraints (RFCR) to the vehicle. There are other devices, however, which can be used in addition to the primary components. Antirotation legs, Australian tethers, Swedish tethers, the ISOFix base, and anti-rebound bars are each designed to change the kinematics of the child restraint in different crash types. Federal Motor Vehicle Safety Standard (FMVSS) No. 213, "Child restraint systems," requires RFCRs to meet the performance requirements of the standard when secured to the standard test seat assembly using (1) the lap belt only or (2) the lower LATCH (Lower Anchorages and Tethers for Children) anchorages only. NHTSA does not use a means supplemental to the lap belt/lower LATCH anchorages, such as a tether or a bar, of securing RFCR to the seat assembly in the agency's compliance test. In the past, NHTSA found that a very high percentage of parents did not use a supplemental tether strap to secure their child seats even when they knew the strap was needed to provide their child protection. The agency concluded that there was a strong likelihood that a tether or a bar would be misused with the seat, and that FMVSS No. 213 should thus require that child restraints must meet minimum requirements of the standard without supplemental tethers. Swedish tethers prevent rear rotation in rear impacts and during rebound in frontal impacts [1]. They link the upper portion of the child restraint to the vehicle floor, and may also have benefits in non-frontal crash types by more rigidly attaching the RFCR to the vehicle [2,3]. They can be attached to built-in anchor points or to the front seat base structure. The tether may reduce excursion in side impacts (lateral) and rollovers (upward/rearward). In addition to the effect it may have in vehicle crashes, the tether may also have benefits during installation. RFCRs have a recommended range of child restraint angles. The RFCR angle should be approximately 45 degrees (with respect to vertical), but no greater [3]. Since young children cannot hold their heads upright due to their weak neck musculature, the reclined angle prevents the head from flopping forward and cutting off the airway. At angles greater than 45 degrees, however, the child restraint provides less support for the head and neck. Variations in child restraint design, vehicle seat design, attachment equipment (LATCH, 3 pt belt), and the location of attachment anchors result in many RFCRs positioned at incorrect angles [4]. The most common method, which allows adjustment of the angle, is to place a spacer (typically rolled up towels or foam noodles) under the base of the RFCR near the seat back (Figure 1). Swedish tethers provide the opportunity to control and easily adjust this angle. Figure 1. Schematic of RFCR with spacer placed under base to correct installation angle.