Rear Crash Protection in SUVs and Pickups
Only 6 of the seat/head
restraint combinations in 44 current model SUVs
are rated good for protection against whiplash
injuries in rear-end crashes. None of the seat/head
restraint designs in 15 pickup truck models earns
a good rating. Overall 4 out of 5 SUV and pickup
seat/head restraints recently evaluated by the
Insurance Institute for Highway Safety are rated
marginal or poor for whiplash protection. This
is the first time the Institute has tested SUV
and pickup seats using a dummy that can measure
forces on the neck during a simulated rear-end
crash.
Only the seats in the Ford Freestyle, Honda Pilot,
Jeep Grand Cherokee, Land Rover LR3, Subaru Forester,
and Volvo XC90 models earn good overall ratings.
Among those earning poor ratings are seat/head
restraints in popular models such as the Chevrolet
TrailBlazer, Ford Explorer, and Toyota 4Runner
SUVs plus the Chevrolet Silverado pickup truck
and some seats in Ford F-150 and Dodge Dakota
pickups.
"Manufacturer advertising often emphasizes
the rugged image of SUVs and pickups," says
Institute president Adrian Lund. "However,
the Institute's evaluations show seats and head
restraints in many models wouldn't do a good job
of protecting most people in a typical rear impact
in everyday commuter traffic."
The Institute evaluates seat/head restraints in
two stages. First restraint geometry is measured
to determine its height and distance behind the
back of the head of an average-size man. Seats
with good or acceptable head restraint geometry
then are tested dynamically on a movable platform
using a dummy that measures forces on the neck.
This sled test simulates a collision in which
a stationary vehicle is struck in the rear by
a vehicle of the same weight going 20 mph. Seats
without good or acceptable geometry are rated
poor overall because they cannot be positioned
to protect many people in rear-end crashes.
Good seat/head restraint design keeps head and
torso moving together in a rear impact: When a
vehicle is struck in the rear and driven forward,
the vehicle seats accelerate occupants' torsos
forward. Unsupported, an occupant's head will
lag behind the forward movement of the torso.
This differential motion causes the neck to bend
back and stretch. The higher the torso acceleration,
the more sudden the motion, the higher the forces
on the neck, and the more likely a neck injury
is to occur.
"The key to reducing whiplash injury risk
is to keep the head and torso moving together,"
Lund explains. "To ensure they move together,
a seat and head restraint have to work in concert
to support an occupant's neck and head, accelerating
them with the torso as the vehicle is driven forward.
To accomplish this, the geometry of the head restraint
has to be adequate, and so do the stiffness characteristics
of the vehicle seat."
A head restraint should extend at least as high
as the center of gravity of the head of the tallest
expected occupant. A restraint also should be
positioned near the back of an occupant's head
so it can contact the head and support it in a
rear-end crash.
If a head restraint isn't positioned behind an
occupant's head, it cannot support the head in
a rear impact, but good restraint geometry by
itself isn't sufficient. A seat also has to be
designed so its head restraint doesn't move backward
in a rear impact because this would prevent the
restraint from catching the head. At the same
time, a vehicle seat cannot be too stiff. It has
to "give" so an occupant will sink into
it, moving the head closer to the restraint. The
evaluation criteria take into account both static
geometry and the dynamic performance of the seats
and head restraints together in the test.
Geometry is improving: The Institute doesn't test
seats with head restraints that are rated marginal
or poor for geometry. These seats automatically
earn a poor rating overall because their head
restraints cannot be positioned to protect many
taller people.
"It's encouraging that only 12 of the 58
seat/head restraint combinations we evaluated
didn't make it to the testing stage because of
marginal or poor geometry," Lund says. "The
auto manufacturers have been working to improve
this aspect of head restraint design."
Rear-end collisions are frequent, and neck injuries
are the most common serious injuries reported
in automobile crashes. They account for 2 million
insurance claims each year costing at least $8.5
billion. Such injuries aren't life-threatening,
but they can be painful and debilitating.
Rating seat/head restraints is international effort
by insurers: Recognizing the improvements in head
restraint geometry and the need to move beyond
ratings based solely on geometry, the Institute
joined with other whiplash injury prevention experts
in late 2000 to organize the International Insurance
Whiplash Prevention Group (IIWPG).
IIWPG conducted extensive research and testing
to develop the procedures for the dynamic tests
and evaluation criteria that have been used by
member research groups, including the Institute,
to rate the performance of seat/head restraint
combinations in vehicles sold in a number of world
markets. Ratings also are being released in Australia,
Canada, and the United Kingdom.
Sled test simulates rear-end collision: Overall
seat/head restraint ratings are based on a two-step
evaluation. In the first step restraint geometry
is rated using measurements of height and distance
from the back of the head of a mannequin that
represents an average-size man. Seats with good
or acceptable geometric ratings are subjected
to a dynamic test conducted on a crash simulation
sled that replicates the forces in a stationary
vehicle that's rear-ended by another vehicle of
the same weight going 20 mph. A dummy specially
designed to assess rear-end crash protection (BioRID)
is used to measure the forces on the neck during
simulated crashes. The sled is a movable steel
platform that runs on fixed rails and can be programmed
to recreate the accelerations that occur inside
vehicles during real-world crashes.
"The sled test simulates the kind of crash
that frequently occurs when one vehicle rear ends
another in commuter traffic," Lund says.
"People think of head restraints as head
rests, but they're not. They're important safety
devices. You're more likely to need the protection
of a good head restraint in a collision than the
other safety devices in your vehicle."
The Institute's dynamic ratings of good, acceptable,
marginal, or poor are derived from two seat design
parameters (peak acceleration of the dummy's torso
and time from impact initiation to head restraint
contact with the dummy's head) plus neck tension
and shear forces recorded on the BioRID dummy
during the test. The sooner a restraint contacts
the dummy's head and the lower the acceleration
of the torso and the forces on the dummy's neck,
the better the dynamic rating.
The Insurance Institute for
Highway Safety is a nonprofit research and
communications organization funded by auto insurers.
For over 30 years the Insurance Institute for
Highway Safety has been a leader in finding out
what works and doesn't work to prevent motor vehicle
crashes in the first place and reduce injuries
in the crashes that still occur. The Institute's
research focuses on countermeasures aimed at all
three factors in motor vehicle crashes (human,
vehicular, and environmental) and on interventions
that can occur before, during, and after crashes
to reduce losses.
(Source: Insurance
Institute of Highway Safety) |