New unibody autos are safer - but more expensive to repair

There's a new driving skill some people are having to learn. It might be called the ''unaligned-car-body tug of war.'' You may have driven behind someone practicing this maneuver. The back wheels of the car can't seem to stay directly behind the front wheels. In fact, they're several inches to the left or right of where they ought to be. So the car looks as of it's trying to travel sideways while the driver fights to keep its nose pointed straight ahead.

While there is more than one reason for this problem, a major one can be traced to the development of ''unibody'' car construction, a technique that began appearing on many European and Japanese cars in the mid-1970s and on US-built cars later in the decade. If you are driving a car built after 1980, it is very likely a unibody, especially if it's a foreign model.

The unibody design was supposed to make the cars safer, a goal that seems to have been reached. But it has also made them much more complicated and costly to repair, requiring consumers to be more careful about finding qualified body shops.

Essentially, unibody takes the traditional car frame - which supported the seats, engine, car body, and other major parts - and discards it. In its place is a complicated system that makes a large underbody panel, a welded body frame, the doors, fenders, bumpers, and even the glass work together for a car that is safer in a crash. In theory, the car will collapse like an accordion, with the sides expanding outward, instead of the body's being compressed above the traditional frame.

The concept appears to have worked. Traffic fatalities in the United States, as compiled by the National Highway Traffic Safety Administration, have dropped from more than 51,000 in 1980 to 42,500 in 1983, even though the number of miles driven increased from 15.3 billion to 16.5 billion. Thus, the rate of fatalities has declined more than 22 percent, from 3.34 per 100 million miles to 2.6.

How much of this good news is due to unibody construction and how much to safer driving may be impossible to measure, but insurance and auto repair experts believe unibody has played a key role.

''It's my opinion that the increase of unibody and the decrease in frame cars is a significant factor in this safety record,'' said James Burcham, director of insurance industry relations at MAACO Enterprises, the nationwide auto painting and body repair company. The 55-an-hour speed limit has been in effect through this entire period, he pointed out, so reduced speeds counted little for the decline within the period.

The integration of much of the automobile into one unit does mean more cars are declared total losses. This, combined with the greater expense involved in repairing a unibody car, has raised car insurance costs, though medical and life insurance claims have been reduced.

But for now, there simply aren't enough qualified people to repair these cars. An improperly repaired car can be difficult or even unsafe to drive. There have been reports of unibody cars that would not steer properly after they left the body shop, or even some that broke apart the first time they hit a serious bump.

While over 80 percent of the cars on US roads are of the unibody type, the percentage of shops and employees that can do major body repairs on them is far lower. There are some 58,000 independent body shops in the US, says Murt VonLeer , damage claim manager at the Kemper Group, the insurance company. Many of the people who work in these shops have only recently started going to unibody repair classes. But about 40 percent of the attendees at Kemper's classes have come from the insurance industry, Mr. VonLeer says, so they presumably will not be repairing cars.

To repair a unibody car properly, the mechanic now has to put all the metal back into its original shape, a process that requires straightening the car's shell through simultaneous pulls at several points, directly opposite from the way it was crushed in the collision. When the job is done, the shell must be within three millimeters of its original position.

Your dealer should be able to tell you if your car is of the unibody type. If it is, there are some ways you can make sure any repairs are done properly.

First, Mr. Burcham says, ask the mechanic if the shop has the unibody specification books for your car's make and model. If it doesn't or he says he does not need them, ''don't allow them to repair your car,'' he says.

Second, make sure the shop knows what a rack system or bench-and-fixture system is and that it has one of these systems to repair your car. The bench-and-fixture system uses metal fixtures that fit into factory-designated control points to pull the car into shape.

Third, make sure it is using a metal inert gas (MIG) welding system, not an oxyacetylene or spot welding system. The high-strength steel being used in unibody cars can only be properly welded with a MIG system, VonLeer says.