Lessons learned on the track today benefit next-generation production cars.
The year 1963 saw the first road-race/ production Chevrolet Corvette, the Z06, a car which no doubt helped popularize the phrase, "What wins on Sunday sells on Monday." Chevy reintroduced the Z06 in 2000, a year after GM Racing began campaigning its C5-R Corvette race cars.
Since then, factory-backed Corvette racing has gained traction with car buyers. So much, in fact, that GM at the end of this year will leave IndyCar racing to focus on its latest C6-R program. "Customers have difficulty 'connecting' with the open-wheel Indy cars and tend to favor productionlike race cars," says Rick Voegelin, communications director for GM Racing. The C6-R Corvettes run in American Le Mans Series (ALMS) road races against such recognizable marques as Porsche, Saleen, Maserati, Ferrari, Audi, and Dodge.
Factory-sponsored racing helps sell more cars, but it is also a vehicle for technology transfer. For example, the 7.0-liter, small-block V8 engine and several key design features of the 2006 Z06 came directly from the C5-R program. At the program's inception, Corvette Chief Engineer Dave Hill insisted that for technology transfer to work, a production engineer should live with the race team. Steve Wesoloski, road-racing group manager at GM Racing, got the job after working as a chassis engineer in Corvette production.
Obviously, demands on race cars and production cars are markedly different and their designs reflect that, though there are many similarities. ALMS rules define how far the race cars can stray from production models. It all starts with what is called a homoligation car.
"It used to be you could build a race car, install a productionlooking interior, and call it your homoligation car," says Wesoloski. "Now, dimensions on which you base the race car length, width, overhangs, wheelbase, and so on must come from the production version, in this case a 2006 Z06." Suspension attach points, for example, must stay within a 25-mm-diameter sphere compared with stock. And race cars can be only 100 mm wider than their stock counterparts. The C6-R is about 25 mm wider than a stock Z06, mostly to accommodate wide racing slicks. The rules are more liberal regarding suspension components. Production Corvettes use composite leaf springs and cast-aluminum control arms. But teams must have the ability to quickly tune a race car's suspension for track conditions and that would be impractical with the leaf-spring design. The race cars instead run coil springs and special, welded-steel control arms. Control arms made of steel are also much stronger and stiffer than aluminum to better withstand lateral accelerations close to 2 g on the track. In contrast, skidpad tests show production Corvettes capable of about 1 g before breaking traction.
STIFFER IS BETTER
High cornering loads also necessitate a stifferthan-stock chassis. In production cars, chassis stiffness is keyed to natural frequency because that is what the driver feels. Here, chassis are designed to avoid vibrations in the 12 to 14-Hz range where wheel hop happens. Engineers shoot for a natural frequency of about 20 to 22 Hz because it's tough to get much over that without adding lots of extra weight for stiffeners.
For a race-car chassis, high static, torsional stiffness trumps driver comfort. "You don't want the chassis to become another spring in series and flex, explains Wesoloski. "All tuning should be done with the springs and dampers. The goal is to make the chassis as stiff as possible without punishing the car. Too stiff and loads go way up." Torsional stiffness is measured like this: fix the rear shock points; place a fulcrum at the center front and a beam across the front shock points; and apply a twisting force. Deflections are measured at four or five points along the length of the chassis.
Both production Corvettes and C6-Rs share hydroformed chassis rails and the tooling used to make them. The perimeter rails go from bumper to bumper and resemble a ladder laid down, much like a truck frame in a single plane. A tunnel down the car centerline boosts torsional stiffness. Shear walls built off the tunnel react loads to the rails.
Despite a weight penalty, race cars use steel rails as opposed to aluminum for production Z06s. The use of steel simplifies joining of the 4130-steel roll cage to the rails. It is important to weld the triangulated-truss roll cage "into" the frame rails because the chassis derives much of its stiffness from the arrangement. Aluminum-to-steel joining methods simply don't give the same level of integrity as welded sections.
Further stiffening comes from the floor panels. Production Z06s have separate, balsa-wood-core composite floor panels and a bolt-in panel tunnel. Composites get their strength from the shear modulus of the core, because the outer sheets try to slide relative to one another under load. It turns out that balsa's shear-modulus-to-weight ratio is better than that of synthetic materials. But balsa also burns easily, making it inappropriate for a race car. The C6-Rs instead use a fire-resistant Nomex core with carbon-fiber sheet. Nomex is a known quantity in racing and well understood. It's also simple to form Nomex into complex shapes. A single 0.75-in.-thick panel goes from the firewall to the frame rails, and to the rear bulkhead. The combination of steel rails, roll cage, and one-piece floor give the C6-R a torsional stiffness of 28,000 lb-ft/deg, compared to about 9,000 to 10,000 lb-ft/deg for a production Z06.
The engine cradle is another key chassis component and one in which production cars are " highertech" than the race cars. Z06s use lightweight, castmagnesium engine cradles, while the C6-Rs opt for steel. The earlier C5-Rs incorporated production cast-aluminum engine cradles. But the castings had provisions for ABS modules, A/C, and other systems that the race cars didn't use and had to be machined off to save weight. Engineers found it easier and cheaper to build custom engine cradles from steel tubing.
Offsetting the added weight of the steel chassis and roll cage is an all-carbon-fiber body shell. Production Z06s as well have carbon-fiber front fenders and inner fenders to cut overall weight and shift it to the rear wheels. The balance of the body is aluminum.
The use of carbon fiber for production body panels started a few years back with hoods on Le Mans Commemorative Edition Z06s. Hoods are a relatively simple shape compared with a fender and considerable development went into bringing the fenders up to Class-A-surface standards. Paint technology has also advanced to allow both standard and carbon-fiber panels to run on the same production line. Just recently they were painted using separate processes. Developing carbon-fiber technology for production Z06s is also considered an important step toward bringing it to more production cars, in the interest of lowering weight and boosting fuel economy.
HIGH SPEED, LOW DRAG
The C6 Corvette body is about 5 in. shorter than its predecessor, mirroring competing Porsche and Mercedes models. Gone are the pop-up headlights of the earlier C5 model. That's a welcome change for design engineers because the feature, even as a muted bulge in the race cars, represented a highpressure area and a source of aerodynamic drag. Flush-mount headlights offer much less drag.
But the shorter body and overhangs (the body sections ahead of the front wheels and behind the rear) also present a challenge. The overhangs act as a moment arm to apply down force from the front splitter and rear wing. Obviously, longer overhangs are better in this case. Engineers were able to use the wing from the C5-R but had to rework the splitter to produce more down force. The horizontal splitter locates low on the front fascia. Additional vertical strakes on the rear diffuser better direct air from beneath the car. The diffuser straightens the airflow and helps prevent churning. For best efficiency, "the air streams coming off the wing and diffuser should smoothly join," Wesoloski explains.
Engineers also found it advantageous to run side windows at all times, not just at high-speed courses. Doing so keeps air attached to the sides of the car where it can act on the rear wing to boost down force, without increasing wing angle and drag. A so-called wicker bill, a vertical piece attachedto the back edge of the wing 15-mm tall ( minimum) and perpendicular to airflow, keeps air attached to the wing and adds down force, but at the expense of drag. It also creates a lowpressure zone behind the wing to accelerate air moving beneath it, which boosts wing efficiency. Engineers adjust wicker-bill height for track type. For example, a tall wicker bill is used on street courses to give high down force at low speeds for cornering. Conversely, highspeed tracks such as Le Mans where speeds top 180 mph require a shorter wicker bill and less wing angle of attack.
A CFD software package called Raven from Raven CFD, Mooresville, N.C., helps engineers identify the basic shapes for aerodynamic surfaces. Raven was originally developed for the modeling of missiles and provides clean, stable solutions at high flow velocities. Conventional off-the-shelf automotive CFD tools, in contrast, are designed to model low-speed flow and tend to fall short at speeds of 150 to 180 mph that the C6-R hits on the track.
CFD points designs in the right direction but it's not the final say. Engineers also physically test the hardware on the track and at a retired air base in northern Michigan. A two-mile-long airplane runway lets race cars run at top speed for about 9 sec, then coast down while potentiometers measure spring displacements. Down force is calculated from known spring rates. Drag is figured from motor torque, rpm, and coast-down time. The threelegged approach CFD, track, and coast-down testing also helps quantify car stability under braking and acceleration.
A central intake is another body feature CFD helped shape. The idea grew from the use of centrally located NACA ducts on C5-Rs and was later refined in wind-tunnel tests. The car nose is the highest pressure point in the flow field and a natural place for an air intake. Locating the intake high on the front fascia also makes it insensitive to pitch and roll. Conversely, a bottom breather sees greater changes to air flow because of its proximity. Braking, for example, causes the car to pitch down, which changes the pressure distribution beneath the car. Both C6-Rs and Z06s have a dedicated central duct for engine breathing, while breathers for the standard C6 Corvette combine engine and radiator air.
The central intake improves airflow by about 20% over bottom breathers, which is extremely important for the C6-R. Rules say all engine must air feed through two 31.5-mm-diameter restrictor plates. Plate size keys to car weight and the C6-R tips the scale at about 2,425 lb, the lower limit. Cars weighing 2,640 lb and above get the next larger restrictor. GM engineers found the advantages of lower weight cornering, braking, and tire life outweigh those of larger restrictors and more horsepower.
PONIES TO SPARE
With the restrictor plates installed, the LS7 7.0-liter small-block V8 race engines make about 590 hp. Without the plates that number climbs to 800 hp. The LS7 comes from the C5-R program and also powers the 2006 Z06. The lower-compression street version is rated at 500 hp.
The use of high-lift, short-duration camshafts in the race engine gives high torque at low rpm. A rev limiter holds engine speeds to 6,200 rpm, though peak revs rarely exceed 5,000 rpm. Both production Z06s and race cars use dry-sump oiling. The system stores oil in a separate tank and a pump feeds it to the engine. The arrangement ensures constant oil flow during hard cornering. It also cuts parasitic losses caused by crank journals sloshing through oil pooled in the oil pan.
Production and race engines also share the same valve angle and valve count. Rules say production heads must bolt to race blocks, and vice versa. Production LS7s get racing goodies from last year's C5-R engines including titanium rods and intake valves, and a billet-steel crankshaft.
Engine development is indicative of the production-racing program as a whole. "We're about a generation behind," Wesoloski explains. "In other words, lessons learned on the current C6-R racing program may be applied to C7 production cars. The logistics of production (tooling, styling freeze) prohibit major changes midcycle."
Those who can't wait that long can get some of the performance parts through GM Performance Vehicle Operations. For example, "Previous-generation racing blocks are available over the counter, though they are pricey," adds Wesoloski.