NHTSA – Rollovers

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.

This failure has led to:

  • Roofs that can bend, crush, or separate from the vehicle,
  • Inadequate or poor welding procedure that can cause a steel roof to collapse and separate, and
  • Structural roof support / roof pillars that may be too weak in strength and defective in design and manufacture

Vehicle 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 Diagram

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 1.3×3.82=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.

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