It all comes down to two sets of digits. Some might say that’s oversimplifying, but the radical improvements in the new Corvette Stingray chassis can be summarized with the figures 57 and 99. These numbers refer, respectively, to the percent of increase in rigidity, and the concomitant reduction in poundage. Taken together they’re even more remarkable, because you’d expect that paring nearly 100 pounds off a frame would result in less torsional strength, not more. And certainly not in a magnitude of more than half again as much.
Switching from steel to aluminum frame members helps to explain this feat, yet that too risks oversimplifying matters, as using a lighter, stronger material alone didn’t do the trick. Balancing light weight and high rigidity in the same foundation is a bit like juggling fire and ice, two competing concerns. To find out how this Corvette alchemy was achieved, and the level of precision required, we spoke with a couple of GM insiders: Michael Bailey and Ed Moss.
02] Did someone say “diagnostic equipment”? When it comes to monitoring test-vehicle dynam
Bailey is a systems engineer who oversees multiple areas. His job is to make sure all parts of the ’14 Corvette Stingray’s frame—tires, wheels, steering, suspension, and more—work together in unison for optimum performance on both the street and track. He’s also a Corvette enthusiast, stung early by the Corvette’s allure. His dad still owns the ’61 model that he purchased in 1970, along with a ’69 Corvette 427. The father-and-son team are currently restoring a ’63 convertible.
“I care a lot about the image of the Corvette, and I know what it’s about,” Bailey notes. “I knew it growing up, but [having worked] on Corvette since 2007, I think I understand what the customer is really after.”
Ed Moss is a veteran engineering group manager, having worked exclusively on the Corvette program in various capacities since 1988. “If you’re going to have a narrow focus, what better than on a Corvette?” he laughs.
He’s essentially an analysis guy, with particular expertise in the body structure. By way of clarification, this reference might sound a bit unclear, since we’re focusing on the Corvette’s frame in this portion of our series on the Stingray. But considering how today’s unibody designs integrate both body and frame, the nomenclature makes more sense.
Moss delineates the Corvette’s overall structure into three main areas: the aluminum frame, the carbon-nano unibody panels bonded to this frame, and the bolt-on body panels. Breaking it down further, he says the frame itself consists of, “Two crash cans [cradles], two chassis nodes, and hydroformed aluminum rails.” We’ll be touching on each of these, but before doing so, how do GM engineers go about creating a clean-sheet chassis?
As with the body and interior design, it all starts with computer modeling, Moss explains. But what’s different in this portion is setting targets for torsional stiffness. The C6 already hit an impressive figure of 9,000 N•m/deg (Newton meters per degree, with each unit equal to about 0.737 lb-ft of force). But the computer projections on the C7 raised the bar substantially to 14,500 N•m/deg, achieving that astounding 57 percent increase mentioned at the outset.
While the reasons for doing so are fairly obvious, they bear highlighting. Less weight means multiple advantages in quickness, handling, and durability. On the other hand, for a chassis engineer, having a stiff, strong foundation is absolutely essential for maintaining precision suspension geometry in a high-performance application.
A heavily disguised C7 test mule undergoes a chassis evaluation on one of GM’s four-wheel
How close did the computer projections come to real-world results? To find out, GM did five rounds of static stiffness testing over a three-year period at Pratt & Miller, and they came as close as within five percent of what the CAD system indicated, Moss says. He points out that all of the C6 and C7 models basically have the structure of a convertible (except for the fixed roofs of the C6 Z06 and ZR1), and the new Stingray benefits from an extra measure of strength. How that was achieved involved using several different approaches and materials.
One way it was enhanced was by completely redesigning the center tunnel, including its close-out panel (basically a box-shaped structure). Simply moving it below the exhaust system increased torsional strength in that area by 20 to 30 percent.
In addition, previously on the C6 Z06 and ZR1, the frame had a constant gauge of 4mm from front to back, so “it was carrying mass in areas that you don’t need,” Moss observes.
On the C7, however, engineers varied the gauge of the aluminum frame from 2mm to 11mm, depending on the location, so it not only dropped pounds, but also enhanced stiffness in specific areas. Note, too, that rather than one-piece hydroformed steel rails, the C7’s chassis has a much more complex assembly, consisting of an extruded crash structure up front, a casting where the cradle mounts, tubing next to the cockpit, another casting for the rear suspension cradle, and a rear crush area.
Also, for the composite bottom of the cockpit tub, instead of balsa wood, there’s now a lighter and stiffer foam-core sandwich. While none of these changes amounted to much individually, taken together they began to add up quickly to those 57 and 99 numbers, in a form of “cascade engineering.”
What role do the carbon-nano panels play here? Again, GM engineers found a way to decrease weight while enhancing stiffness. Normally a decrease in poundage (by lowering the panels’ density) might make them more flexible, Moss acknowledges, but not so in this case.
The carbon-nano clay’s smaller particles interlock better, “like platelets,” he says. When used in various ratios with resin and glass, the specific gravity drops from 1.5 to 1.2, while the carbon maintains about the same strength and stiffness.
03] As noted, the new Stingray chassis is nearly 100 pounds lighter than the previous desi
04] Varying the thickness of the frame allowed the engineers to concentrate stiffness wher
05] The floor panels now use a foam-core sandwich construction, which is both lighter and
Stepping back from the engineering details for a moment, it’s important to realize that GM engineers function as a “basket-weave organization,” Bailey points out, alluding not only to the team effort required, but also the interlocking design demands. Aristotle’s phrase about the whole being greater than the sum of its parts applies here, considering how individual components have to serve several different masters.
For instance, there’s one big change in the front cradle, a 5x3-inch rectangular frame that supports the control arms and shock absorbers, among other components. It also plays a role in crash protection and body structure. In the ’14 Stingray, however, this key component is made of hollow members, with solid “nodes” used where stressed components are attached.
While this innovative approach is not entirely new, it didn’t appear on Corvette until around 2005 in the magnesium cradle of the ’06 Z06, Bailey points out. Unlike casting a solid beam, hollow castings require a vacuum and a low-pressured, assisted permanent mold. The extra tooling and manufacturing required (done at DMI, an outside vendor) is well worth the extra effort, as it allows for creating intricate profiles that result in a performance payoff.
To be specific, in addition to the functions already noted, “It allows us to move more mass outboard,” Bailey explains. “If you took all of the mass on the C6’s [solid] cradle, and moved it outboard, you’d end up with too much stiffness.”
06] In a break with past practice, all ’14-up Corvette frames will be assembled in-house a
Another change involved the lower control arms, which are also now hollow, saving about 9 pounds. Besides weight reduction, there’s another benefit to this configuration. On the C6, the control arms’ bushings tended to migrate outward in high-load situations, typically during hard cornering of braking. The switch to hollow arms, along with the addition of cams, holds in the bushings more securely.
Interestingly, even though the track width on the C7 is slightly wider, the Michelin Pilot Super Sport tires are narrower than on the C6. The effect is less wind resistance, along with no penalty to (and in fact a slight increase in) lateral grip.
As for the brakes, lessons learned from the C6.R racing program were applied. The Stingray features a new “dual cast” rotor, with an inner aluminum web and an outer cast-iron rotor body. (For an in-depth look at the C7’s new binders, see “Area Z51,” in our Aug. ’13 issue.) This configuration allows for independent thermal expansion of the rotor, so it doesn’t affect hub geometry.
Also new is the elimination of drilled holes in the rotors, which can produce cracks over time. Even so, the base rotors have less mass than did the C6 units, and they feature arched slots, like those of the C6.R’s competition brakes. The calipers are now fixed instead of floating, for more-even wear, less drag, and better modulation through the pedal.
Although the base brakes are slightly smaller overall, they have 35 percent more swept area. The Z51 package has bigger front rotors, enlarged 1 inch in diameter. All of which means the stoppers on the new Stingray can bring it to a clenching halt at least 10 feet shorter than the C6.
In addition, the steering system is now at least four times stiffer (not five times, as previously reported elsewhere), largely for better feel. Bailey says these parts, taken together, tend to function like a torsional spring, causing a loss of input. By creating a more rigid connection between the column, coupling, intermediate shaft, and cradle components, the feel and response markedly improve.
07] The new BG chassis facility is truly state-of-the-art. Here, a worker uses a laser sca
As for the composite monoleaf transverse springs, those haven’t changed in a material sense, but they are now longer due to the C7’s wider track (73.9 inches, versus 72.6 on the C6). That means they also have a slightly different rate and frequency, along with a modified mounting.
“We designed the car to not take the spring loads in the shock attachment, but into the cradle instead,” Bailey explains. That’s because, as already noted about the cradle, the monoleaf springs serve multiple functions, such as providing a lower center of gravity, serving as a stabilizer bar, setting trim height, and reducing mass.
The Magnetic Selective Ride Control—what Bailey calls “Mride shocks” for short—can’t be installed in the race car, due to rules restrictions, but that’s obviously not an issue on the street. They optimize the damping of roll and pitch, especially on “low body vertical speed events.” (This expression doesn’t refer to vehicle velocity, but to the body-versus-wheel motion in a slalom situation.) “The Mride puts the body where you want it to be,” Bailey says.
Another key component that the race cars don’t get is the ELSD (Electronic Limited Slip Differential). This unit improves the traction effort of the rear tires and greatly enhances high-speed control when cornering. Basically it prevents the inside wheel from getting all the power, routing some of it to the outside wheel instead. We’ll be covering that high-tech feature in more detail in a later installment on the Stingray driveline, so stay tuned. We still have way more insider material to reveal in upcoming issues.