NHTSA - ROLLOVER & ROOF CRUSH INJURIES
ROOF SUPPORT & ROOF PILLAR
COLLAPSE IN PASSENGER CAR, TRUCK, VAN, & SUV ROLLOVER ACCIDENTS OFTEN
RESULT IN SEVERE
HEAD INJURIES, PARALYSIS & DEATHS
TO THE OCCUPANTS
The most dangerous suv and auto vehicle
accidents with the highest number of deaths and serious injuries occur in
rollover accidents. Because of the speed and the forces involved, rollover
crashes are usually very destructive deadly events. Vehicle damage often
includes deformation of the roof and its supporting structures. Head and neck
injuries resulting in paralysis (both paraplegia and quadriplegia) are
common,  and
are directly associated with roof deformation. In October 2001,
U.S. highway safety regulators announced that they may toughen roof crush
resistance standards on car and truck roofs to reduce injuries in rollovers.
Roof crush during a rollover is the most dangerous type of wreck for
sport-utility vehicles (SUVs), pickups and minivans.
The National Highway Traffic
Safety Administration (NHTSA) is considering changes after finding roof
crush damage involved in serious or fatal injury to 26 percent of passengers in
rollover crashes. NHTSA said in a statement that a study of 1995-1999 accident
data showed that passengers were almost six times more likely to die in a
rollover crash than a front-end wreck.
The roof crush review is the
agency's third step this year to evaluate rollovers in SUV, trucks, & vans.
The agency issued its first vehicle rollover ratings this year and considered
the effect of rollovers of Ford Motor Co.'s Explorer sport-utilities as part of
an inquiry into failures of Bridgestone Corp.'s Firestone tires linked to 271
highway deaths.
ROOF CRUSH RESISTANCE
STANDARD TOO WEAK
"The standard really needs
to be upgraded," said Joan Claybrook, president of Public Citizen, a
consumer advocacy group and a former NHTSA Administrator. "The standard
right now is a static test that piles 1.5 times the weight of the car onto the
roof to see how
much it is crushed. We need a dynamic test. Piling weight on the roof isn't
enough." Roof crush tests designed in 1973 fail to meet needs of
changing auto designs, especially for passenger trucks and sport-utilities that
are most likely to roll over in an accident. The Alliance of Automobile
Manufacturers, whose members make 90 percent of U.S.-sold vehicles, said roofs
already work to protect passengers. "Current roof systems are performing
extremely well," said the group's spokesman, Scott Schmidt. "For
unbelted occupants, increased roof crush doesn't help anything. If you put a lot
of structure (weight) in the area roof it increases or raises the
center of gravity." A vehicle's structural support system and the roof
creates a "non-encroachment zone" or "survival space" that
should protect occupants in a crash. If a roof crushes substantially in an
accident, occupants may suffer disabling head or neck injuries. Most
vehicles do not have enough headroom to allow for more than three to four inches
of crush without significantly increasing the risk of serious injury.
Research shows that more roof
crush resistance could reduce injuries to belted passengers in a rollover crash.
One way to increase the resistance is to add high density foam or other material
to the inside of the roof pillars. When the currently empty spaces
are then filled with the foam, or even a honey combed cardboard type of
material, the crush resistance increases dramatically and the injuries and deaths from the
formerly collapsing roof structure, decreases dramatically.
NHTSA's FMVSS 216, which sets the minimum
strength requirements for a vehicle's roof crush resistance, does not
require manufacturers to conduct dynamic rollover tests on roofs. The federal standard also fails to consider what material the roof is
made of and how it is constructed.
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| 1. |
Roofs that can bend, crush,
or separate from the vehicle, |
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| 2. |
Inadequate or poor welding
procedure that can cause a steel roof to collapse and separate, and |
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| 3. |
Structural roof support /
roof pillars that may be too weak in strength and defective
in design and manufacture
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Vertical Motion During Rollover
During the initial trip event and during each subsequent
contact with the ground the vehicle will be subjected to horizontal
decelerations. Robinette et al (1993) found that the typical horizontal
deceleration (apparently averaged of the entire event) was 0.43g. The actual
horizontal velocity profile will be step-like, with the steep portions
corresponding to "corner" contacts with the ground. Assuming that
the transverse velocity is zero at the completion of a full roll, then the
change in horizontal velocity during each ground contact would be about 2m/s,
for an initial velocity of 11m/s.
Note that during the initial tripping event it is likely
that there will be substantial horizontal deceleration which will tend to
throw the occupants in the direction of the roll. This appears to be the
mechanism of ejection for the unrestrained dummy in the paper by Habberstad et
al (1986) - the dummy was ejected before the vehicle reached the first quarter
of the roll.
Vertical Motion of Vehicle's Center of
Gravity (CG or C of G) During Rollover
The method is to plot the approximate height of the
center of gravity of the vehicle during various stages of the rollover. The
velocity and acceleration of the center of gravity can then be roughly
approximated for a given roll duration figure 1.
Rollover - Roof Crush & Acceleration Analysis
Explained
As the vehicle rolls over a "corner" the C of
G reaches its highest point and, if the speed of the roll is sufficient, the
acceleration of the C of G might go positive (note that the effect of Earth's
gravity has been included in the above values). This indicates that the
vehicle loses contact with the ground. In some of the rally crashes this
effect was so strong that the roof did not contact the ground at all during
the first half roll.
The highest vertical acceleration occurs when the
underside of the vehicle is in contact with the ground, at the start of the
roll and at the end of a full roll. This is a manifestation of the location of
the C of G of the vehicle, which is closer to the underside of the vehicle
than the roof. In typical passenger cars the vertical distance from the C of G
to the roof is similar to the transverse distance from the C of G to the side
of the vehicle and therefore there is relatively little vertical motion of the
C of G as the vehicle rolls from its side to the roof to the other side. Low
aspect ratio vehicles such as sports cars have a smaller distance between the
C of G and the roof and therefore the vertical loads occurring when the roof
is in contact with the ground can expected to be higher. This might partly
explain the finding by Moffatt (AAAM 1995) that high roof vehicles have
generally less roof damage than low roof sports cars. The
change in vertical velocity during each quarter turn is estimated about 2m/s
for this scenario.
Horizontal motion of Vehicle C of G During a
Roll
During the initial trip event and during each subsequent
contact with the ground the vehicle will be subjected to horizontal
decelerations. Robinette et al (1993) found that the typical horizontal
deceleration (apparently averaged of the entire event) was 0.43g. The actual
horizontal velocity profile will be step-like, with the steep portions
corresponding to "corner" contacts with the ground. Assuming that
the transverse velocity is zero at the completion of a full roll, then the
change in horizontal velocity during each ground contact would be about 2m/s,
for an initial velocity of 11m/s.
Note that during the initial tripping event it is likely
that there will be substantial horizontal deceleration which will tend to
throw the occupants in the direction of the roll. This appears to be the
mechanism of ejection for the unrestrained dummy in the paper by Habberstad et
al (1986) - the dummy was ejected before the vehicle reached the first quarter
of the roll.
Rotational Motion During Rollovers
A complete revolution in 1.7 seconds indicates an
average angular velocity of 3.8 radians per second. The distance from the C of
G to the corner of the roof is about 1.3m for a typical passenger car
therefore the tangential velocity of the corner of the roof will be 5 m/s. The
average radial acceleration ("centrifugal force") experienced by an
object at this point will be 1.3x3.82=19m/s/s or about 2g.
    
After the tripping event which initiated the roll the
vehicle will be moving sideways with a typical horizontal velocity of 11 m/s
(assuming the trip speed was just sufficient to cause the roll - see
Gillespie, 1992 p326) therefore the first two contacts of the roof with the
ground will probably involve relative speeds of about 6 m/s (11 - 5) and the
impact will tend to increase the speed of rotation of the vehicle. In
effect, the occupants will experience angular accelerations in a direction
opposite to the direction of the roll during these first two roof contacts
(and opposite to the direction in which they were thrown at the start of the
roll).
As the horizontal speed of the vehicle drops (due to the
braking effect of the ground impacts) and the rotational speed increases, the
tangential speed of the corner of the roof may eventually exceed the
horizontal speed of the vehicle and the impact will tend to decrease the
speed of rotation of the vehicle. In effect, the occupants will then
experience an angular acceleration in the same direction as the roll. The
observation of rally crashes where the occupants head and arms are extended
outside the side window are probably due to this angular deceleration, the
peak of which would usually occur as the vehicle tips over on its wheels near
the end of the first roll or at the start of the second roll. At each of these
points the C of G of the vehicle is passing through its highest point
therefore the occupants tend to become "weightless". An unrestrained
occupant has a high risk of being ejected at this point, if the side window is
open or broken.
Rotational speed will have a similar step-like profile
to horizontal velocity. To obtain a rough estimate of the change in rotational
speed during each ground contact, assume that the maximum rotational speed is
reached after half a revolution (at the third ground contact). For a maximum
rotational speed of 6 rad/s (based on a average of 3.8 rad/s for a full roll)
the change during each contact will therefore be about 2 rad/s. This is
equivalent to a linear velocity change of about 2m/s in the region of an
occupant's head, in a direction tangential to the C of G of the vehicle.
Combined Effect in Rollover
The combination of vertical acceleration/deceleration,
horizontal decelerations and rotational acceleration/deceleration generally
results in complex occupant kinematics during a roll-over occupants are thrown
from side-to-side and up and down in a chaotic manner.
 Partial
ejection during rollover through open or broken side windows is a strong
possibility, even for restrained occupants. Subject to the limitations of the
approximations used, it was found that the changes in velocity during each
contact with the ground were similar for each of the three motions: vertical,
horizontal and rotational (tangential). For a trip speed of 11m/s (a typical
minimum for a passenger car), these three changes in velocity are estimated to
be about 2m/s. The effects of the rotational motion tend to either reinforce
or cancel the other two motions, depending on the phase of each. Therefore
peak velocity changes of about 4m/s could occur. Head strikes with an
unyielding surface at this velocity are likely to produce severe head injuries
(Friedman).
Summary of Analysis of Roof-Crush
and Related Injuries in Rollovers:
For simple side-on rollovers the vertical impacts with
the ground are likely to be of sufficiently low speed to not be a direct
problem with typical roofs.
End-over-end rollovers and launching rollovers, where
the car C of G gains significant height above the ground, involve much
larger vertical impacts and roof strength is more critical.
Roof Shifting (side-ways) is a cause for concern.
Possible effects are that the side window may shatter and that the occupant
is exposed to greater risk of direct contact with the ground or partial
ejection.
Restrained occupants who are partially ejected may be
subjected to a whipping action which forces their upper body out of the side
window. They are then violently pulled back inside the vehicle and at this
time the inboard side of their head may contact the outside
upper edge of the side window frame.
Most of the
energy absorption appears to take place when the wheels, springs, shocks and
underside of the vehicle contact the ground. Therefore a stiff roof
structure is unlikely to prolong the roll, but provide additional safety for
its occupants.
ROLLOVER & ROOF CRUSH
DEFECTS EVALUATION
 If
you or a member of
your family has been involved in a SUV rollover accident, van
rollover, truck rollover with a severe roof crush, roof pillar
collapse failure or any other serious accident or failure and
you have questions about whether the vehicle or roof design may
caused or contributed to the accident or injuries, then call us now. Mr.
Willis is a Board Certified Personal Injury Attorney with over 20 years of
product liability and rollover litigation experience. Call the Willis Law
Firm at 1-800-883-9858 for a Free & Confidential
Consultation or Click
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