Steel body loses weight to stay in the automotive race

By Russell Williamson

LIGHTWEIGHT cars and steel have traditionally never been two products that have sat well together.

As carmakers strive to reduce the weight of vehicles to improve efficiency and meet ever-more stringent environmental legislation, aluminum and other light-metal alloys have been the preferred choice.

However, the steel industry is fighting back with the development of the Ultra Light Steel Auto Body (ULSAB) project.

The project brings together more than 30 of the world's steel manufacturers on five continents, aiming to build a significantly lighter, stronger and cheaper car than existing steel-bodied vehicles.

Pictured here is a computer generated design of what that car could look like.

The styling was done by AD Concepts in Michigan, the United States.

By adhering strictly to the design of the ULSAB body-in-white, AD Concepts have styled a vehicle that combines the practicality of the ULSAB design into an attractive, modern five-passenger, four-door sedan.

Creating a "face" for the ULSAB is the first major development in Phase II of the project, which will enable further detailed engineering analysis and testing of ULSAB prototypes which will be built, not by hand, but by off tooling, therefore replicating actual production.

The development of the ULSAB began in 1992 when the U.S.-based Porsche Engineering Services was commissioned by the American Iron and Steel Institute to work with Ford North America to ascertain the weight reduction possible in the Ford Taurus DN5 model.

A design analysis found that the weight of the body-in-white -- the car's skeletal frame without the hood, trunk, front quarter panels and doors -- could be reduced by 19 percent.

However, Porsche thought a far greater weight reduction could be achieved if they were to design a car similar in size and shape to a BMW 5-Series or Honda Accord (four-door, five- passenger, front-engined medium sized sedan) with a clean slate, and so Phase I of the ULSAB project was initiated.

Porsche's brief was to design a lightweight steel body which maintained present standards of structural integrity, durability and crash worthiness and would be easy to manufacture in volume.

In order to establish a design benchmark, the ULSAB engineers first analyzed a range of criteria of similar-sized vehicles to the one they were designing.

By gathering data on a range of Japanese, European and United States-built vehicles, including BMW 5-Series, Chevrolet Lumina, Acura Legend, Ford Taurus, Honda Accord, Lexus LS400, Mazda 929, Mercedes-Benz 190E and Toyota Cressida, the Porsche engineers established an average reference measurement.

They then set design and performance targets that represented a hypothetical midsize sedan capable of being mass-produced in the not-too-distant future.

Central to the ULSAB design project was the concept of holistic engineering which regards the body-in-white as an integrated system, rather than an assembly of individual components.

It was this design which was vital in reducing the weight of the car frame, while the use of advanced technology in production techniques and state-of-the-art automotive steel were of further benefit.

In using advanced technology such as "hydroforming", tailored blanks and laser welding to create the body-in-white, the Porsche engineers still had to be mindful of the fact that this design was intended for mass production.

To meet these needs, all the technology used by the ULSAB designers is available for car manufacturers at present.

Hydroforming was used in the roof rail and eliminates the need and subsequent weight of flanges for spot welding. The roof rail is formed by placing a steel tube into a preshaped "jelly mold" and inflating the tube with high pressure liquid.

Tailored blanks were used in areas such as the front and rear rails and door sills and enables weight reduction without a loss of structural integrity. The blanks consist of combinations of various thickness and strength steels laser-welded together before being pressed into the final shape.

Laser-welding parts such as the blanks and the roof panel to the roof rail uses a continuous weld, thereby removing the need for flanges and improving strength as spot welds are strongest only at the actual weld points.

In the early stages of the ULSAB's development, structural rigidity and weight reduction drove the process.

Once these goals came close to being realized other factors, such as crash worthiness and cost of production, became increasingly important.

However, vehicle weight remained the most important consideration and although the ULSAB did not quite reach its target, the reduction of 66kg, compared with the present benchmark of 271kg, was significant.

A reduction in weight of the body-in-white not only improves fuel economy and subsequent exhaust emissions, but secondary weight and cost savings can be achieved through smaller engines, lighter brakes, suspension and wheels and tires for similar performance standards.

Other significant improvements in the ULSAB were made in torsional rigidity, which was increased by more than 65 percent compared with the current reference average, and the body-in- white mode.

By improving both the torsional and bending rigidity of the car frame, the ULSAB offers the potential for vastly improved noise, vibration and harshness (NVH) levels, ride comfort and handling.

Torsional rigidity refers to the amount the vehicle twists along its front to rear axis and bending rigidity is the amount of bending along the same axis and determines the rigidity of the car's body.

The body-in-white mode frequency for the ULSAB was 51 hertz against the benchmark average of 38 hertz. The body-in-white mode frequency is the lowest frequency at which the body of the car will resonate, therefore the higher the body-in-white mode frequency for a car, the lower the chance of the car frame vibrating.

This lowers the NVH of the car and it is generally understood by engineers that if a car has a body-in-white mode frequency of more than 40 hertz, any vibration detected is not emanating from the car's frame.

Although there has yet to be a decision on whether Phase III -- the actual construction of a running prototype -- will be developed, the results so far can only be positive and auger well for the continued use of steel in automotive production.