Trojan Horsepower

GM prefers the term “common architecture” for describing two distinct products with mutual attributes and components. To gain insight into the real meaning of those words, AI selected the 2004 Cadillac XLR destined for showrooms this summer for analysis. This two-seat, folding-hardtop sports car started life as a 1999 concept car called Evoq. Upper management approved the leap from the show car to a production model in April of that year.

Shortly thereafter, the idea of building Cadillac’s future flagship over Chevrolet Corvette architecture bubbled out of GM’s product-planning cauldron. Marrying the most prestigious car in the corporate lineup to a feisty sports car from the opposite side of the tracks at first seems like risky business. But not according to Vehicle Line Executive Dave Hill, who’s responsible for both cars.

“Actually, there’s a high degree of synergy here,” Hill says. “Searching for ways to use our manufacturing plant more efficiently, the Corvette team investigated the possibility of building a car for another GM division. Meanwhile Cadillac was reinventing itself by creating new products that didn’t merely follow established models. So the seeds of this architecture-sharing project germinated in two different areas. John Middlebrook [GM’s vice president of brand marketing and corporate advertising], served as the match maker by accepting our rationale that a new Cadillac could be created without detracting from an existing Chevrolet.”

Cadillac XLR Chief Engineer Dave Leone is convinced that this Cadillac-Chevrolet marriage will thrive. “From a chassis-systems standpoint,” says Leone, “the current Corvette’s performance-car architecture offers excellent vehicle dynamics and road-holding capability. Our intention was to improve on that great performance in the new Cadillac. We recognized that several changes would be necessary to suit the XLR’s totally different market segment [luxury sports roadsters such as the $62,600 Lexus SC430, the $75,000 Jaguar XK convertible and the $86,655 Mercedes-Benz SL500]. We needed more road isolation, a quieter passenger environment and a more refined power delivery. Since this is a low-volume program, we had to be financially responsible and spend our engineering budget wisely. Architecture sharing made perfect sense from that standpoint.”

While starting with an existing design can be a major time saver, there was no rush to market here because the XLR is a niche model with minimal impact on GM’s bottom line. Belt-tightening during economic downturns forced postponement of some investments necessary to advance the XLR cause.

As a result, its gestation will run a full four years — from management approval to first customer deliveries — or nearly twice as long as the most fleet of foot makers might need. Nevertheless, Leone does credit architecture sharing with saving time and money.

“We trimmed at least a year of engineering time and time is money,” he says. “Since we started with such a strong foundation, we were able to focus more attention on refinements. When you start from ground zero, you’ve got to develop all the little components and details that affect every aspect of performance throughout the car — the steering and suspension hardware in particular.

XLR’s interior is a hotbed of leather-adorned technology. Membrane-switch door releases, navigation system, satellite radio, infrared night vision, active cruise control and heated/cooled seats are only a partial list of the Caddy’s credentials.

Leone goes on to say that the Corvette was the ideal foundation because it was originally designed with break-through structural attributes.

He notes that in the past, open cars inevitably suffered from inadequate torsional stiffness because the roof is missing. The fifth-generation Corvette solved that problem with an innovative frame structure consisting of two hydroformed-steel rails integrated with a large tunnel down the center of the car. GM tests show that in terms of both torsional and beaming stiffness, the structure is superior to all of its key competitors.

Structural stiffness is especially important in a luxury convertible because it determines whether or not the finished product creaks, groans and shakes over uneven pavement. When prices depart the mainstream, (expectations are that the Cadillac XLR will cost $70,000), customers justifiably expect rocksolid construction: doors that slam with a bank-vault report and a general absence of the jitters and jive that plague the clappedout convertibles driven by college kids.

The traditional means of bolstering an open car’s stiffness is to add stout reinforcements in the door-sill and windshield-pillar areas. The down side is excess weight, poor performance and abysmal gas mileage. That would never cut it in a Corvette and Leone is justifiably proud that his Cadillac XLR will be stiff AND light.

“We will definitely be lighter, and therefore more nimble feeling, than our competitors,” Leone predicts. “The target is 3,600 pounds, versus well over 4,000 pounds for the Jaguar and Mercedes and 3,850 for the Lexus.”

Apparently, his team didn’t miss its ambitious target by much. GM’s preliminary specifications sheet lists a curb weight of 3,650 pounds.

While the foundation is Corvette, the house is distinctively Cadillac. Leone explains, “To preserve the visual theme established by the Evoq concept, we stretched the Corvette’s wheelbase by 30mm [1.2-inches] and trimmed 50mm (2.0- inches) from the front overhang. The beauty of our hydroforming process is that we can make both the Corvette and the Cadillac rails in the same set of tools. They become two distinct parts when we laser trim the Cadillac part shorter than the Corvette’s rail.”

Since the powertrain has such a dominant influence on the character of any car, the two soul mates also diverge in this category. In place of the Corvette’s sledgehammer approach — a 5.7L pushrod V-8 pounding out 405 horsepower — Cadillac emphasized suave sophistication. According to Leone, the next-generation of the 4.6L DOHC Northstar 8 (currently powering Cadillac DeVilles and Sevilles) will provide a few less horsepower but still commendable performance with exceptionally smooth and quiet operation and good fuel economy.

Leone adds, “The cachet of dual-overhead camshafts is well established in luxury vehicles so the Corvette’s engine isn’t suitable for the XLR. Instead of its hard-hitting torque and throttle response, we’re tuning our engine to deliver a more refined rush that keeps building until you reach the 155-mph speed limiter. “Preparing the Northstar V-8 for this application was no small task. We shifted it from transverse to a longitudinal orientation, added variable valve timing, changed the crankshaft from a casting to a forging in the interests of quietness and boosted both power and torque. For all intents, this is the third-generation version of the engine Cadillac introduced ten years ago. We also engineered a five-speed automatic transmission for the XLR [versus the current Corvette’s choice between a four-speed automatic or six-speed manual gearbox].”

Installing an engine that’s greater in length, width and height prompted several chassis disruptions. The cast aluminum member that cradles the engine plus steering and front suspension components is new to relocate both the rack-and-pinion steering gear and the front wheels forward a bit. That in turn forced new steering arms on the front knuckles. While they were at it, engineers tuned up the steering geometry slightly with extra caster to enhance the XLR’s on-center road sense.

Notes Leone, “While the basic design is the same as the Corvette, several of the suspension parts are new to suit the Cadillac’s specific needs. Essentially, we were building accumulated knowledge here. But since the XLR is heavier by design, we had to revalidate everything anyway. The Cadillac brakes Brembo vented rotors, PBR calipers) are significantly larger and the Michelin tires we selected are completely different from the Corvette’s Goodyear tires.”

XLR’s retractable hardtop from Car Top Systems is fully automatic. As front latches release, the deck lid and package tray lifts. Quarter windows fold into the roof as it retracts into the tonneau area and the deck lid closes to conceal the top.

In place of the Corvette’s manually erected soft top, Leone’s team engineered an exotic hard top that folds under the decklid at the touch of a button. A cast magnesium bracket nicknamed “the origami” because of its involved shape supports the top mechanism and reinforces the rear of the XLR. The folding top assembly weighs over 200 pounds, which is substantially more than the Corvette convertible’s rudimentary top but there is a silver lining: superior weight distribution, the result of adding mass at the rear. According to Leone, the XLR is neatly balanced at 50 percent front and rear while the Corvette carries 52 percent of its weight on the front wheels. Like the Corvette, the XLR uses composite- plastic body panels made using various molding techniques. Leone acknowledges that an aluminum body was considered and rejected for a number of reasons, including compatibility with processing in place at the Corvette’s Bowling Green, Ky., manufacturing plant.

“Don’t forget that composites are hundreds of pounds lighter and more dent resistant,” Leone stresses. “Building bodies in steel or aluminum would also have forced compromises in the shape and crispness of certain exterior design details we couldn’t accept. The minimum edge radius possible in composites is 1.5mm versus nearly 3mm in metal.”

Compared to the stripped-for-speed Corvette, the Cadillac XLR’s cockpit is festooned with creature comforts and safety systems galore — a key fob that unlocks the doors and disables security measures as you walk towards the car; membrane-switch door releases; seats that heat, cool and croon stereo music; a nine-speaker Bose audio system sharing an in-dash display screen with a DVD-based navigation system; satellite radio; infrared night vision; radar-based cruise control that warns the driver of looming traffic in a head-up display; OnStar communications and concierge services; eucalyptus-veneer and anodized-aluminum trim; and side airbags that guard against head and thorax injuries in a lateral collision.

Some of this gear and most of the engineering upgrades will likely embellish the sixth-generation Corvette due late next year as a 2005 model. The evolutionary nature of GM’s architecture sharing makes it less a game of hand-me-downs and more a relay race where each runner advances the cause before passing the baton. Leone concludes, “Customers don’t know or care whether we begin from scratch or build on our success. But they do expect outstanding cars and shared architecture is an excellent means of giving them exactly that.”

The new Cadillac shares basic structure with the Corvette. Suspension parts are unique to XLR and the front structure has been modified to accommodate the larger box volume of the Cadillac Northstar engine. Both cars are skinned with composite panels.