When General Motors executives unveiled the latest version of the Corvette Stingray at the recent Detroit Auto Show, they highlighted a floor display showing the C7 sports car’s new multicomponent, all-aluminum chassis, which they said is 99 lb (45 kg) lighter and 57% stiffer than the current C6 Vette’s hydroformed steel rail-based frame.
The new lightweight base structure boosts performance and fuel efficiency, the execs claimed, while its greater torsional rigidity lessens noise and improves ride and handling. The aluminum frame gives the 2014 car “an optimal 50/50 front/rear weight balance” and a “world-class power-to-weight ratio” that they said bests those of the Porsche 911 Carrera and Audi R8.
“For the C7, we decided to go with aluminum rather than steel since aluminum can provide significant weight advantages,” said Ed Moss, Engineering Group Manager for Body Structure, as he pointed out the features of the shiny (clear-coated) aluminum chassis/passenger cell structure. “Our job was to choose the right material and part-production process for each function. In this case, we came up with a structure that includes 10 castings, 38 extrusions, 76 stampings, and three hydroformed parts,” the veteran Corvette frame engineer explained, sweeping his hand across the exhibit.
Mass-efficiency was, of course, only one goal for the new chassis, he continued. “The C7, whether coupe or convertible, is basically an open-air design that has no roof structure to add extra support,” Moss said. But ensuring stiffness is the key to a solid ride and superior handling, “so we worked hard to make the frame stiffer than the C6. As always, it was a tough trade-off between stiffness and mass. Luckily, there are a couple of dials that we can turn to fine-tune the equation,” he said.
Add in other engineering considerations such as crashworthiness and the need to be “innovative, but relatively affordable,” and the task becomes even more complex. All the design data, Moss noted, were optimized and validated using finite-element analysis.
In addition to the new aluminum structure, GM engineers also have increased the use of carbon fiber in the Corvette’s exterior body panels, as part of a corporate strategy to boost lightweight composites applications. (See http://www.sae.org/mags/aei/11722)
Low-mass materials exercise
“The new Corvette frame is a good exercise in the structural materials, forming operations, and joining techniques that GM engineers have available to choose from,” said Ron Harbour, Senior Partner, Global Automotive Manufacturing at the global management consulting firm Oliver Wyman. As such, the new structure gives some idea of automakers’ palette of low-mass materials and processes. In general, the use of extrusions avoids the need to produce costly stamping-series dies, while castings offer the prospect of parts consolidation and fewer assembly processes, he said.
“The new [frame] shows the balance of functionality and cost-effectiveness that’s required to do a good engineering job,” he noted, adding that “it reminds me of the Cadillac XLR space frame,” which indeed looks very similar but included both steel and aluminum components.
Whereas the previous C6 Corvette featured hydroformed steel-tube main frame rails with a constant 2-mm (0.08-in) wall thickness, the C7’s chassis employs rails composed of five customized aluminum segments, each tailored to have the gauge, form, and strength properties to achieve the required properties, Moss said. Wall thicknesses range from 2 to 11 mm (0.08 to 0.43 in). The chassis is composed of two perimeter main frame rails, an enclosed box beam-like “tunnel” structure, and a cockpit assembly.
Moss noted that the freshly developed frame is produced in an all-new body shop at the Bowling Green Assembly Plant in Kentucky. GM had previously announced a $131 million investment at Bowling Green, including $52 million for the body shop to manufacture the C7 understructure using, among other methods, a new computer-controlled, precision laser welding process that can hold tolerances of 0.025 mm (0.001 in).
Aluminum frame rail
Moss then enumerated in sequence the components of one of the perimeter rails: “At the front we have a crush-zone extrusion with a double figure-eight cross-section that is made from a high-yield, high-strength 7000-series aluminum alloy,” he began. “It folds up like an accordion during a collision to absorb the impact energy.”
Then comes a hollow-cast node at the suspension-cradle interface point that is composed of a high-strength A356 aluminum alloy. “Everything hangs off the node,” he said. “The casting provides good dimensional control—plus or minus 1 mm—for easier assembly.” The cast-aluminum nodes, which were fabricated by Diversified Machine Montague Operation of Montague, MI, have “little or no porosity because DMI knows how to get good mold-flow during casting.”
Next on the rail comes a hydroformed aluminum-tube center section, he said. Hydroforming is a special kind of die-forming operation that uses a high-pressure hydraulic fluid to force room-temperature metal sheet into a die. The remaining seam is then welded to form a high-strength structural tube member.
Moving toward the rear, the tube is followed by another hollow-cast A356 node and “another double-figure-eight” crush-beam, this one composed of a 6000-series (high-yield, high-strength) aluminum alloy. The front and rear nodes support hollow cast-aluminum suspension cradles that are said to be approximately one-quarter lighter and one-fifth stiffer than the solid cradles used on the C6 car’s structure.
Sitting in the middle of the frame structure is an aluminum box beam-like assembly with a high-stiffness shear-wall structure that is strengthened with four thin-walled plate reinforcements. Moss said that the key support plates were vacuum die-cast at Ryobi Die Casting Inc. in Shelbyville, IN.
The Corvette’s chassis assembly features part interfaces that were made using various joining methods including conventional welding, a new patented spot-welding method developed by GM (go to www.sae.org/mags/aei/11408 to read more about this process), as well as screw-bolts reinforced with adhesives.
“Before this, GM had seemed reluctant to use much adhesive bonding, as, say, Ford does,” said Richard Schultz, Managing Director of the automotive practice at Ducker Worldwide.
Aluminum vs. high-strength steel
The frame is just another indication of the ongoing efforts by the steel and aluminum industries to supply the automakers with lightweight structural materials, mostly to meet upcoming new fuel-economy regulations, Schultz said. Overall, steel use in light vehicles will drop to less than 50% of the curb weight over the next decade, he continued.
“In the long run, dual-phase and boron steels will contribute to that weight savings, but most of the reduction will come from aluminum,” he said.
Although aluminum is lighter than steel, it also will always be more costly, he noted. “The new high-strength steel grades cost about 50 cents for each pound of weight saved,” Schultz said. “For aluminum, it’s more like $1.50 and $2 per pound.”
Although GM spokesmen did not reveal the new Vette’s curb weight, they did indicate that it will weigh a bit less than the current vehicle’s 3208 lb (1455 kg).