Despite what the members of Earth First! might think, body-on-frame architecture is still necessary for those vehicles that need true off-road capability, towing ability, and which may see severe duty. This is not to say that they can’t afford to lose a little weight. However, it’s hard to reduce weight when crash standards are increased. By softening the front structure–through the “down gauging” of the front frame rails–car-to-truck crash compatibility is improved, although overall frame stiffness is reduced. Making the rest of the frame thicker recovers the lost rigidity, but at the cost of increased weight. Which increases the overall weight of the truck, and increases the energy it brings to the crash event. Which leads to a reevaluation of the materials used, and a call for alternatives that are lighter. Unfortunately, these alternatives often carry a heavy price premium that leads automakers back to known designs made from a known material–steel, which is presumed to be heavy.
The work of the Auto/Steel Partnership’s Lightweight SUV Frame Team is proof that steel doesn’t have to be heavy. Funded, in part, by the U.S. Department of Energy (DOE), this project removed all constraints but two: (1) not the most advanced design, but a known quantity, the frame had to replace the baseline frame (from a Ford Expedition/Lincoln Navigator) without major assembly and packaging issues, and (2) it had to do so for a minimal increase in cost. The team engaged Altair Engineering to design and engineer the frame, and Oxford Automotive to carry out a detailed manufacturing and assembly cost analysis. The result was a 100% steel frame of non-traditional shape that is 23% lighter than the baseline, meets or exceeds its performance, reduces overall frame weld length by 50%, and has a 31¢ per pound price premium.
At the outset, a number of steel types were investigated (metallic coated, boron, sandwich, metal foam), but high strength steel (HSS) and advanced high strength steel (AHSS) were ultimately chosen as the primary metals for the job. “We increased the content of HSS from about 5% in a typical frame to over 60% in this design,” says Jim Cran, project manager for the program. Driving the increase was the adoption of a “through rail” joining system similar to that found on the 2004 Ford F-150 pickup where a round tube intersects a rectangular tube and is welded on each side. “This technique gives a joint that is 10 times stronger than attaching the cross piece to the side of the frame rail,” says Cran. “Plus, it tolerates greater use of thinner gauges of high strength steel with no loss in strength.”
Optimization analyses were used to determine the structures with the greatest mass savings at given performance levels (measured in terms of bending and torsional stiffness, modal response, and peak stress), create a two-dimensional feasibility study that compared loads against the outline of the baseline frame, and define the package space. A three-dimensional analysis set the location of the frame rails and cross-members. This produced a basic design in which straight front and rear frame rails are joined to an hourglass center section. Based on the manufacturing requirements, the frame retained this overall shape, but moved from upper and lower rails connected by triangulating members to a design with an open C-channel center section and front and rear hydroformed rails.
“The baseline frame weighs 226 kg.,” says Cran, “and the new frame weighs just 174.4 kg., for a savings of 51.6 kg.” Bending and torsional stiffness are 31% greater than the baseline, with a natural frequency of 25.0 Hz in torsion and 27.8 Hz in vertical bending. The hydroformed front rails are a 2.25 mm hexagonal design made from HSLA 420/480 steel, and 18% lighter than the baseline frame’s front rails. Cran admits the need to conduct more simulation work to validate the manufacturing process, however. “There’s nothing in the design that can’t be made, or isn’t being done,” he says. “Most of the pieces clearly are manufacturable, but others have never been used before, so it will take some validation testing to make people comfortable with the design.” And some understanding of the materials properties to achieve the same goal. “We may be able to take the engineering community to a new level of design understanding,” says Ron Krupitzer, senior director, Automotive Applications Group, American Iron and Steel Institute, “but we then must come up with processes that work and make certain we have the materials in the thickness and width necessary to produce this design.”
Based on the strong correlation between the models and prototypes for the Ultralight Steel Auto Body program (http://www.autofieldguide.com/articles/040305.html), Krupitzer expects the lightweight SUV frame to perform as expected. “Based on current modeling techniques,” he says, “we think it is very predictable, but we can’t always know the limitations until we test them.” That is the goal of Phase II, while Phase III will build prototype frames for validation and testing. Even if funding never materializes to take the program to that level, elements of the project are certain to make it into production. Both the engineering firms and steel companies are free to work with any OEM willing to follow this path, a direction most are expected to follow. “The new standards for car-to-truck crash compatibility will require many OEMs to redesign their body-on-frame vehicles,” says Cran, “which is the perfect time to adopt the architecture and build processes associated with this project.” Despite a 31¢ premium compared to current frame designs, Cran says the hourglass frame is well below the 70¢ per pound premium DOE allows for a 25% weight savings, and the cost could drop more. “I think it’s fair to say that you could take other measures to mitigate this increase in a clean-sheet design, and combine it with gains earned from designing the body and frame holistically. It will happen.” Log on to www.a-sp.org for more information regarding this project.