Four Score

Also from Issue 40

  • 2009 C6 ZR1 debut
  • 1965 big block at Goodwood Revival
  • Comparison Test: 2007 vs. 2008 Z06
  • 2009 Indy Pace Car
  • Saddle Tan Split-Window
  • Rare Aztec Gold 1998 coupe
  • Market Report: C4
  • CRC’s C1-look C5 convertible
  • Callaway B2K at Bloomington
  • How-To: C2 radiator support repair
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The original C4 uniframe weighed just over 350 pounds complete—very light for the day, particularly given its strength. Much of that success came down to the liberal use of HSLA steel. The front and rear frame rails and roof bow were all made of HSLA steel, joined by conventional mild sheet steel elsewhere.

GM had used HSLA steel before—in the X-, J-, and A -Cars, as well as some pickup tailgates—but the Corvette job was different. The sheet widths were more finicky; the panels had to be galvanized inside and out; the sub-pieces them­selves were unusually complex; and new welding technologies had to be developed for joining the various stampings.

Regarding the latter, engineer Walt Jaeger, who worked under Ron Burns, put it this way: “We discovered that welding HSLA to itself takes much higher welding currents plus higher cycle times than normal mild-steel welds. We found we had to go with what we call a dual-pulse weld, with up-slope and down-slope controls. Normal arc-welding temperatures would spatter the galvanized coatings, so we first hit the metal with a much hotter, controlled, in­creasing-energy pulse—the up-slope. Then we drop the current, then cut it off completely, and finally we give it one more jolt. All these pulses are controlled by computer. It took a lot of time to figure out the best current profiles. We ended up with a computerized, automated welding technique that uses four to five times more energy than a typical weld, but it certainly does the trick.”

COMPUTERS WERE INDISPENSIBLE

Chevrolet, already a leader in computer-aided design, relied even more heavily on these digital tools when it came to creating the first truly modern Corvette.

By the late 1970s, everyone said their cars were designed by computer—it sounded good. But in the case of the C4, it also happened to be true. Computer modeling cut the development time of some of the car’s elements in half; in other areas, by two-thirds.

Before hand-building the first C4 pre-prototypes for testing, the team “built” uncounted numerical models on computer. These models were devised from drawings the design engineers “concepted out” for this purpose. By using finite modeling analysis, the computer could then test various alternatives and calculate their probable real-world performance.

In principle, this was a faster, easier method of testing than making prototype parts out of metal and driving them on a proving ground. But that’s not how the C4 team used it: Instead, they invested even more labor in sampling a much larger number of possible solutions to each design problem. Their goal was to find the best answer for every component—not just the least-flawed of a few different hand-fabricated test parts.

To assist with that task, Chevrolet hired a series of outside computing firms, among them the Detroit branch of Structural Dynamics Re­search Corporation. SDRC digitized many of Chevrolet’s drawings and calculated things like their structural stiffness, how rigid various joints would be, the ideal firmness of bushings and other components, the effects of different suspension geometries, and how to best shape the car’s removable roof. Grumman also lent lots of computer support, especially in the areas of structure and crash testing.

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