They don't look like much, but old-fashioned chassis frames are the very foundation for some of the toughest and most popular vehicles in North America.

Most new cars feature unibody construction in which large stamped metal sheets are welded together to form a strong, relatively lightweight body shell. But most light trucks and sport/utility vehicles (SUVs) still use traditional body-on-frame chassis architecture, which is heavier but considered more rugged.

Some of the newest, smaller SUVs are switching over to unibody construction to reduce weight, improve fuel economy and provide a more car-like ride. But to the typical buyer of full-size pickups and their derivatives, unibodies are for sissies. So automakers and suppliers now are developing new ways to take out weight and cost of traditional ladder-frame chassis without sacrificing toughness.

The hottest--and most controversial--new manufacturing method is hydroforming, a process that uses water or other fluids under very high pressure to precisely shape metal tubes into hollow structures.

Most major chassis and frame suppliers now have hydroforming capabilities, but insiders say Magna International Inc. is one of the auto industry's biggest proponents of hydroforming and is winning substantial new business as a result. Magna's latest coup: a $370 million contract from competitor Dana Corp. to produce more than one million frames annually for General Motors Corp.'s next-generation C/K platform trucks beginning in 1998. However, some sources suggest that Magna and others may be overselling the technology a bit.

Hydroforming has been used in aerospace and other industries for decades, but it's just now catching on for structural applications in the auto industry. In the structural arena, hydrotormed tubes are being used to replace box section assemblies typically made of stamped steel components welded together.

It is claimed to be an ideal process for making car subframes, structural parts such as engine cradles, radiator surrounds and supports, lower and upper longitudinal body rails, instrument panel support beams, D-pillars for station wagons and various body crossmembers.

The American Iron and Steel Institute says less than 10% of the steel in the typical North American-built family vehicle was in tubular form in 1981, but that has grown to more than 15% today thanks to the popularity of body-on-frame vehicles such as pickup trucks, SUVs and full-size vans that have the highest content of tubular steel.

The hydroforming process starts with a round metal tube placed in a forming die. The tube is then filled with a fluid--usually water--under high pressure that forces it to conform to the shape of the die cavity.

Proponents say hydroforming lowers assembly and tooling costs and cuts die wear while reducing weight and increasing strength and stiffness. Based on some estimates, hydroformed steel parts are 10% to 15% lighter than comparable stamped steel assemblies. Industry analysts estimate that GM's new C/K truck frames could be 25 lbs. (11.3 kg) lighter and cost $60 less thanks to hydroforming.

But other industry experts caution that hydroforming isn't right for every application and can have drawbacks.

A.O. Smith Automotive Products Co. soon will be producing hydroformed engine cradles and several other parts, but Steve Kretschmer, vice president of product engineering, says the benefits vary from application to application.

"There are a variety of technologies . . . conventional stamping, roll forming, hydroforming. All are important technologies, but none is right for everything," he emphasizes. "Successful companies will be those that pick and choose among their technologies to get the right assembly at the right cost and weight.

"One of the things folks will tell you is that hydroformed parts are lighter, but it's real tough to make general statements like that," Mr. Kretschmer says. The process doesn't allow as much flexibility in shape design as conventional stamping, and that can lead to design compromises or a structure that is not as stiff as a conventional structure of the same weight, he says.

Changes in the "designed-in" openings in frames also can be difficult to accommodate with hydroform tooling. "Truck frames have a zillion holes that are constantly being moved," he says.

Not surprisingly, Mr. Kretschmer touts a new patented A.O. Smith forming technique called "Simulform" as another new way to chop weight and cost from vehicle structures. It allows the forming and joining of all major structural sections of a boxed assembly, such as an engine cradle, in just two press strokes, he says. Applications include floorpan assemblies, subframes and cradles, space frames, crossmembers and front/rear sections of full frames.

A.O. Smith will use the new Simulform process to produce a new engine cradle later this year that's expected to be substantially cheaper than the part it replaces, which currently is hydroformed, Mr. Kretschmer says.