Seat belt tension in child restraint system (CRS) installations with and without belt tensioning and lock-off features

Child restraint systems (CRS) are installed too loosely in 44–58% of vehicles, which increases injury risk. Some CRS include seat belt tensioning and lock-off features—typically a built-in clasp that clamps over the seat belt to produce tension with a mechanical advantage and/or lock the belt into place within the CRS shell. The objectives of this study are to define (1) how much belt tension can be produced across a variety of CRS designs with belt tensioning and lock-off features, and (2) how these features affect consumer misuse and opinions. The outboard seat belt anchor of a second row minivan seat was replaced with a tension load cell. For Aim 1, a convenience sample of 29 CRS was installed in the instrumented vehicle seat by a researcher. Before engaging the lock-off, the tension in the belt was pulled by hand to “pre-locked” tensions of either 2, 10, 20, 30, 40, 50, or 60 N. After engaging the lock-off, the “locked tension” in the belt was recorded. Then weighted plates were placed in the CRS to represent typical occupant masses and the locked belt tensions were recorded again. For Aim 2, 30 adult participants installed a subset of four CRS into the instrumented vehicle seat. Three CRS included a lock-off (two rear-facing only (RFO) infant bases and one forward-facing convertible) and one CRS did not include a lock-off (RFO infant base). Belt tensions and installation errors were compared across CRS and participant feedback was collected <i>via</i> survey. Seven lock-off designs did not create additional belt tension while the others produced tensions that were three to 10 times greater than the Federal Motor Vehicle Safety Standard (FMVSS) 213 testing range of 53.5–67.0 N. Adding occupant mass to the installations reduced the locked belt tensions by approximately 21, 40, and 61 N for the masses of the CRABI 12 mo, HIII 3yo, and HIII 6yo, respectively. For the participant installations, CRS with belt tensioning lock-offs had significantly higher tension levels (66.5 ± 25.1 and 66.2 ± 35.9 N) compared to a lock-off design which simply locked the belt in place (32.8 ± 24.8 N) and the CRS without a lock-off (18.6 ± 15.1 N) (<i>p</i> &lt; 0.05). Before engaging the lock-off, participants pulled on the belt with averages of 9.4 and 29.7 N of tension for each of the two RFO CRS but very little tension (∼ 0 N) for the forward-facing CRS. Errors of incorrect belt path and incorrect lock-off use were noted for several participants. CRS belt lock-offs, especially those that produce additional tension, help consumers achieve tight installations. However, manufacturers’ instructions should be sure to address common misuse scenarios which may be introduced by lock-offs.