Corvette Body Materials - From Fiberglass To Carbon Fiber
Tracing Corvette's pioneering use of lightweight materials
From the August, 2011 issue of Vette
By Barry Kluczyk
Photography by General Motors
We equate Corvette with fiberglass like we do pinstripes on a Yankees uniform or red on a can of Coca-Cola. It's just always been that way.
Fiberglass isn't the primary body material on Vettes these days, but the car continues a tradition of non-traditional, lightweight bodywork that began with the very first one off the assembly line in 1953. In fact, the reason conventional fiberglass isn't used any longer is because it's been replaced with better, lighter composite materials.
Beneath the skin, the Corvette has long been a pioneer in lightweight technology, from the use of aluminum wheels and suspension parts, to powertrain and chassis parts, and more. That heritage was advanced with the launch of the C5, which used materials including balsawood in the floor to minimize the car's overall mass; the C6 went several steps further, with the introduction of the Z06's aluminum chassis and the ZR1's extensive use of carbon-fiber body panels.
Importantly, the Corvette wasn't the first car to feature a fiberglass body, but it was the first mass-produced model. Similarly, the other lightweight components used in its construction led the way for the rest of the industry, slotting in between the ultra-expensive, low-production exotics and the high-production, value-priced family cars.
Fiberglass was first considered for use on a GM vehicle by legendary designer Harley Earl. Besides a certain "exoticness" for the early '50s and the undeniable weight advantage, fiberglass offered an economical way to create the low-volume Corvette without investing in expensive sheetmetal-stamping dies.
GM's Parts Fabrication Operation was tasked with figuring out the intricate requirements for the '53 Corvette's body, while supplier Molded Fiber Glass Company (with some support from Owens Corning Fiberglass) got the contract to build them. Amazingly, the company was awarded the business in April 1953 and was asked to deliver the bodies by the June 1953 start-of-production deadline--and they had never built a car body previously.
The use of a fiberglass body...
The use of a fiberglass body was new enough in 1953 that, reportedly, the first couple of Corvettes off the Flint, Michigan, assembly line didn’t start because they weren’t properly grounded.
Not surprisingly, there were plenty of unknowns when it came to designing and building those first Corvettes. Tests had to be done to determine how many layers of fiberglass were needed for the various areas of body, as well as the interior "tub." And for products so large, the correct ratio of resin and plastic hardener had to be determined, not to mention figuring out how long it would take for each hand-laid body to cure. Also, it wasn't clear at first how many separate parts would be required to construct each body, and a surface finish suitable for production-quality paint was far from assured.
The parts were produced with the "chop gun" method, in which pieces of fiberglass matting and polyester resin were "blown" into a mold to build up the part, layer by layer. With the benefit of nearly 60 years of hindsight, it's easy to criticize the quality of the early Corvettes' fiberglass bodies, but considering they represented the genesis of a production method that basically didn't exist previously, it was a feat that should be lauded.
Building an entire mass-produced...
Building an entire mass-produced body out of fiberglass was a technological landmark in 1953, but it brought challenges. The quality wasn’t stellar at first, but it improved dramatically with time.
Starting with the C3 generation in 1968, body parts were manufactured with a press-mold process, whereby the fiberglass material and resin were shaped in a die-like tool that produced smoother parts more quickly. It was a significant jump in forming technology and laid the groundwork for a change in the body panels' material in 1973. That year, the composition changed from conventional fiberglass to sheet-molded compound (SMC), which was composed of fiberglass, resin, and a catalyst formed under high heat and pressure. The ratio of resin to fiberglass was reduced with SMC, while the fiberglass itself was a bit coarser. The new material helped produce panels that were even smoother right out of the mold, meaning they required less surface finishing prior to painting. It also helped create a better final paint finish.
Technically, all Corvettes since 1973 have used SMC body panels, but the material composition has changed dramatically, featuring less traditional fiberglass and more lightweight plastic. The early SMC material created parts that were stronger and more rigid, but more brittle. As SMC technology and production experience evolved, Corvette engineers were able to alter the material composition and the body parts' specifications in order to trim the car's curb weight--an endeavor that was certainly welcomed though the later C3 years, as engine output kept eroding. Mostly, that meant making thinner body panels, because SMC was denser and stronger than conventional fiberglass.
The ’63 Corvette was the first...
The ’63 Corvette was the first to incorporate a full roof as part of the fiberglass body matrix. There was a greater steel structure to support it underneath, but advances in the production of the body panels meant they were a bit thinner than C1 panels, which helped save weight.
The C3 generation introduced...
The C3 generation introduced the use of press molds to form the body panels, which produced smoother, more uniform parts and did it more quickly. The backsides of these panels are generally smooth, especially when compared with C1 and C2 panels.
A new formulation for the...
A new formulation for the body material was introduced in 1973. Although fiberglass was still the primary component, the material was known as sheet-molded compound, or SMC, and was molded using heat and pressure.
The exposed flank of this...
The exposed flank of this ’73 Corvette reveals the layers of its SMC bodywork. It looks like conventional fiberglass, because that was the primary ingredient at the time. In the generations to follow, the amount of fiberglass used would shrink, thanks to an increased reliance on other plastic compounds.
The assembly-plant changeover from St. Louis to Bowling Green in 1981 brought a switch to a more plastic-infused formula for the SMC body panels. Published figures vary, so without the capability to weigh both factory-stock '80 and '81 models, it's difficult to provide an accurate comparison of the weight differences between traditional fiberglass and SMC-bodied cars. From the numbers we researched, it appears the '81s were about 100 pounds lighter than their '80 counterparts. That's not all attributable to the SMC bodywork, however, since the change to a fiberglass mono-leaf rear spring for '81 accounted for about a 35-pound reduction. Nevertheless, newly formulated body panels helped lighten the Corvette--and it was the same basic formula used throughout the C4 generation.
C5: Getting Serious About Weight Savings
The introduction of the C5 in 1997 represented one of the rare instances in the automotive world where the next-generation model weighed less than its predecessor. Even more impressive, the '97 Corvette was larger overall--longer and wider--than the '96 model, yet it tipped the scale at about 3,220 pounds with a manual transmission, compared with the '96's 3,300-pound curb weight.
When production moved to Bowling...
When production moved to Bowling Green at the end of the C3 generation, the body material for the Corvette evolved, too, to SMC with greater composite content. The panels were thinner and lighter than ever.
A number of contributors helped drive down the C5's weight, including the use of SMC body panels that had a higher content of plastic than ever before. The material, which is basically the same as used in the C6, was composed of about 40 percent resin--polyester, vinyl ester, styrene, or a blend of all three--33 percent calcium-carbonate filler, 20 percent chopped fiberglass, and the remaining 7 percent resins and hardeners that improve the out-of-mold surface finish.
The C5's panels were exceptionally light, but so was the Corvette's all-new chassis, which used beefy rails and hydroformed sections to provide strength with less complexity and weight. In fact, the floor sections used a sandwich of materials including featherweight balsawood to minimize mass, a feature that continues with the C6.
The Gen III small-block can't be discounted in the weight savings and overall greater balance of the C5. Compared with the old-school small-block it replaced, it delivered a lightweight aluminum cylinder block and a composite intake manifold that weighed less than 10 pounds.
The C4 generation didn’t bring...
The C4 generation didn’t bring any significant changes or advancements in body material technology, but the greater use of aluminum suspension and powertrain components helped keep the car’s weight to a minimum, even while heavy safety and electronic features were added almost every year.
In 1999, the Corvette "hardtop" model took weight savings and performance to a new level. By replacing the signature--and heavy--hatchback glass with a more formal-looking roof section made of an even lighter-weight SMC material, curb weight dropped to about 3,155 pounds. There hadn't been such a lightweight Corvette since the early C2 days, when comparatively heavy airbag modules, electronic chassis-control systems, and structural crash-safety features weren't even sparkles in the eyes of engineers.
The hardtop model, of course, was the foundation for the C5 Z06, whose weight was further pared to 3,120 pounds by shedding some of the base models' luxury features for a more purposeful driving experience. The '04 Z06 Commemorative Edition was lighter still, with a carbon-fiber hood that was 10.6 pounds lighter than the standard SMC piece. Taking that weight off the nose of the car improved its overall balance as well. It was the perfect send-off for the influential and technologically advanced C5 generation.
C6: Aluminum Chassis, Carbon Fiber, and More
Nearly 15 years before the...
Nearly 15 years before the first carbon- fiber body panel was used on a production Corvette, the CERV-3 concept debuted in 1990 with a fully carbon body. Now, more than 20 years later, the material retains its exotic aura and only a few cars feature it.
Although the C5 and C6 generations share basic chassis layouts, the C6 brought additional changes in the quest for reduced weight, not the least of which was the elimination of the Corvette's trademark retractable headlamps in favor of simpler, lighter-weight fixed units. The plastic-intensive SMC body panels remained, although surprisingly, the rear fenders were made of conventional steel. That's right--steel fenders on a Corvette. It was a first.
The base '05 Vette weighed in at 3,240 pounds--only about 20 pounds more than the first C5 models of 1997, despite more safety-enhancing structure under the skin and generally more standard content. A year later, the C6 Z06 brought with it an aluminum-based chassis structure and carbon-fiber body panels that represented the most significant targeted weight-reduction initiative in the Corvette's history. With its 505-horsepower engine and a curb weight of less than 3,200 pounds, the Z06 had an enviable power-to-weight ratio that most higher-priced European exotics couldn't match.
A more plasticized SMC body...
A more plasticized SMC body material covered a comparatively lightweight space frame for the C5 generation. It debuted as a slightly larger model than the C4, but at a lower curb weight. Its all-aluminum LS1 engine also helped keep the pounds off.
The ’99 hardtop model weighed...
The ’99 hardtop model weighed only about 3,155 pounds. Its fixed roof used a combination of conventional SMC on the outer panels and lighter, low-density SMC—made with hollow glass microspheres instead of calcium carbonate—for the structural sections of the roof. The total SMC content added up to only 23 pounds.
A carbon-fiber first: The...
A carbon-fiber first: The ’04 Z06 Commemorative Edition featured the Corvette’s first production carbon part. It was the hood, which weighed less than 30 pounds—more than 10 pounds less than the standard SMC hood.
More weight-reducing elements...
More weight-reducing elements were incorporated into the C6 generation, including the use of fixed headlamps for the first time since 1962. The car weighed only about 20 pounds more than the ’97 C5, and virtually the same as the outgoing ’04 coupe.
Despite looking like the steel chassis of the base Corvette, the C6 Z06's aluminum frame weighs nearly one-third less. And besides the core-material difference, it features a unique manufacturing process that incorporates MIG and laser welding, as well as self-piercing rivets, while the base Corvette frame is assembled with conventional spot-welding techniques. Additionally, magnesium is used for the engine cradle and some of the other suspension attachment points, further contributing to mass reduction.
On the outside, the Z06 differs from base Corvettes with carbon-fiber panels used for the front fenders, front wheelhouses, hood, and rear fenders. A check of Chevrolet's latest press data shows the car's curb weight at a svelte 3,175 pounds. Interestingly, the base Corvette is listed at 3,208 pounds, a mere 33-pound difference. If that doesn't seem like much of a trade-off for an aluminum chassis and carbon-fiber body panels, keep in mind that the Z06 packs some beefier components, including brake rotors that are about 10 percent larger and a larger rear-axle assembly. It also features a dry-sump-type oiling system, which has a separate reservoir tank and about twice the oil capacity of the base car. So, the low-mass structure elements don't merely cut the weight of the car; they work to offset the weight of the heavier higher-performance elements.
Weighing less than 3,200 pounds,...
Weighing less than 3,200 pounds, but packing more than 500 hp, the C6 Z06 set a new standard for an exceptional power-to-weight ratio. A unique aluminum chassis and carbon-fiber body panels offset the greater mass of higher-performance drivetrain components.
The Corvette ZR1 uses the same aluminum chassis structure as the Z06 and incorporates even more carbon-fiber body parts, including the roof panel, but the there's a weight penalty for the LS9 engine's intercooled supercharger system. The ZR1 tips the scales at 3,333 pounds, but all that extra weight isn't attributable to the blower. It has an even larger rear-differential carrier than the Z06, as well as other heavier-duty--emphasis on heavier--drivetrain components. Then again, with a power-to-weight ratio of 5.22:1, or 1 hp for every 5.22 pounds, the ZR1 can outpace all but a few race-car-derived production vehicles.
In case you hadn't noticed, cars have been getting obscenely heavy in recent years, with much of the excessive poundage coming from dense steel used in crash protection, along with the seemingly dozens of airbag modules and miles of wiring for chassis-control systems. A new Camaro SS with an automatic transmission weighs more than 3,900 pounds, while a family car such as the Buick LaCrosse tips the scales at more than two tons--and we won't even bother listing the tank-like weights of popular crossovers.
The C5 and C6 models owe much...
The C5 and C6 models owe much of their low curb weights to their similar underlying body structures, which feature hydroformed, box-type framerails. Aluminum is used strategically, for the engine cradle and some suspension mounts. The strong structure required less of a steel skeleton than was used in the C4, which helped save weight.
That the Corvette has remained unequivocally a lightweight car in this day and age is remarkable. But despite its heritage as a technology leader, particularly in lightweight materials, Corvette engineers are under the same pressure as the rest of the industry to deliver on tougher-than-ever crash standards--and the car isn't about to lose any of those high-tech electronic control systems, either. Maintaining its low-mass credentials, then, will require the continued use of lightweight body panels, but with carbon fiber still considerably more expensive than SMC, it's likely the plastic panels will remain on at least the base models.
As for the chassis, we'd be surprised if separate frames will be justified for base and higher-performance models under the new austerity of post-bankruptcy GM. With luck, that could mean the trickle-down effect will deliver a lighter-weight chassis for the base models. It seems to us that maintaining the base-model coupe's curb weight in the 3,200-pound range will be difficult. But based on the Corvette's track record of continual advancements, particularly over the last 15 years, we wouldn't bet against it.
The C6 ZR1 weighs more than...
The C6 ZR1 weighs more than the Z06, but boasts about 130 more horses and features even more carbon body components, including the fixed roof panel. It’s finished with a unique clearcoat that allows the natural weave pattern to shine through without being affected by ultraviolet light—something that can’t be said of “bare” carbon fiber.
Dana Corp. manufactures the...
Dana Corp. manufactures the aluminum chassis used on the C6 Z06 and ZR1. Rather than the spot welds of the base Corvettes’ steel frame, it’s manufactured with MIG and laser welds and self-piercing rivets. Compared with most other aluminum parts used in auto construction, the Corvette chassis is at least three times thicker, which required the pioneering of new laser welding equipment.
For 2011, the limited-edition...
For 2011, the limited-edition Carbon Edition takes the Z06 to a higher performance level, with the lightweight carbon ceramic disc rotors and various carbon-fiber parts from the ZR1. Weight savings are offset somewhat by the inclusion of the ZR1’s larger wheels and tires, but the package still maintains a low curb weight and exceptional power-to-weight ratio.
|Corvettes Compared: '53 Roadster vs. '11 Convertible
||Four-wheel disc w/ABS
|Curb weight (lbs.)
Corvette's Pioneering Paint Systems
While most of our main story focused on the advanced technology that has gone into the Corvette's lightweight body panels, we thought it also worth mentioning that the car has been a pioneer in the material that covers the panels: the paint. It started with the change in manufacturing locations from St. Louis to Bowling Green in 1981. That's when lacquer paint was abandoned in favor of the enamel-based basecoat/clearcoat system that would become the standard for the industry within a few years. And when the C5 was introduced for 1997, Bowling Green again led the way with the changeover to a more environmentally friendly waterborne paint system that reduced solvents in the mix from about 60 percent to around 10 percent. It also fostered richer colors in the palette. Today, waterborne paint systems are the norm at assembly plants all over North America.
The modern basecoat/clearcoat...
The modern basecoat/clearcoat paint system was born at the end of the C3 generation.
Low-solvent waterborne paint...
Low-solvent waterborne paint was introduced with the C5 generation. It’s now the industry standard.