February 1, 2011
The Society of Automotive Engineers resumed its ongoing boxing match over injury causation in rollovers at last week’s SAE Government Industry meeting. In Malibu’s corner was Wayne State and University of Michigan’s Transportation Safety Institute, presenting research supporting the theory of occupant diving as the mechanism of head and neck injury in rollovers – regardless of roof crush.
(For those of you who haven’t followed this 25-year-old scrum, Malibu refers to two sets of experimental rollover tests General Motors conducted in 1983 and 1987 on Chevrolet Malibus. Known as Malibu I and II, the tests were conducted to validate the theory that occupants don’t suffer head and neck injuries because the roof collapses on them, but because the force of the crash propels them into the roof. Over the years, automakers have clung to the Malibu results, despite crash data showing that the number of deaths and injuries in rollover accidents has risen disproportionately, with more than quarter of the accidents involving a serious roof intrusion.)
On the other side was NHTSA, arguing that roof strength is related to injury. It’s refreshing – if ironic – to see NHTSA champion a relationship between intrusion and injury. The agency is a late convert to this view; after years as an adherent of the Holy Gospel of Malibu.
Meanwhile, over at the Transportation Research Board’s Annual Meeting – also last week – research from less likely suspects supported the need for stronger roofs.
Florida Requires Real-World Strenuous Roof Strength Requirements for Transit Buses
In August 2007, Florida upgraded its requirements for rollover occupant protection in all transit buses used in the state. Previously, transit buses were required to meet the FMVSS 220 School Bus Rollover Protection static roof strength compliance test, similar to the single-sided FMVSS 216 procedure. After reviewing the data on transit buses from both test methodologies, Florida increased the stringency of the requirements to a modified version of the UN-ECE Regulation 66 (ECE-R66) Dynamic Dolly Rollover test to better protect occupants.
At the TRB meeting, Jerry Wekezer, a professor from Florida A&M University and Florida State University presented numerical analyses of both test procedures, as part of the state’s ongoing bus testing and research. Under the old state regulations, buses passed the quasi-static roof resistance test by withstanding a prescribed force without large deflection. However, under the tougher ECE requirements, the same bus significantly failed the dynamic rollover procedure when sidewall deformed into the occupant space. Transit busses are designed with strong roofbows to pass FMVSS 220, but the sidewalls, which have no design restrictions, are weaker, resulting in severe intrusion into the occupant’s survival space in the real-world dynamic test.
Wekezer argued that, contrary to NHTSA’s claims, a quasi-static load resistance test of the roof structure does not give sufficient indication on how the structure will behave during a real world rollover. He argued that the simple, repeatable FMVSS 220 procedure may result in ignoring significant real-world rollover factors, since the load in the old test is initially applied symmetrically onto the roof structure in an unrealistic way.
Prominent auto safety researcher Raphael Grzebieta, of the University of New South Wales in Australia, expressed his dismay in the US’s continued use of the static FMVSS 220 test requirements when 44 other countries have already adopted the more real-world dynamic test procedure. In Australia, the adoption of dynamic test and 3-point belt requirements in transit buses has eliminated almost all of fatalities.
Obviously, this fellow has never heard of the super-duper protective qualities of compartmentalization. And what does this say about our passenger vehicle roof strength standards since FMVSS 216 is based on the same premise? We’re sure NHTSA will get around to it in a few decades.
Door Number 1: Diving
Back to the main bout. At the government-industry meeting, Jingwne Hu from UMTRI presented detailed modeling studies evaluating occupant kinematics and injury in rollovers. He concluded from visual inspectionpeak neck load occurs at beginning of roof crush, long before significant intrusion occurs. that the dummy dives into the roof rather than the roof collapsing down on the dummy’s head. He also analyzed roof crush and neck load vs. time and determined that in both strong and regular roofs, the
Those of us who follow dummy design understand that you have to be careful about interpreting these results since the HIII dummy Hu used was not designed for rollover-type of loading conditions. In addition to the lack of biofidelity in flexion and extension, there are many studies showing that the HIII neck is much stiffer in compression than the human neck, which can alter occupant kinematics in a rollover. Further, the ability to predict injury potential cannot be reliable if the dummy is not measuring values that are representative of the injury mechanisms being evaluated. For example, peak neck load in Malibu was measured at the upper neck load cell. Many neck injuries seen in rollovers are at the lower cervical spine, which is not adequately evaluated by this measurement.
Hu acknowledged that near-side roof stiffness affects far-side injury risk because a weak roof causes the far-side roof to contact the ground earlier and remains in contact with the ground longer, increasing the injury risk for the far-side occupant. The University of Virginia’s review of CIREN data for belted non-ejected occupants in single vehicle rollovers validated that roof crush is related to head and neck injury risk for the far-side occupant and chest injury for the near-side occupant.
Hu recommended low-friction padding is a countermeasure for head and neck injury potential for all occupants in rollovers. With energy-absorbing foam with a zero coefficient of friction, the head will slide away, lowering the risk of neck injury. As the friction increases, neck injury risk increases.
Hu argued that lap belt design needs to be improved to reduce vertical excursion and keep the dummy off the roof – something safety advocates have been saying for years. He specifically recommended that lap belt stretch be decreased, slack be removed with a pretensioner, and sliding latch plates be replaced with cinching latch plates. Hu also mentioned that UMTRI is researching a double lap belt design to reduce rotation of the lap belt.
Essentially, Hu stressed the need for a combination of more effective vehicle restraint systems, stronger roof structures, and roof interior design optimization for occupant protection in rollovers.
“This is exactly what we as auto safety experts have been recommending for years,” said biomechanics expert Salena Zellers.
Door Number 2: Roof Strength
NHTSA’s National Center for Statistics and Analysis (NCSA) presented the opposing view. The agency research compared roof strength to roof deformation and injury in real-world rollovers using NASS CDS data. They demonstrated a statistically significant relationship between the peak strength-to-weight ratio and the maximum vertical roof intrusion. This work supports earlier NHTSA work (and research from the Insurance Institute for Highway Safety) demonstrating a relationship between vertical roof intrusion and injury risk in rollovers. NCSA argued that this supports the validity of strength-to-weight ratio as a measure of real world roof strength. (The NHTSA/IIHS unity on the link between injury and roof crush is a brief moment of harmony in a relationship that has gotten a bit testy of late over data disputes.)
The Bottom Line
Automakers have known for decades that their seat belts do not restrain occupants effectively in rollovers. Instead of focusing on retaining the occupant survival space and designing seat belts that prevent injurious head contact with the roof structures, they have moaned and groaned that roof crush doesn’t cause injury, so why bother preventing it? They blame the injury to belted occupants in rollovers not on the roof, but rather on their own defective seat belts and poor vehicle designs that lack of sufficient head room.
To keep this in perspective: your vehicle design engineer tells you that the seat belts he designed will not keep the driver from impacting the steering wheel at force levels high enough to cause severe torso injuries in a 30 mph frontal impact. He argues that it doesn’t matter if the steering wheel is moving rearward toward the driver from the impact, the driver will impact it anyway because the seat belts will not restrain him effectively in that direction.
You say: “You’re fired!” And then you find a way to increase the occupant space by preventing rearward displacement of the steering wheel and design a belt system that can restrain the driver from impacting the steering wheel in that amount of distance – just like the auto manufacturers accomplished decades ago.
Tell us again why it’s different for the roof?