As Safety Standards Rise, So Will The Air Bag Count

AT the 1995 Detroit auto show, Mercedes-Benz rolled out a safety research vehicle equipped with 17 air bags, an elaborate demonstration of how cars of the future might protect passengers from highway mishaps of almost any description.

At the time, show visitors were taken aback by the seemingly implausible number of safety devices built into the X-bag, as Mercedes called its overachiever of a luxury sedan. But what appeared excessive a decade ago is not nearly so far-fetched today; vehicles with eight air bags are available now in showrooms, and the quantity is likely to increase.

Among the factors driving this trend are increasingly stringent safety regulations, especially those involving side-impact collisions. Last month, the National Highway Traffic Safety Administration proposed new test procedures and regulations that would, for the first time, set standards for head protection in side crashes.

Though the government does not stipulate the technology or type of devices automakers must use to meet standards, safety engineers generally agree that the new rules would effectively mandate the installation of side-impact air bags. Since automakers already have a voluntary commitment to make head protection air bags standard in most vehicles by the end of the decade, the proposals are considered likely to proceed through the approval process next year, though some details of the test procedures may be modified.

Shielding passengers from injury in a side impact is a more vexing problem than protecting them in a frontal collision. In a head-on impact, the fenders, hood and suspension all fold up, helping to disperse the forces; the flanks of a vehicle offer far less vehicle structure -- only the thickness of a door -- to absorb the energy of a crash.

''Side impacts, compared to frontal impacts, are much more challenging,'' said Sean Ryan, a supervisor of safety systems at Ford Motor Company. Additionally, Mr. Ryan explained, the limited crush space reduces the time in which the system can react to an impact and inflate the air bag.

Several configurations of side-impact air bags are being used by automakers. Some, like the thorax bags designed primarily to protect the upper body, are tucked behind the door panels. Others are mounted inside the seat backrests; these designs may also provide coverage of the pelvis at the lower end or have extended cushions at the top to shield the head.

Other designs are specifically intended to provide head protection. One is a long cylindrical tube that inflates across the side window area to prevent head contact with solid parts of the car's interior. Another overhead design, the curtain air bag, inflates downward from the ceiling trim and covers the side windows entirely. According to the Alliance of Automobile Manufacturers, more than half of model year 2004 cars and trucks are available with head protection air bags.

The side curtain air bag is regarded by many engineers to be the design best suited to meeting future safety standards, in part because its slim design helps reduce inflation time.

''It's a relatively small-volume bag, so the time it takes to fill and provide effective restraint is much quicker,'' said Patrick Jarboe, a senior engineer at Autoliv, a large producer of auto safety systems. ''We can deploy a side curtain air bag in 10 milliseconds and fill it in about 10 more milliseconds,'' Mr. Jarboe said.

Still, no single air bag design can cover all the bases. ''Generally, a vehicle with a side curtain air bag would provide good protection against head injury,'' said Doug Campbell, the vice president for occupant safety systems engineering at TRW, a major supplier of air bags. ''But the curtain air bag is not large enough for the entire upper body, so you may also need thorax bag or a thorax and pelvic bag,'' Mr. Campbell said. A large S.U.V. that is high off the ground may only need a side curtain air bag, he said.

The proposed changes to the regulations, which affect Federal Motor Vehicle Safety Standard 214, would also require modifications to the crash test dummies currently in use. A new female crash test dummy called for in the proposal represents a 4-foot-11-inch woman. Use of this dummy, along with a more advanced male dummy, will help automakers develop better side-impact safety systems for a broader percentage of the population, according to the safety administration.

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