Drive General Motors Corp.'s all-new Chevrolet Silverado/GMC Sierra full-size pickups and one immediately perceives a marked improvement in structural "feel." It's largely due to the automaker's newly developed design - and production process - for the GMT800's ladder frame.

Customers and the jaded press have offered mostly raves about the new structure - with some notable exceptions (see sidebar, p.96). But for the most part, the innovative undercarriage is having a similar pronounced effect across the industry.

It's known around the small Ontario town of St. Thomas, where the GMT800 frame is built by Magna International Inc., that Ford Motor Co. has inquired often about the supplier's hydroforming technology. "They've knocked several times," confides one Magna executive.

There's reason for the interest. Although the hydroforming process has been much-hyped, the modularity of the new light trucks' platform is at least equally significant. The modular design should provide GM with several long-term manufacturing benefits - including reduced buildtime, less changeover downtime and improved model differentiation - as well as a multi-year lead on competitors.

That's because the GMT800 frame uses three box-shaped modules to replace the traditional full-length frame rails. These box selections can be mixed-and-matched to produce the maximum number of model variations with a minimal number of parts.

Employing four front modules, four midsections and two rear frame sections, GM can create the 14 models to be introduced this year off the GMT800 platform. Just an additional three midrail and two rear frame sections are necessary to accommodate the remaining 26 models scheduled to launch over the next few years. This flexibility is in stark contrast to more traditional truck-frame manufacturing that requires relatively intensive tooling changes for a large percentage of the myriad chassis variants the truck market demands.

The keys to the GM/Magna manufacturing flexibility are roll-forming and draw-bending; the former process gradually pulls a flat coil of steel through a set of rollers into the desired finished section, and the latter clamps the midrail in place. The frame also features tubular crossmembers that replicate previous stamped components but deliver increased rigidity.

"It's this strategy where we feel we've got an advantage over anything we've done in the past as well as the competition, particularly in the midrail area," says Frank Casali, GM engineering group manager, full-size trucks.

"We offer four wheelbases on these pickups, and all that wheelbase difference is taken up in the midrail. With roll-forming and draw-bending, if the marketing comes in sometime down the road and it indicates we want to try a different wheelbase or there's a niche market we want to go after, it's simply a matter of us roll-forming a shorter or longer length and adjusting the kickup position with the existing tooling.

"In the old manner of a stamped, single-piece rear rail," he continues, "we would have had to buy a whole new set of tools. That requires a very long lead time and is expensive. This is just of a matter of, 'OK, roll it out a little bit longer and adjust where the kickup (part of the rail) is located,'" asserts Mr. Casali.

That kind of manufacturing flexibility reportedly has other automakers eager to strike a deal with Magna, which built the 1 million-sq.-ft. (93,000-sq.-m) facility after winning the GM contract in 1994. "We think we have a significant lead" over other suppliers, says Dennis Bausch, Magna executive vice president-marketing and planning.

Each day, some 500 robots and 1,000 employees on four assembly lines churn out 3,000 frames that wind through an overhead conveyor system nearly 6 miles (9.6 km) long. Total annual capacity at the plant will grow to 1.5 million units when construction is completed on two more assembly lines.

Other benefits: the GMT800 frame manufacturing process reduces scrap metal by 44 lbs. (20 kg) per vehicle and overall weight by 15 lbs. (7 kg) because the rails no longer have to overlap. Out-of-the-factory front end alignment is improved due to more accurate welds; it also eliminates extensive heat-shielding packages.

And for good or ill, GM reduced its number of frame suppliers from three - A.O. Smith Automotive Products Co., Dana Corp. and Tower Automotive Inc. - for the previous-generation C/K pickups to just Magna for the GMT800 program. O