Using a new dynamic test and a dummy designed especially for rear impact testing, the Institute has rated 73 seat/head restraint combinations available in 63 car models sold in the U.S. market. The ratings of good, acceptable, marginal, or poor indicate the range of occupant protection from whiplash injury in rear-end crashes at low to moderate speeds.
Starting points for the ratings are the evaluations of head restraint geometry the Institute has been conducting since 1995 (see "Saving our necks in car crashes," Sept. 16, 1995). Now seats with head restraints that have good or acceptable geometry are being tested dynamically to compare their protection against neck injury in rear impacts. These seat/head restraint combinations earn overall ratings based on both geometry and dynamic test results. The Institute isn't testing seats with head restraints rated marginal or poor for geometry because such seats won't protect taller people. They're rated poor overall, based on geometry.
Only 8 of the 73 seat/head restraints that were dynamically tested earned overall ratings of good. Sixteen are acceptable, and 19 are rated marginal. The other 30 seat/head restraint combinations that were tested are rated poor, as are 24 seats that weren't tested because of inadequate geometry. The seat/head restraints that were dynamically tested together with those that weren't represent available seats in current car models the Institute has evaluated in its high-speed frontal offset crash test program.
Institute ratings of seats and head restraints in cars sold in the U.S. market are part of an international program that includes ratings of additional seat/head restraints in the Canadian, Australian, and European markets.
"Consumers in markets worldwide can use the new ratings to buy cars that provide better protection in rear-end crashes," says Adrian Lund, the Institute's chief operating officer.
Winners and losers
Among the seat/head restraints that were tested dynamically, the winners are the ones in Volvos (all models) and Saab 9-2X and 9-3 models. These are rated good. So are the seat/head restraints in the Jaguar S-Type, Subaru Impreza, and some Volkswagen New Beetles. The dynamic test performance of the 2004 Toyota Corolla's seat/head restraint also was good, but this car's overall rating is acceptable because the head restraint's geometry is rated acceptable.
A total of 54 seat/head restraint combinations are rated poor overall.
"It's obvious that some automakers are doing a better job than others of designing seats and head restraints to protect their customers' necks in rear crashes," Lund says. "Especially disappointing is that so many car models still have head restraints with poor or marginal geometry. Good geometry is a simple and necessary first step toward adequate protection, and seats with bad geometry cannot begin to protect many taller occupants." Two-thirds of the 24 seats that weren't tested dynamically because of inadequate head restraint geometry are in General Motors cars.
Neck injuries sustained in rear-end crashes seldom are life-threatening, but they can be painful. They occur frequently and are expensive. In the United States alone, they cost at least $7 billion in insurance claims per year.
Importance of a good seat/head restraint
When a vehicle is struck in the rear and driven forward, the vehicle seats accelerate occupants' torsos forward. Unsupported, the occupants' heads will lag behind the forward movement of their torsos. 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 following a rear impact. 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," Lund adds. "But good head restraint geometry by itself isn't sufficient. A seat also has to be designed so it doesn't rotate backward in a rear impact because this would move the head restraint away from 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 new evaluation criteria take into account both static restraint geometry and the dynamic performance of seats and head restraints together in tests."
New versus old evaluation procedures
Since 1995 the Institute has been rating the geometry of head restraints in passenger vehicles based on how close the restraints are to the back of the head of an average-size man. In publishing the first ratings, the Institute explained that "good geometry is necessary but not sufficient for good protection. The relative stiffness of the seatbacks also helps determine effectiveness."
Assessing seatback stiffness and other characteristics of whiplash injury prevention requires crash testing or other dynamic assessments that weren't practical in the mid- to late-1990s. Very few head restraints back then had geometry sufficient to warrant dynamic testing. The geometry of most head restraints was marginal or poor. Such restraints cannot provide adequate protection because they cannot be positioned to support many people's heads during crashes.
Another reason dynamic tests weren't conducted is that there wasn't a test dummy with a realistic spine and neck configuration designed for testing in rear-end crashes at low to moderate speeds. Existing dummies in the mid- to late-1990s had rigid spines and necks that weren't designed to produce human-like responses to rear crash forces.
Since then a new test dummy, BioRID, has been developed that's designed specifically for rear crash testing. Representing an average-size man, BioRID is beginning to be widely used. Plus automakers have improved the geometry of head restraints. The Institute's first evaluations, which involved 1995 models, found only 5 seats with good geometry. In contrast, 80 percent of the head restraints in 2004 models have good or acceptable geometry. Some models also are being equipped with new head restraints designed to move closer to the backs of people's heads during rear impacts. Dynamic testing is required to evaluate these "active" restraints and seatbacks that are specially designed to reduce acceleration forces.
Two-step rating procedure
Geometry: First the height and backset of head restraints are evaluated according to this protocol, which is used internationally.
Dynamic test: Then seats with good or acceptable head restraint geometry are evaluated in a simulated rear impact using measures recorded on a BioRID dummy. Overall ratings reflect both restraint geometry and test performance.
No dynamic test: Seats with marginal or poor head restraint geometry, such as the one below, aren't tested dynamically. They're rated poor overall because their geometry is inadequate to protect taller people.
Ratings are released internationally
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 the following research organizations supported by automobile insurers: Thatcham in the United Kingdom; Allianz Centre for Technology in Germany and the German Insurance Institute for Traffic Engineering; Folksam Insurance in Sweden; ICBC in Canada; Insurance Australia Group; and CESVIMap in Spain.
IIWPG conducted extensive research and testing to develop the procedures for the dynamic test and evaluation criteria that have been used by member research groups, including the Institute, to rate the performance of more than 200 seat/head restraint combinations in vehicles sold in a number of world markets. Earlier this week, the ratings were released simultaneously by IIWPG partners in Australia, Canada, Germany, and the United Kingdom as well as by the Institute in the United States.
IIWPG rating procedures
Overall seat/head restraint ratings are based on a two-step evaluation. In the first step restraint geometry is rated, using the same procedures as before. Seats with good or acceptable geometric ratings then are subjected to a dynamic test conducted on a sled that simulates the forces in a stationary vehicle that's rear-ended by another vehicle of the same weight going 20 mph.
The dynamic test 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 tension and shear forces recorded on BioRID 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 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 73 overall ratings (see previous pages) represent more seat/head restraint combinations than are listed. When the ratings for a car model's seat options are the same, these ratings are combined.
Sled test sets tougher standard
Nine seat/head restraints rated good for geometry and another 21 with acceptable geometry turned in poor performances in the dynamic test.
"The principal reason for the failing dynamic performances of these seats was that the seatbacks rotated backward in the test," Lund says. "This moved the head restraint farther from the dummy's head, so initial contact with the head restraint took longer. The result was that the dummy's head wasn't supported in time to reduce the differential motion of the head and torso that leads to neck injury. So, although the auto manufacturers have been improving the geometry of the head restraints in their cars, in many cases they need to make further improvements to their seats and head restraints."
Saabs and Volvos are winners
The seat/head restraint combinations in two Saab models and three Volvos are rated good, but the designs of these systems aren't the same. As an occupant's torso sinks into a Saab 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. Volvo took a different approach, designing seatbacks with a special hinge to reduce the forward acceleration of an occupant's torso.
"The designs are different, but the result is the same," Lund points out. "Both Volvo and Saab have found a way to reduce the differential motion of an occupant's head and torso that causes neck injury in rear crashes. This is what we want every automaker to do."
Institute research released in 2002 indicated that fewer neck injury claims are filed for Volvos and Saabs with the advanced seat/head restraint systems, compared with older models of the same cars without such systems (see "Not your father's head restraint: New designs reduce neck injuries," Oct. 26, 2002).