The smooth, flowing lines of most new cars and trucks conceal bumpers so well consumers rarely give them a second thought. But back into a pole in a Wal-Mart parking lot on a busy weekend and bumper design suddenly becomes an important part of your life.

If you ram the pole in a Hyundai Elantra going 5 mph (8 km/h), you're looking at more than $1,700 in damage and all the hassle that entails: dropping the car off at the body shop, fighting with the insurance company, mooching rides to work for a week while they fix your car.

If you're driving a Volkswagen New Beetle, you curse the moron who stuck a steel pole right behind your parking space and drive away without a scratch. Unfortunately, bumpers are one of the few automotive components that are not consistently getting better over time - although there are exceptions, says the Insurance Institute for Highway Safety (IIHS).

Federal government standards used to prohibit all except minor cosmetic damage to the bumper itself in 5 mph tests. But in 1982, standards were rolled back to 2.5 mph (4 km/h), and allowed unlimited damage to the bumper and attachments. Carmakers loved the change, although some cars and vans still meet 5-mph standards. Insurance companies hate it.

They may not look like much, but bumpers are the focal point of a major long-term battle that pits automakers against auto insurers. Consumers and suppliers are stuck in themiddle. Automakers want to make lighter, lower-cost (and higher-profit) vehicles with sleek cab-forward designs and short overhangs. Insurers want big, strong bumpers that stick out ahead of the rest of the body to provide maximum protection - and maximum profits for the insurance industry.

Consumers say they want vehicles that will withstand a 5-mph crash without damage, but they'll rarely pick one vehicle over another just because it has better bumpers. Meanwhile material suppliers are battling as never before over market share in what is a huge segment of the automotive components business.

Bumpers typically consist of a plastic cover with a reinforcement bar made of steel, aluminum, or plastic. It also typically includes an energy absorber between the outside cover and the reinforcement bar, usually a polypropylene foam or plastic honeycomb.

All together, these surprisingly complex assemblies represent a huge business to dozens of different automotive suppliers. Greg Janicki, an analyst at CSM Corp., a company that tracks the vehicle and components industry, says giant bumper system suppliers have not yet emerged as they have in automotive interiors, brakes and other component systems. Instead many suppliers still compete for business. The competition is lively.

The American Iron and Steel Institute (AISI) actually put out a press release recently boasting that steel expects to increase its share of the car and truck bumper market from a current 76% to 84% by 2001. "This percentage may surprise automotive observers, because many bumper systems have plastic fascias. However, a hidden steel beam actually provides the structural strength," AISI points out.

That's true, but styling changes, aggressive plastics producers and aluminum parts are making the steel industry nervous. A number of new plastic designs replace traditional metal beams with plastic for major structural and energy absorbing components.

The bumper business is changing rapidly thanks mainly to the growing popularity of light trucks, says CSM's Mr. Janicki. Automakers are smoothing out the lines of trucks such as the Lincoln Navigator, GMC Denali and others by replacing old-fashioned exposed bumpers with new designs that encase the bumper beam in a smooth plastic fascia.

But they are continuing to use reinforcement beams made of steel because it's low cost, and steelmak-ers have done a good job of developing ultra high-strength steels that can be formed into lighter weight bumper beams.

Aluminum, the lightest bumper beam material available, appears to be losing ground, probably because it also is the highest cost.

Insurers don't care what they are made of; they just want beefier bumpers. And the newest designs aren't always the best. The New Beetle bumpers which perform so admirably in the IIHS tests, for example, use an old design featuring hydraulic shock absorbers, says Mike Ciccone, special projects coordinator at IIHS. Several other cars that have scored well on IIHS's tests also feature the old hydraulic absorbers, he says, but he cautions that some new designs also fare well in some cases.

The Hyundai Elantra did poorly because its bumpers have little energy-absorbing materials, Mr. Ciccone says, but other bumpers perform poorly for other reasons. Some BMW and Mercedes models - renowned for safety in high-speed crashes - incur expensive repairs during 5-mph impacts because styling doesn't allow bumpers to stick out far enough, he says.

"If you don't have enough overhang, no matter what you have, you're going to have problems," he says. "You need overhang, energy-absorbing material and a strong enough bumper beam. Those are three main elements."