Mention “lightweight design,” and supercar specialists Ferrari, McLaren, and Porsche probably top your hit parade. General Motors, if you think of them at all, surely lives down this list. But now that every maker is striving to satisfy future fuel-economy obligations, GM is emerging as a leader of the lightweight pack, especially in terms of mass-produced models that most of us can actually afford.
Charlie Klein, GM’s executive director responsible for CO2 strategy, energy, mass, and aerodynamics (what else matters?), recently summarized the progress America’s largest car producer has made. Using GM’s own figures, seven fresh production models are said to be lighter by an average of 350 pounds compared with their immediate predecessors. Using manufacturer-claimed base-vehicle weights, GM touts that the Cadillac XT5 is some 650 pounds lighter than the Mercedes-Benz GLE-class. Packing evidence into his case, Klein revealed the strategies behind GM’s efficiency initiative and shared a few advanced technologies under examination for future implementation.
“Seven fresh production models are lighter by an average 350 pounds compared with their immediate predecessors”
Six to eight years ago, GM’s top engineering management concluded that lighter products not only were essential to meet more stringent mileage standards, but also to compete successfully against global competitors in performance. Once that realization trickled down to the R&D trenches, Klein and his engineering team elevated lightweighting to the same priority as advanced powertrain technologies, aerodynamic refinement, and the optimization of every component and system.
Lightweighting isn’t the wholesale conversion of steel bodies to aluminum or carbon-fiber construction. Because cost and other variables—such as crash safety, interior space, and noise and vibration—are involved, this engineering responsibility gets complicated fast. So that everyone involved understood the war against weight, some clever GM manager coined a concise battle cry: Every engineer, part, and gram matters!
Fortunately, GM is armed with clever experts and powerful design and development tools. Computer-aided engineering (CAE) is arguably the handiest wrench in the GM toolbox; it facilitates the evaluation of 1001 good ideas before choosing a few for the prototype stage. CAE helps optimize load paths, structural stiffness, crash performance, aerodynamic efficiency, and curb weight. GM’s team of 140 simulation engineers invested 19 million CAE computational hours to help shave weight in the Camaro and Malibu. Taking the incumbent German sport sedans seriously, another 50 million computational hours were invested in assuring that the Cadillac CT6 could meet or beat the Audi A6, BMW 5-series, and Mercedes-Benz E-class.
Some of this effort is basic attention to details. The Cadillac ATS, for example, is lighter because many panels are liberally punched with holes, and flanges are trimmed and scalloped around spot weld locations to shed unnecessary material. The new Malibu is larger and structurally stiffer than its predecessor, but it’s also hundreds of pounds lighter because half of its unibody consists of high-strength steel; seven different grades were selected to put the most appropriate steel at every location.
The Cadillac CT6 is a crazy quilt of aluminum, steel, magnesium, and plastic—11 different materials in all. Front longitudinal members that must absorb collision energy are aluminum extrusions. Castings, which save weight and trim the total parts count, are used for major joints such as the front hinge pillar. Sheet aluminum covers the roof, fenders, and doors. Various grades of steel comprise the passenger cell’s safety cage. To assemble the CT6’s parts into a cohesive unibody, GM uses eight different joining methods: 900 feet of structural adhesive, 30 feet of aluminum arc welds, 7 feet of aluminum laser welds, 2 feet of steel brazing, 1624 steel spot welds, 1469 aluminum spot welds, 745 flow-form screws, and 333 self-piercing rivets.
Audi, Jaguar, and others have manufactured aluminum bodies for years, but GM ventured beyond their methodology to make the CT6 lighter, stiffer, and reasonably affordable. One major stride is avoiding use of rivets for joining panels, which adds weight and cost. Instead, GM perfected spot-welding techniques to unite the various forms of aluminum. This required new electrode tip designs and careful programming of the electrical current and voltage. Nine GM plants applied the lessons learned to begin manufacturing aluminum liftgate, door, and body-structure components.
The next step is joining aluminum and steel with spot welding. A key issue is that aluminum melts at 1200 degrees Fahrenheit while steel melts at 2800 degrees. Also, a thin inter-metallic layer forms between the two metals and the oxide coating the aluminum sheet must be dealt with. By using simulation tools to model the underlying physics, GM engineers identified the weld parameters that would allow traditional methodology—spot welding—to be used in Cadillac’s newest sedan. After validation testing is completed, this patented technique will be used to join two sheets of aluminum to a single sheet of steel in the CT6’s seat backs. The second near-future application is spot welding an aluminum inner hood panel to its steel reinforcement.
Laser welding is another attractive technology with problem areas. Conventional techniques diminish quality with weld spatter and porosity issues. GM’s patented solution is using dual laser beams to create a quiescent pool of molten metal at the weld location. The second problem is that the laser’s aperture plate is damaged by energy reflected from the weld area. That concern was resolved by shaping the plate into a V so that energy bounces harmlessly off without damaging the aperture plate.
Magnesium is an attractive structural material because it’s one-third lighter than aluminum. A few makers are already making inner-door assemblies of this material; Klein’s show-and-tell compared a seven-piece steel assembly weighing 20 pounds to single-piece magnesium die-casting weighing half that much. Also, sheet magnesium is attractive for roof panels and other areas of the car. Three years ago, GM began manufacturing Cadillac SLS decklids in China consisting of a magnesium inner layer joined to an aluminum outer layer. Heating the magnesium to 850 degrees F softened it to the extent that it could be formed into a die with a blast of high-pressure air. A few hundred production cars were built to gain experience with this methodology.
Without saying exactly when, GM engineers acknowledge that they have sufficient experience testing carbon-composite wheels to offer them soon as a regular production option. This will save 35 pounds per car in unsprung weight and rotating inertia. Another study area is replacing most of a unibody’s floor pan with a molded-fiberglass panel. A demonstration piece revealed a spot-welded sandwich of steel and fiberglass in key areas where seats, reinforcements, and cross members attach. Advantages are reduced parts count, lighter weight, and compatibility with current assembly methods. To save even more weight, substituting carbon fiber for the fiberglass also is feasible.
Tallying up the gains, GM claims a net savings of 15-million gallons of fuel and 150,000 tons of CO2 per year for seven new production models: Buick LaCrosse; Cadillac XT5; Chevrolet’s Camaro, Cruze, Malibu, and Volt; and GMC Acadia. This bodes well for the future. Lessons learned, methods in place, and technology under development will lift GM’s standing on everyone’s respect list. And don’t be surprised if future GM products—most notably the 2019 Corvette—rival the best Ferrari, McLaren, and Porsche supercars in terms of both weight and performance.