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Special Report: Automotive

Corvette backbone

By J. Neiland Pennington

Above: The new C8 Corvette is a 490-hp track-ready rocket, capable of 190 miles per hour out of the box. Photo: National Corvette Museum

Aluminum die castings replace fabricated alloy sheet assemblies on eighth-generation sports car

November 2020 - For at least 20 years, aficionados of the Chevrolet Corvette have speculated on the possibility of the car evolving from a front-engine configuration to mid-engine. Almost all of Europe’s supercars have the engine mounted behind the seats. With the the exception of a few one-off efforts, Corvette was the American holdout.

Speculate no more. For its eighth generation, the Corvette does indeed introduce the much-anticipated and long-awaited mid-engine design. The C8 Stingray is a total break from a 67-year front-engine tradition, now putting it in league with Ferrari, Lamborghini, McLaren, Lotus and many of the other European cars that appear on teenage boys’ posters.

It was no secret that Zora Arkus-Duntov, the Corvette’s first chief engineer, wanted to build a mid-engine sports car. Albeit many years posthumously, he has his wish. It isn’t that he and his engineers didn’t try. In 1960, they built the Chevrolet Engineering Research Vehicle (the CERV I), a single-seat, open-wheel vehicle closely resembling a Formula 1 racer. That exercise ended with the CERV III in 1990, but the mid-engine concept wouldn’t go quietly. Chevrolet’s code name for the 2020 Corvette is ZERV, a tip of the hat to Arkus-Duntov and his first mid-engine engineering exercise.

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Corvette assembly line at Chassis 3. This stage of assembly is after fluid fill, just before inspection. Photo: National Corvette Museum

Eleven years ago. both this magazine (July/August 2009) and its sister publication, Modern Metals (May 2009), published in-depth features about building an aluminum version of the Corvette’s steel space frame. Designed for the ZR1 and Z06, the highest performance models, the conversion cut weight by 62.5 kilograms or 137.8 pounds.

Fast-forward to 2020. The mid-engine structure is now a center-tunnel backbone design, and is revolutionary for the Corvette. Nevertheless, it was experience with the original aluminum structure that laid the groundwork for the current C8.

The result, Automoblog reports, is “a light, stiff structure as the foundation for everything else in the car. The engine and transaxle sit solidly in the middle of the backbone, and the suspension system hangs off the ends. The rigidity allows the suspension to perform as it should, and not be burdened by chassis flex.” It also lowers the vehicle’s center of gravity, essential to any high-performance car.

Tadge Juechter is Corvette’s executive chief engineer, and only the fifth person to be named chief. The aluminum structure he and his colleagues created over a decade ago for the C6 was largely aluminum sheet, with hydroformed components plus high- and low-pressure castings. Assembly was primarily by MIG welding, with fabrication at the time by Dana Structural Products Group, Hopkinsville, Kentucky, near the Bowling Green, Kentucky, assembly plant where Corvettes were built. That assembly was later moved in-house; production of the current structure continues in Bowling Green.

No more MIG welding

The new model’s center-tunnel backbone is based on 20 high-pressure die castings, six of them major structural members. MIG welding has been eliminated; laser welding, adhesive bonding and screws connect the components. Aluminum extrusions, stampings and hydroformed sections are used, as well as carbon-fiber components.

In an interview with the online magazine Jalopnik, Juechter called the C8 the car with “the most use of high-pressure die casting in General Motors’ history.” The castings, he noted, reduce the number of joints, increase stiffness and reduce mass.

They do not, however, reduce the overall weight of the vehicle. At a net weight of 3,366 pounds, the new car is actually 68 pounds heavier than its C7 predecessor. Weight saving isn’t always an advantage, as the new structure makes the C8 the stiffest Corvette ever, 10 percent more rigid than the previous generation.

Without a rigid platform, you cannot build a high-performance automobile, and you also need precise weight distribution. The C8 has front/rear weight distribution of 43.5/56.5 percent, and the rearward weight bias produces superior high-speed cornering. The purpose is to reduce “plow,” the tendency of a car to continue straight ahead in a curve, but not enough to induce excess slip from the rear tires—what old-time sportscar racers termed “the dreaded oversteer.”

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Todd Haight, left, of Walker Chevrolet, Franklin, Tennessee, presents the keys of this assembled-to-order Stingray to the Quartreys of Cane Ridge, Tennessee. No C8s are made for dealer inventory. Photo: Neiland Pennington

The Bedford Six

The major castings in the backbone assembly are called the Bedford Six for GM’s Bedford, Indiana, casting operation. Although Chevrolet did not reveal the weights of the Six, Bedford has produced some prodigious parts over the years. For example, in the 1990s, the plant die cast the upper engine blocks for the Cadillac Northstar V8 and the so-called “Shortstar” V6 for the Oldsmobile Aurora. (The sidebar on page XX gives an idea of the casting operation’s capabilities.)

Accommodating seniors

The rocker panels are lower on the new Stingray without compromising weight-bearing capacity and structural integrity, making entry and exit easier. It’s no secret that Stingrays are, in many cases, the pride of drivers who have worked an entire career to afford one, so ease of ingress and egress is a significant selling point for older owners.

The backbone design has sufficient stiffness to permit removable transparent or carbon fiber panels, according to Chevrolet. Chevy noted that the backbone also allows a right-hand drive version and increased interior and front trunk cargo space.

Crashworthiness—remember, this is a high-performance car—is enhanced by the tunnel-formed backbone. Rails that bear the load in a collision are more closely spaced. Chevy points out that four beams carry impact loads into the tunnel for enhanced occupant safety.

As a car whose advancements were largely evolutionary, the new Corvette Stingray is a hugely revolutionary redesign. It’s about time, insist many ’Vette fans, and this should keep them satisfied for many years to come.

But not everyone will be mollified. The Corvette engine is still an overhead-valve, pushrod V8 that traces its origin to then-engineer Ed Cole’s original small-block V8, a 265-cubic-inch engine first fitted to the ’Vette in 1955. It’s time, some argue, for an overhead-cam, multiple-valve V8. Stingray technology would then compete with Europe on engine sophistication as well as location. And the idea of a hyper-hybrid car, mirroring the astounding Porsche 918 Spyder, has also been bruited about.

With the rapid pace of automotive development, it is reasonably certain the industry will see advancements from the Bow Tie brand sooner than later. We shouldn’t have to wait until 2087 for Corvette’s next big thing. FFJ

Thinking big at Bedford

GM’s Bedford, Indiana, casting operation has long had the chops to produce massive castings. Our first encounter with its capabilities was 28 years ago, when the operation won the 1992 Grand Award for aluminum die casting in the International Casting Competition, sponsored by Modern Metals. The prize was for the alloy 380 upper casting for Cadillac’s groundbreaking 4.6-liter, four-cam Northstar V8 engine.

The specifications for the part are still impressive. The block was produced in a casting machine with 3,000 tons of clamping force, carrying a four-slide die weighing 65 tons. The shot weighed 76.5 pounds and filled the die cavity in two-tenths of a second, using an adaptive shot control. The trimmed block alone weighed 59 pounds, and it contained eight cast-in grey iron cylinder liners that added 18 pounds.

The judges’ assessment: “Each feature on the part is fluid dynamically designed. This is no conversion from another casting process. It was designed from the outset as a die casting, and a lot of work has been done on the process.”

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