How GM Is Cutting Vehicle Weight

What are the things that General Motors is doing to make its body structures lighter? Terry Swartzell, Underbody System Architect at the automaker, spelled many of them out during a presentation at the recent Great Designs in Steel conference (autosteel.org) in the context of what they did for the new midsize car architecture that is being used for the 2016 Chevrolet Malibu and the 2017 Buick LaCrosse.

Elements that he described as constituting the “Mass Reduction Hierarchy” are:
  • Control of architectural bandwidth
  • Efficient integration of systems
  • Aggressive CAE iteration and optimization
  • Right performance targets
  • Right steel grades
  • Execution excellence.

For example, he noted that the LaCrosse and the Malibu, while on the same platform, have different sizes (e.g., the wheelbase of the Malibu is 111.4 inches and the LaCrosse is 114.4 inches).

Historically, he explains, they would have the approach of “one underbody fits all.” A consequence of that is that they’d optimize the structure for the biggest case, whether that’s in overall size or in terms 
of powertrain.

While the LaCrosse will come with a standard 3.6-liter V6, the Malibu is offered only with a four-cylinder engine (1.5-, 1.8- or 2.0-liter).

A consequence is that there is what Swartzell describes as “scar mass”: the smaller, less powerful Malibu has to carry the weight for what is engineered in for the larger LaCrosse.

So in the new approach, that’s not acceptable. He admits, “Yes, there’s less part sharing. But that’s a price we’re willing to pay for mass savings.”

In addition to which, he points out that even for a given vehicle they’re looking not at the most-extreme scenario when engineering the structure, but the one that is expected to be the biggest seller. So for the Malibu, it is a vehicle with the 1.5-liter engine (it would make little sense to optimize it for the 1.8-liter, as that is part of a two-mode hybrid system that’s being made available on the Malibu, and while it is a quite impressive hybrid—offering 47 mpg city—given low gas prices, that’s not going to be a high runner anytime soon). They find it more effective to add mass where they need to rather than using mass across the board, even in cases where it isn’t needed.

Addressing some of the other points, Swartzell said that they’re making extensive use of computer-aided engineering—they’ve established six virtual “gates” that each model must go through during development, and they’ve creased 2,832 standard cases to address vehicle characteristics, ranging from structures to loads to thermodynamics.

They’re using lightweight materials where they can, and that because they’re using thin sections they’re using bulkheads in places for attachment (he cited airplane wing attachments as an analogy) and using adhesives to supplement spot welds (in the cases of the Malibu and LaCrosse, structural adhesive use is on the order of 54 meters each). Flanges are minimized, edges are scalloped, lightening holes are punched into parts in the pursuit of mass reduction.

Swartzell emphasized that when it comes to the six concepts listed, “It is not just one or two of these concepts—it is all of these concepts.” It is a holistic approach, not individual selections from a menu.—GSV