After 60 years of alternating redesign and evolution, the seventh-generation Chevrolet Corvette has entered production, this time all new and wearing the iconic Stingray nameplate. The 2014 C7 continues Corvette’s tradition as General Motors’ performance and technology flagship, and it's the pacesetter for advanced lightweight materials and structures that are emerging steadily on other GM vehicle programs.
Besides the new aluminum frame, carbon-fiber and lighter-density SMC (sheet molding compound) exterior body panels, and extensively upgraded Gen 5 LT-1 V8 previously detailed by AEI, the C7 also benefits from a thoroughly revamped assembly plant and almost new work force in Bowling Green, KY. With a base price of $52,000, the C7 boasts the following significant engineering features:
• Industry-first use of a shape-memory alloy wire in place of a motorized actuator to open and close the hatch vent that releases air from the trunk. The lightweight material, developed by GM, saves weight and allows the rear hatch lid to close more easily;
• Industry-first pairing of a manual transmission (the new seven-speed Tremec TR6070) with cylinder deactivation is a key to the C7’s 29-mpg highway EPA rating and its ability to achieve up to 35 mpg in optimum conditions. The torque of the 6.2-L V8 in four-cylinder mode enables the 3.1-L “four” to propel the car to 90 mph (145 km/h);
• Industry-first use of aerogel, a NASA-developed lightweight material with excellent thermal-insulation properties that lines the inside of the Corvette’s transmission tunnel;
• Magnesium seat frames help the Lear Corp.-supplied seats shed mass. The new seats are genuinely supportive, comfortable, and well made—a key asset to the equally well executed interior;
• A closed-loop thermal management system with integrated ducting in the vehicle body to cool the rear-mounted transaxle. The setup also features an optional electronically controlled limited-slip differential (e-LSD). The cooled axle was pioneered by that used on the LeMans-winning C6R racecar, as were the slotted brake rotors and hood-ported radiator;
• First Corvette use of electric-assist power steering (a belt-driven ZF unit) with variable ratio and adjustable effort;
• First GM use of an integrated tire temperature/chassis control system. Thermal data from the Michelin tires provided indirectly by the tire pressure sensors are used to more precisely control the ABS and e-LSD. Three tire temperature modes are used: cold (below 45°F), warm (45-115°F) and hot (above 115°F). “Cold" mode prompts the ABS to intervene sooner and more progressively, while the differential locks up quicker to minimize spin on the inside wheels. As temperatures rise, ABS control intervenes later while the e-diff engagement is phased in more slowly.
• C7 moves to the GM Common Global A electrical architecture, the corporate system that is tried and true and provides a lot of functionality and far more robustness. But it’s heavier than the previous C6 architecture.
• And for the first time, the FE1 suspension that Chevrolet calls the “grand touring” package is capable of generating 1 g on the skidpad—compliance, comfort, and now grip that was previously only available in the track-focused Z-51 setup.
Responsible for C7’s development and delivering the car on time and budget is Chief Engineer Tadge Juechter, a 20-year Corvette team veteran and highly respected as a “go-to guy” within GM engineering. Riding as passenger with AEI during the C7’s U.S. media launch in California in late August, Juechter shared insights into the new Stingray’s development program, which was green-lighted in 2009.
“Beyond it being better in every way than the C6—with a major focus on the interior—we wanted to make it a car that any driver, not just pro-level guys, can go fast in pretty quickly, and feel comfortable doing it,” he said. “The e-LSD, the overall balance of the car, and the Michelin tires’ progressive feeling at the limit are huge contributors to that feeling of driver confidence.”
Reducing development iterations during the program was a goal but proved to be a challenge due to the all-new aluminum structure. “Its design is far more sophisticated and ambitious,” Juechter noted. “We had a clean-sheet approach that enabled us to thoroughly investigate the best materials and processes. So, how were we going to optimize it?”
He explained that the analytical tools available to GM and its suppliers had gotten much better, so iterating to arrive at a final design that meets all the requirements was faster. But the process still required many physical structures to be created for testing.
“The virtual models would crash perfectly as shown by the perfect animations of the crash,” he recalled. “Then we’d do a physical test and it didn’t look anything like the model.”
He noted the example of 7000-series aluminum used in the Stingray’s front and rear clips. The material is known as the strongest aluminum alloy available—it lends itself to very thin-wall sections and is ideal for energy absorption. In the early C7 structure models, the 7000-series parts “collapsed perfectly axially, like an accordion, and absorbed energy as we predicted,” Juechter said.
Then came drop-silo testing—again, perfect collapse. (Drop-silo tests, which are usually destructive, determine the ultimate load-carrying capacity of a component and identify global and local failure modes if any.)
But in early crash tests of the entire car, with the 7000-series components now welded into an assembly and being asked to perform in real-world conditions with fascias and bumper assemblies, the collapse mode changed completely.
“In fact, instead of collapsing on itself, a piece of it tore off and it started peeling apart like a can. That wasn’t what we expected,” he said.
It took three major design iterations “and a lot of tweaking” before the production structure was frozen, Juechter noted.
Aerogel and meeting ped-pro
Everyone involved with C7 engineering (up to 1000 people at the program’s peak) is responsible for keeping track of new technologies they encounter and flagging them for further investigation. Aerogel is one example of a technology found outside the auto industry and brought into GM—not without considerable fuss.
“It’s a ‘wonder’ material for thermal transfer, developed for space suits," said Juechter. "It comes from a supplier with no previous auto experience. It’s expensive, but we tried other materials for the thermal insulation and nothing else comes close to the stuff in terms of performance.”
Juechter noted that it took the course of the program to validate the aerogel because the supplier wasn’t really interested in becoming a Tier 1 to GM. Another problem was that the powdery material is tricky to ship. It leaves a messy residue and has to be encapsulated in a vacuum-sealed bag.
“The stuff has content that, if it got into our paint systems, it would screw up our paint; it’s not like a sheet good that can be cut and pasted onto the car,” he said. “So we had to convince HP Pelzer and IAC, our two interior component and trim suppliers, to work with this stuff. And they wanted nothing to do with it! But no advanced technology is easy to implement.”
Another challenge was looming pedestrian-protection regulations. According to Juechter, the so-called “ped-pro” regulations are “monstrously intrusive” in the way they affect front-end design, particularly in a sports car. “Ped-pro had us worried that we wouldn’t be able to go to Europe with the car,” he said.
Pyrotechnic solutions as used on some European cars were ruled out due to added mass and incompatibility with Corvette’s traditional rear-opening hood. The solution was found in the hood material. Carbon fiber’s material properties can be tailored to help absorb the impact energy in the ped-pro test, Juechter explained.
Once testing began, the team found the carbon hood enabled “really tight” front-end packaging around the engine compartment with sections of the hood lower than on the C6. A close look at the C7 front end reveals multiple contact points for a 32-degree angle plane where it touches the vehicle. Juechter wagers that no other car in the industry has come out with a model that has sections of its hood lower than the previous generation model.
“We didn’t have to abandon all that lovely shape and contour—and we’re going to Europe with it,” he asserted.