DETROIT – How to get weight out of vehicles that already have been made significantly lighter is a conundrum facing suppliers and automakers alike.

Unfortunately, there is no silver-bullet solution that will make the process easier going forward, say panelists at a Society of Automotive Analysts Lightweighting Summit here.

“It’s unlikely there are any breakthrough materials awaiting discovery,” says Abey Abraham, director-Ducker Worldwide at the conference.

In the face of increasingly stringent global fuel economy and emissions regulations, including in the U.S. where a 54.5-mpg (4.3 L/100 km) fleet fuel-economy bogey is set for 2025, suppliers and automakers already have been able to remove tens – in some cases, hundreds of pounds of mass when creating new vehicles. One or more alternatives to traditional grades of steel – aluminum, magnesium, carbon fiber, and most commonly ultra-high-strength steel – make up a growing percentage of the bodies of most new cars and light trucks in the U.S.

But continuing to put vehicles on a diet to improve economy and emissions is making some industry players lightheaded.

“We can’t turn around next year and make a brand-new version of (an existing vehicle architecture),” says General Motors’ Rob Peckham, technical integration engineer-mass strategy and tool development. Using the Chevy Equinox as an example, he says GM removed nearly 400 lbs. (181 kg) from the vehicle with the latest generation. “We just came up with a brand-new car. Several years later, if you were going to do a facelift, how would you find enough mass to get out 125 lbs. (57 kg), or up to 500 lbs. (227 kg), for SUVs and trucks?”

While Peckham’s question has no easy answers, many panelists say a mixed-materials strategy, and optimizing designs will continue to be the way forward.

By using topology optimization software to determine the most effective load paths, suppliers and automakers are able to remove material that is unnecessary to the structural integrity of even the smallest parts.

Ford’s Shawn Morgans, global manager-systems architectures, says optimization software led his engineers to determine “40% of the material wasn’t serving any purpose” in a small, boxy reinforcement part, designed to transfer load from the outside to the inside of the rocker. “And on a small part that fits in the palm of my hand we took out over a kilogram (2.2 lbs.) of mass per vehicle,” he says. “So it’s the small steps, and the big steps, that are getting us where we need to go.”

For areas where mass cannot be removed, use of steel alternatives will continue to gain ground, panelists says.

Mixing materials is not new – Morgans reminds the crowd the Model T had steel panels, an aluminum hood and a wood frame – but the trend will become more popular, says Abraham. “Mixed materials is really the pathway (to mass reduction),” he says.

Ducker sees aluminum as the fastest-growing automotive material, going from 397 lbs. (180 kg) per vehicle in 2015 to 565 lbs. (256 kg) per vehicle by 2028, when it will comprise 16% of total vehicle weight.

While Ford probably is the automaker most closely associated with aluminum – after famously taking as much as 700 lbs. (318 kg) out of the F-150 pickup by switching from steel to aluminum body panels, others also have been using the durable, lightweight material extensively.

Aluminum has been a popular replacement for steel for hoods but also is being employed in other areas.

While Peckham notes the Equinox, Chevy Malibu and Bolt all have aluminum hoods, the Bolt also has an aluminum liftgate and doors. The Chevy Camaro employs cast aluminum shock towers and both the Bolt and Malibu have select chassis components made of the material.

The fast growth of aluminum should be music to the ears of supplier Novelis, but Gary Gallo, manager-automotive recycling for the aluminum powerhouse, notes the increase isn’t without downsides.

“The aluminum industry has never seen the volume of aluminum-scrap generation that we’re seeing today,” he says, driven by the rapid growth in the use of aluminum for closures.

The scrap, being generated largely by GM, FCA and Toyota and not part of Novelis’ closed-loop recycling program in which Ford and Jaguar Land Rover participate, could put downward pressure on pricing.

“There is no historical reference for what may happen to pricing as a result of that,” Gallo says. “We do believe that scrap sales going forward, that are left to a spot market, will be at significant risk.”