Tech Briefs
Automotive dieting

Combating vehicle weight gain has been a losing battle for the auto industry. In the past 20 years, the average European car's curbside body weight has risen by 20% despite the best efforts of designers and engineers. The reasons for this center mainly on legislative requirements for enhanced safety, with resultant additions to structures and also to airbags (front, side, and, in some cars other areas) and seatbelts for all occupants. It is also due to the momentum of market trends towards higher specifications; air conditioning is now becoming relatively common in Europe, whereas even five years ago it was something of a rarity on any car other than a luxury model.

The problem of keeping weight under control is being addressed in several ways, including through the use of aluminum. While aluminum is used for the production of only about 1% of all cars worldwide, Audi demonstrated with the big A8 that the material could be used for low-volume production. The company is now starting manufacture of the A2, a medium-volume (at least 50,000 units per annum) five-door car. Frank Walker, Manager of Technical Coordination at Corus, an international metals group created by the merger of British Steel and Koninklijke Hoogovens, does not envision it becoming the material of choice for mass production within the auto industry. Walker believes the industry will stay with steel but that it will be increasingly demanding with regard to the material's performance.

Walker is also Technical Committee Chairman of the ULSAB-AVC (UltraLight Steel Auto Body Advanced Vehicle Concepts) project, a pan-continental effort by a consortium of 33 steel producers to meet vehicle crash safety standards and cut the curb weight of a typical C-class car to that of a smaller B-class model by 2004. An earlier ULSAB body project demonstrated last year that it could achieve significant body weight savings of at least 25% via advanced technology steels. Walker regards maintaining this level and meeting future crash legislation as "one of the most difficult challenges facing the auto industry today." However, the consortium has challenged itself to meet the world's toughest safety standards anticipated for 2004. The future is likely to hold higher crash-test speeds, changes to offset tests and oblique offsets, and tests such as side impact by a telegraph pole cross-section.

Although the ULSAB-AVC project may achieve its objective, component manufacturers may be unable or unwilling to cut weight. And there is another interesting set of social figures. "Despite a heightened awareness of the need for dietary constraint and increased exercise, the average weight of the human frame is increasing," said Walker. Quite simply, people are getting bigger—and heavier.

The focus of the new UltraLight Steel Auto Body Advanced Vehicle Concepts (ULSAB-AVC) includes safety features.

These conflicting factors make the OEM's task difficult, but the ULSAB-AVC is confident that it can meet future safety criteria and apply it to smaller cars. "We will demonstrate that at present it is only advanced steels that can cost-effectively address the conflicting demand for lower weight—and thus lower fuel consumption—for mass production vehicles while enhancing safety, performance, and retaining comfort levels," said Walker. "It has become a major technological challenge to make cars not only strong and safe but also as lightweight and affordable as possible."

Walker stated that without using high-strength steels and laser-welded tailored blanks to meet safety requirements, the auto industry stands to lose half the weight savings already demonstrated through the original ULSAB project because of increasing curb weights. He is convinced that despite Audi's demonstration of aluminum's possibilities, in 2010 steel will remain the most common material for the global vehicle park. The third-largest steel producer globally, Corus is also the fifth-largest producer of aluminum. With expertise in the use of both materials, the company is able to advise on appropriate applications and even a composite of the two metals. But what leads to Walker's conviction regarding the continuing dominance of steel is that the evolution of steel technology has been impressive. High-strength steels are now up to five times stronger than mild steels, but most of the technology that has achieved this is very new: 80% of today's steel grades were developed in the 1990s. Metallurgical properties of steel include strain-hardening, the influence of which increases with speed-of-impact to absorb even more energy.

The ULSAB-AVC project will be carried out by Porsche Engineering Services in Troy, MI, to achieve the requirements for application in 2004. It is scheduled to be completed next year, and the consortium plans to be far more open about its progress than was the case with the ULSAB project. The phase plans for the ULSAB-AVC project is in several stages. Stages one to three—introduction to program, CAE (computer aided engineering) analysis for crashworthiness, and benchmarking/target setting—have been completed. The next stage is styling. Then will come engine, transmission, powertrain, and platform concepts; development of data, subsystem concept design, and manufacturing technologies; and, finally, results.

Advanced steels and technologies In the nearly two years since the global steel industry unveiled its original UltraLight Steel Auto Body (ULSAB) prototype, automakers have been increasing steadily their uses of advanced steel technologies. ULSAB demonstrated that through the use of advanced steels, the latest in process technologies, and holistic engineering, it is possible to reduce the mass of body structures substantially using steel. Doing so helps vehicle makers to capitalize on the benefits of steel including safety, low cost, and environmental friendliness, while reducing emissions and increasing fuel economy. ULSAB used more than 90% high-strength and ultra-high-strength steels. Below are production vehicles that use high-strength steels. The 1999 BMW 3 Series has a body structure weighing 230 kg (507 lb) with a high-strength steel content of 50%. The previous model contained less than 5%. Ford's new Focus uses high-strength steel for both the body structure and exterior body panels. The new Mercedes-Benz S-Class uses 38% high-strength steel, which contributes to a lower body weight, an increase in torsional stiffness of 70%, and improved crash performance. Toyota's latest subcompact car, the Vitz, uses high-strength steel in 48% of the mass of its 253-kg (558-lb) body-in-white, which weighs 17 kg (37 lb) less than its predecessor, the Starlet. Ford's Windstar utilizes almost 60% high-strength steel. ULSAB used 14 tailor-welded blanks (TWBs), representing 45% of the body structure mass. Other cars that use TWBs include the following. The new GM G platform (Cadillac DeVille, Buick LeSabre, Pontiac Bonneville, and Oldsmobile Aurora) uses a TWB for the bodyside inner panel, similar to the bodyside outer panel used in the ULSAB prototype. The G platform also uses North America's first nonlinear TWB application in the floorpan. The Volkswagen Golf uses 14 - 21 TWB parts, depending on the specific version. Nearly all Chrysler platforms now use TWB. The Jeep Grand Cherokee uses nine, four of which are in the body structure. The BMW 3 Series (with a diagonal-running weld line), Mercedes S-Class, 2001 Ford Explorer, Dodge Durango, Chrysler LH platform, Jeep Grand Cherokee, Honda Accord, Dodge Neon, Chrysler PT Cruiser, Cadillac Seville, and VW Golf all use TWBs in the door inner panel. The ULSAB prototype demonstrated a hydroformed side roof rail that runs from the A-pillar along the B- and C-pillars into the rear floor panel. Instead of using a standard tube with a relatively low diameter to thickness ratio (D/t), the ULSAB roof rail is constructed from a tube with a diameter of 96 mm (3.8 in) and a thickness of 1 mm (0.04 in), resulting in a D/t ratio of 96. GM uses a hydroformed roof rail in the Buick Park Avenue and Cadillac Seville. GM also uses hydroforming for the front part of the main frame members for the Sierra/Silverado trucks. Currently the main hydroforming applications are in engine cradles, suspension, radiator supports, and IP beams. The ULSAB body structure has 18 m (59 ft) of laser welding, approximately 60% of which is required to join the hydroformed side roof rails to the roof. This approach accommodates one-sided access and enhances the stiffness of the body-in-white. Volvo began using laser welding for the roof in the 850 (the predecessor of the V70), and was followed by other European automakers including BMW, Volkswagen, and Mercedes. Jean L. Broge

Typical C-class vehicles include the Opel Astra, BMW 3 Series, and Audi A3. The ULSAB-AVC project has selected the Ford Focus as the benchmark C-class vehicle to represent current safety standards in the sector. One aspect of technology being studied is how far a common platform can be stretched to be applied to different size vehicles and still meet all required criteria. The earlier ULSAB prototype achieved a best-in-class 203-kg (447-lb) body structure for an E-class vehicle (typically a Ford Taurus or BMW 5 Series) whereas the ULSAB-AVC target is for a 183-kg (403-lb) body structure for the C-class. This means shedding 24 kg (53 lb) from a typical current C-class bodyshell. "We do not believe we have dug sufficiently deep into new technology potentials, and so we are confident that what we are doing is achievable," said Walker.

To do so will require the auto industry to increase use of higher-strength steels, 2-D laser-welded tailored blanks, laser welding for body assembly, and hydroformed components (see box). "Hydroforming in the auto industry is in its infancy," claimed Walker.

Stuart Birch