November 15, 2004
Most cars sold in the U.S. do a poor job of protecting against neck injuries in rear-end collisions, according to a new dynamic test using a dummy designed especially for rear impact testing.
Only 8 of the 73 seat/head restraints tested by the Insurance Institute for Highway Safety (IIHS) earned overall ratings of good. Sixteen were acceptable and 19 were rated marginal. The other 30 were rated poor, as were 24 seats that weren't tested because of inadequate geometry.
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.
Among the few winners were the head restraints in all Volvo models and those in the Saab 9-2X and 9-3 models, which were rated good. Also winning good ratings were the 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 only acceptable because the head restraint's geometry is rated acceptable.
"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," said Adrian Lund, the institute's chief operating officer.
"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."
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.
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."
Although Saabs and Volvos were both winners in the texts, they used different methods to get there.
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.
The complete listing of test results is available on the Institute's Web site (Adobe Acrobat required).
