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.
Head and torso must move together
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 close to the back of an occupant's head so it can contact the head and support it early 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.
Seat/head restraints in the Volvo XC90 and Subaru Forester earn good overall ratings, in part because of their advanced designs that help keep the head and torso moving together in a crash. As an occupant's torso sinks into the Subaru seat during a rear crash, a mechanism in the seatback is designed to push the head restraint up and toward the back of the head. The goal of the Volvo seat is the same, but the design is different. In the XC90, the seatback includes a special hinge to reduce the forward acceleration of an occupant's torso.
The seats in the Mercedes M class are rated marginal by the Institute, but recent tests by an insurer group in the United Kingdom produced a good overall rating for M class seats fitted with an optional "active" restraint designed to move up and toward the head during a crash. Unfortunately, seats with this better head restraint design aren't yet available in M class models sold in the United States - not even as an option. A similar seat design is standard equipment in some Mercedes car models sold in the U.S. market, and the Institute will evaluate these early in 2006.
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. A seat/head restraint's dynamic rating is combined with its geometric rating to produce an overall rating.
"The seats from Subaru and Volvo work well, but dynamic tests are showing that not all of these advanced designs result in improved protection," Lund points out. "For example, active head restraints in three models from Nissan - XTerra, Pathfinder, and Infiniti FX - are marginal or poor overall. In contrast, seats in the Ford Freestyle are rated good even without the bells and whistles of the advanced designs."
Rating 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). In addition to the Institute, IIWPG members include Thatcham in the United Kingdom; Allianz Centre for Technology in Germany and the German Insurance Institute for Traffic Engineering; Folksam Insurance in Sweden; Insurance Corporation of British Columbia in Canada; Insurance Australia Group; and CESVIMap in Spain. These are all research organizations supported by automobile insurers.
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 because rear- end crashes are so common."