In September, the journal Traffic Injury Prevention published an article by Maltese, et al., titled “An evaluation of front seat distance from rear facing child restraint systems in prevention of injury in frontal crashes.” As the title suggests, it is the long-awaited paper that provides the scientific backing for Dorel’s summer 2023 announcement requiring its CRs used rear facing to gently touch the vehicle seat ahead whenever possible.
Even prior to publication of the paper, Dorel shared with CPSTs the basic findings of the research (which the company had funded) at conferences and in webinars. In particular, it said the testing found evidence that head injury criterion, or HIC, scores (which predict the likelihood of head injury) could be reduced (improved) by installing a rear-facing CR without leaving a gap between the CR and the front seatback. It explained that this was because the CR and seatback move in tandem during a crash when they start out abutting one another, whereas a gap allows for a moment when the top of the CR strikes the seatback hard, which is more injurious to the child.
Editor’s Note: While the study described in this article touts a certain installation situation, continue to follow the manufacturers’ instructions, both for the vehicle and CR. To date, only Dorel has modified instructions in reaction to this study. Other manufacturers are aware of and have access to these published findings, and SRN will report if any of them weigh in on this topic. Also, click here for an article on practical considerations for following Dorel’s guidance in the field.
Even before it was published, the research’s findings prompted Dorel to add seatback-touching requirements to the owner’s manuals for all of its brands. The changes were phased into manuals beginning late 2023, but Dorel also directed CPSTs to apply the change retroactively to older CRs. (Find an in-depth article covering these changes here and in the January/February 2024 issue of SRN and a related article on tips for CPSTs here and in this Sept/Oct 2024 issue.)
The published study covers the findings that Dorel has already shared while illuminating certain details of interest. Here is a summary of the main points.
What test fixture did researchers use? The tests were conducted using the new FMVSS 213b test bench, modified to include the generic front seatback developed by Consumer Reports in 2014, on the accelerator sled test system at Calspan, Inc. (a third-party test lab regularly used by NHSTA and others for testing).
Which CRs did researchers test? Safety 1st convertibles Grow and Go and Go Sprint and RF-only Comfort 35 were tested; each individual CR was subjected to only one test.
Which test dummies were seated in the CRs? All CRs were tested with a Q1.5 ATD, which represents an 18-month-old. Using the instrumented dummy, researchers looked at HIC15, neck tension, neck shear and compression force, and flexion/extension moment (the reaction force of compression).
How many tests did researchers conduct? Thirty-four: 16 tests of the RF convertibles and 18 of the RF-only CR (four of which were baseless installations).
What speeds were tested? Most tests had a 30 mph change in velocity (as specified in FMVSS 213), but two were conducted at a higher speed of 35 mph.
How did the researchers install the CRs? The convertibles and the RF-only base were installed using the LA attachment; the baseless RF-only CR was installed using a lap-shoulder belt. Proper installation requirements were followed (including how the crash dummies were harnessed). The researchers used the Consumer Reports testing protocol to position the front seat to simulate various starting-gap or no-gap conditions relative to the CR.
What gap distances were tested? Tests were conducted starting with 15, 70, 170, and 300 mm gaps between the top of the CR and the simulated front seat. In inches, this range is from roughly a half inch to just shy of a foot of gap. They also tested conditions where the CR was “gently touching” (no gap) and “bracing” (pressing into the front seat 20 mm, or about three-quarters of an inch) the simulated seatback.
What did the testing find? The abstract for the study summarizes its most important findings this way:
“Placing a rear facing CRS, both convertibles and infant carriers, against or close to the seatback of the seat immediately in front of the CRS reduces head and tensile neck injury criteria in ATDs (test dummies). The amount of gap between the front seat and the rear facing CRs is strongly and positively correlated with HIC for both convertibles and infant carriers. RF infant carriers with and without a base yield comparable injury metrics and kinematics when touching or nearly touching the back of the front vehicle seat.”
Essentially, the testing suggests that a child (simulated by an 18-month-old ATD) is much less likely to suffer head and neck injuries when the CR they ride in starts out touching or close to the seatback ahead in any type of rear-facing situation. (However, note that at all gap distances, the HIC score for tests run at 30 mph [the speed used in FMVSS 213 testing] was well below the 1,000 point maximum to meet the federal standard.)
SRN is especially happy to learn the objective measurements for “close” and “far.” Using scientific modeling techniques (Bayesian regression) on the test results, the researchers could point to a gap of roughly 100 mm (about 4 inches) as the worst-case gap, at a HIC score of 200 points above average. (Remember, these are relative measurements to improve safety; the situations in all these tests would have passed FMVSS 213.) By comparison, a no-gap installation resulted in HIC scores that were 200 points below average. At gap distances greater than 100 mm, the model produced HIC scores tapering lower. (Though only one test was run at a gap over 140 mm, this test of a 300 mm gap did reflect the researchers’ predicted decrease in HIC scores.)
What about head strikes? When the dummy was seated in the RF-only CR, head contact reaction force, which measures head strikes to the CR or vehicle seatback, was significantly lower when the CR started out touching the seatback or was within 15 mm (about a half inch) compared to tests of gaps of 70 or 140 mm (2.8 to 5.5 inches). (Note: Tests were not conducted for any distance between a 15 and 70 mm gap.)
What about bracing? As mentioned, the researchers also tested CRs that were braced (pushing about .75 inches into the seatback), and these tests resulted in the best CR performances of the study, though only slightly better than the no-gap (“lightly touching”) scenario. However, it is important to note that these findings are from sled tests in a lab; Dorel has wisely opted not to recommend (and, in fact, prohibits) bracing their CRs against a seatback due to the many safety issues this could cause in actual vehicles. Bracing a car seat against a front seat violates the instructions of many vehicle owner’s manuals.
What about CRs installed in third rows? Because the study uses a simulated front seat, it discusses outcomes relative to various front seat gaps only. However, note that Dorel’s instructions requiring touching the seat ahead apply to a CR installed in any rear row.
What about RF-only CRs used baseless? A stated research objective was to determine whether baseless or with-base outcomes would differ significantly. The researchers tested Comfort 35 with and without its base. From the four baseless installations tested in the study, the researchers concluded that the no-gap installation was safer for RF-only models whether or not the base was used.
What is the bottom line for CPSTs? While this study provides information that CR (and possibly vehicle) safety engineers can add to their body of knowledge regarding CRs, the information is mostly just “nice to know” for CPSTs. As always, follow the manufacturer’s installation instructions and check for updates; SRN will also alert readers if any manufacturers other than Dorel weigh in on this topic in any way.
However, one takeaway from the study can help CPSTs as they apply Dorel’s instructions. Because the Dorel instructions say to install a CR “as close as possible” to a seatback if it can’t completely touch, it is helpful to know some of the objective measures from the study. The study finds a gap of about 4 inches produces the highest (worst) HIC15 scores. Therefore, although an objective figure for a safe amount of gap is not specified in the study, it certainly appears that a gap of less than, say, a couple of inches would be safer than a gap of 4 inches, considering HIC and other testing parameters. The study also suggests that if a gap of 4 inches is the closest the CR can get to the seat ahead, it may be better to widen the gap instead, if possible.
Again, it is important to remember that the study was conducted using Dorel CRs in a lab, and only Dorel has reacted to it (by making changes to instructions). What other companies will decide to do with this data (if anything) remains to be seen. Another article in this Sept/Oct 2024 issue discusses many things CPSTs should consider when installing any rear-facing CR that touches the seat ahead, especially a front seat.
Reference: Maltese, M., et al. “An evaluation of front seat distance from rear facing child restraint systems in prevention of injury in frontal crash tests.” Traffic Injury Prevention. September 26, 2024. DOI: 10.1080/15389588.2024.2391453