Licensed Professional Engineers
FORENSIC CLUES # 43 - "Vehicle Rollover and Roof Crush" by John L. Ryan
A newsletter dedicated to keeping attorneys informed of the technical side of product liability cases.
Issue 43: Vol. 1 May/June 2011
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( 479) 549-4860
Rollover crashes have the highest fatality rate of any type of automobile collision, and claim over ten thousand lives each year. Ejection from overturning vehicles results in the most fatalities annually, followed closely by passenger contact with the roof. Over half of the people injured or killed in overturns are belted. The roof structure of automobiles is crucial to occupant safety in rollover incidents. An inadequately designed roof structure in a rollover will result in excessive deflection of the roof into the passenger compartment.
This issue of Clues will examine the history of the standards concerning roof crush strength, the current federal requirements, future federal requirements, and a brief overview of an engineering approach to a roof crush standard.
History of Federal Roof Crush Standards
In the early 1970’s, National Highway and Traffic Safety Administration (NHTSA) began to create a standard that would set performance requirements of automobile roof structures. The original draft apparently called for live rollover tests involving two complete revolutions, with windows open, and the requirement that anthropomorphic dummies be contained during the rollover test. Manufacturers fought these proposed requirements until the live roll test requirement was removed. In 1973 the Federal Motor Vehicle Safety Standard (FMVSS) 216 – Roof Crush Resistance was created. This standard set strength requirements for the roof structure of passenger vehicles weighing 6,000 pounds or less. This was reported to be a temporary alternative to meeting FMVSS 208’s live rollover requirements that was never withdrawn.
Current Federal Roof Crush Standards
Currently vehicles must meet FMVSS 208’s rollover requirements, or FMVSS 216’s. The test procedure in 208 involves a live rollover test. The vehicle is positioned on an inclined platform that is accelerated to 30 mph laterally, then quickly decelerated, causing the vehicle to overturn. The vehicle must keep the test dummy contained in the vehicle, and the head injury coefficient (HIC – a measure of the deceleration of the head) must be 1,000 or less. A clause in FMVSS 208 gives manufacturers the option of meeting seatbelt criteria in a frontal collision instead of meeting the overturn criteria.
FMVSS 216—Roof Crush Resistance Standard
FMVSS 216 applies to all passenger vehicles lighter than 6,000 pounds. This lets heavier vehicles be manufactured with inadequate roof structures. The high center of gravity of heavy trucks and SUV’s result in higher rollover rates than cars, yet their roof structure does not have to meet FMVSS 216 criteria, and FMVSS 208’s criteria can be bypassed. FMVSS 216 involves loading the roof of a vehicle with 1.5 times the unloaded weight of the vehicle with a plate angled to simulate a real world direction of force that would be acting on the roof in a rollover.
To someone unfamiliar with the concept of dynamic forces caused by acceleration and deceleration, 1.5 times the vehicle weight may sound like a reasonable and safe amount of force. When a vehicle impacts the ground in a rollover, it is decelerated rapidly. That deceleration causes a magnification of the forces occurring in the impact. Automobile collisions involve extremely high rates of deceleration expressed in g’s. Free falling objects accelerate at the rate of one “g”. Each additional g essentially adds an amount of force equivalent to the weight of the object. Using 1.5 times the vehicle weight does not simulate real world rollover conditions. To put this in perspective, federal standards require a maximum of 60 g’s of acceleration for vehicle occupants. Frontal crash tests into rigid barriers at between 35 and 40 mph results in decelerations between 20 and 50 g’s. FMVSS 216 requires that the roof of the vehicle must not deflect more than 127 millimeters (5.0 inches) when loaded with 1.5 times the vehicle weight. Check how much space is between the top of your head and the roof of your vehicle the next time you sit in your car - I bet it will be less than five inches.
Proposed Changes to Federal Roof Crush Standards
Changes to make 216 require more stringent rollover criteria have been proposed, but to date have yet to be implemented. Proposals have been made to increase the static loading from
1. 5 times the vehicle weight to 2.5 times the vehicle weight. Another proposal is using 3 times the vehicle weight. A significant proposal involves requiring no contact with a 50th percentile test dummy in the test. This is a more logical requirement, since the current five inch maximum deflection would result in contact with an occupant in some vehicles, and no contact in others.
An Engineering Perspective
From an engineering perspective, any structural element that is critical to human life must be designed not to fail under foreseeable operating conditions. Some wrecks will be unsurvivable due to high rates of deceleration regardless of any intrusion into the passenger compartment. Rollovers that involve survivable decelerations should not claim lives. Vehicles must have roof structures that will prevent impact of the occupants’ heads with the vehicle roof.
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