Car's Type of Frame Makes a Difference

Times Staff Writer

Question: I was advised to buy a car with a full-perimeter frame as opposed to a car with unit-body construction, if I was at all concerned with safety. Can you explain the difference? Which popular cars today are built with full-perimeter frames?--M.E.

Answer: The vast majority of new cars are produced with what is called unit-body or unibody design, in which the body itself acts as the frame. In a unibody car, the sheet-metal floor pan, the fenders and the roof, among other parts, are welded and bolted together to carry the load of the car. The wheels are simply attached to the body.

A dying breed of car, however, features full-body frames. These cars have two heavy steel beams that run the length of the car. The wheels are attached to this frame, and it bears the load of the entire car.

Automobiles that have full frames are often touted as being safer than unibody cars. It is not always true, though, that a unibody car is unsafe in a moderate collision. But you can be sure that a unibody car will almost always sustain much greater physical damage in a collision than a full-frame car.

Automobile manufacturers have attempted to make cars safer by designing them so that the body will collapse in an accident, thereby absorbing some of the force of the impact. That helps make large, front-wheel-drive cars with unibody frames safer, but it also makes them vulnerable to great damage in only a small collision. The alignment on some unibody-designed cars can be thrown off in only minor collisions, and repair bills can be astronomical even when damage appears minor.

In a car that has a full frame, the frame itself will bear much of the load in a major accident. The frame may bend, but it can be straightened much more effectively than a unibody can be straightened. Usually, full-frame cars are full-size models that are the heaviest cars on the road, which gives occupants in them an additional measure of protection.

As for subcompacts, almost all of them are rated by the federal government as among the least-safe cars. Many of these cars barely weigh a ton, compared to some larger cars that weigh twice that much.

Finally, you may want to purchase a publication that lists the results of government crash tests on various car models. Those government tests show that the size and weight of cars are not the only factors that affect safety. The ability of the car's windows to stay put and the placement of steering wheels and dashboards also are critically important to passenger safety.

Some of the cars that still have full frames are the Chevrolet Caprice, Ford LTD, Lincoln Town Car, Mercury Grand Marquis and the Cadillac Fleetwood. Some pickup trucks and passenger trucks also feature full frames.

Q: I always wonder what a sign that says "Diesel No. 2" means. Is there such a thing as diesel No. 1, and is it better or worse than diesel No. 2? Do trucks use the same diesel as is sold for passenger cars, and if not, what is the difference?--M.V.

A: Diesel No. 1 does exist but it is mostly sold during winter in colder climates. Diesel No. 2 has more wax in it and does not flow well at very low temperatures. That can make for hard starting and poor performance. Trucks use the same diesel fuel as passenger cars.

Ralph Vartabedian cannot answer mail personally but will respond in this column to automotive questions of general interest. Do not telephone. Write to Your Wheels, You section, The Times, Times Mirror Square, Los Angeles 90053.

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