Body-in-Lite

Chrysler LLC believes it has a jump on the competition in reducing vehicle weight, thanks to a new approach in body engineering the auto maker says will begin to pay big dividends in cars and trucks slated to hit the market in the next three to five years.

The new holistic approach to body-in-white (BIW) engineering was developed by Chrysler and Daimler AG's Mercedes-Benz and MBTech in collaboration with the American Iron and Steel Institute and promises to cut the weight of a vehicle's “structural backbone” some 13%.

Driving the push to reduce BIW mass and improve structural integrity are upcoming safety standards to protect against side impacts and rollovers, as well as growing pressure from consumers and regulatory agencies to boost fuel economy.

Bill Grabowski, director-body core engineering for Chrysler, says the BIW typically accounts for about 20%-25% of vehicle weight, or roughly 1,000 lbs. (454 kg), and that every 120 lbs. (54 kg) of mass removed results in a 1% gain in fuel economy.

By that measure, a 13% reduction in body weight would improve fuel economy a precious 0.2 mpg (1.5 L/100 km) in a 20 mpg (11.8 L/100 km) vehicle.

The new computer-aided engineering technology, dubbed “biomimetic topology” because it mimics the way nature evolves efficient structures such as bones and trees, simultaneously takes into account the entire vehicle body and all stress points during the design process.

Today, engineers approach each part of the body separately, then fit the pieces — typically 250-300 components — into the structure. Certain front-end parts may be optimized for frontal impacts, while roof rails are engineered to withstand other forces, with each system designed in isolation.

With the new process, all those forces are considered at once, load paths are optimized and high-strength steels (HSS) strategically utilized, while the integration of all the components occurs up front.

That, Chrysler says, allows it to trim unnecessary weight. Think of the skeleton of a bird — lightweight, but designed for performance, Grabowski says.

In one example of employing the process, Chrysler says it was able to redesign a front shock tower, shaving 1.12 lbs. (0.5 kg) from the component.

Ultimately, the result will be greater use of HSS, which Chrysler says will go to an average of 30% of its current BIW structures by weight to 60% within the next three to five years.

Although pieces of the new design process have been used in existing models — the railing system in the new Chrysler Sebring, for example — a complete BIW from the technology won't emerge until the '10-'13 model year, Grabowski says.

Cost associated with the shift to more HSS steel is an issue. But Grabowski says Chrysler is “constantly checking the value equation. And there may be opportunities to use less of a material.

“It's unclear where this will wash out,” he says. “But we think the steel industry will begin to increase capacity (and ultimately lower the price of HSS). Our goal is to be cost-neutral or produce a cost savings.”

The design process is proprietary to Chrysler and Mercedes, which Grabowski believes gives the companies a leg up in the weight-reduction game.

“Other auto makers will make similar claims,” he says. “But we think this (process) is a proprietary advantage unique to us.”

For its part, the AISI says its steel-making members, which account for 75% of U.S. and North American steel capacity, are ready for the coming boom in HSS.

Citing a report from Ducker Research, it says the industry's use of high-strength, low-alloy steels has grown from 3.6% of the average vehicle by weight to 11.9% in 2007. By 2015, Ducker says, application of more conventional mild steels in vehicle bodies and closures will decline to 29.0% from 54.9% today. HSS steels will account for 68.5%, up from 44%, the researcher says, including advanced HSS.