Shift to Unitized Body No Slam Dunk

In This Time of Record-High Fuel prices, the idea certainly sounds good: migrate some of the industry's big-volume models — SUVs and pickups — from their body-on-frame architectures to the lighter, unitized body structures on which most cars are built.

With a 35-mpg (6.7 L/100 km) fuel-economy mandate now on the books, everything must get lighter, and if SUVs and pickups are going to remain viable, it seems logical to try eliminating their bulky frames, a single (though multi-part) component that by itself weighs several hundred pounds.

As soon as fuel prices started spiking, in fact, several auto makers quickly trotted out signals of such thinking.

At January's North American International Auto Show in Detroit, Ford Motor Co. unveiled the Explorer America concept, a new-age redesign of the seminal and big-selling Explorer — “new-age” because the concept rode on a unibody platform. And Toyota debuted the A-BAT, a compact pickup, also sporting a lighter unibody structure.

By the time of February's Chicago auto show, General Motors Corp.'s truck division, GMC, was ready with the GMC Denali XT Hybrid concept, a unitized-body quasi-pickup recalling the Chevrolet El Camino car-based pickup popular in the 1960s and 1970s.

GM's Pontiac division actually will carry that ball first, it seems, with the recently announced G8 ST — an El Camino-ized variant of the new G8 rear-wheel-drive sedan that will be in showrooms late next year.

So what's not to like about switching to lighter structures? After all, many compact and midsize SUVs and pickups originated on full-frame structures largely for manufacturing convenience that also kept investment low, not because typical buyers needed the ruggedness a frame imparts.

It's tempting and currently in vogue to target that heavy steel frame, but some engineers caution there's plenty to be considered before the let's-cut-weight stampede to unibodies begins, some of which has nothing to do with engineering.

Refining Definitions, Understanding Needs

First, can pickups and SUVs be readily “converted” from body-on-frame designs to unibody structures?

Absolutely, but only to a point. Industry insiders say the first step is defining what SUVs — and to a lesser extent, pickups — must be in the future.

“You do have an ‘architectural’ need for the frame,” with pickups, says Frank Gabbianelli, chief engineer at Cosma International, the metal-forming unit of supplier giant Magna International Inc.

Cosma produces about 45% of the light-vehicle frames for North America. Gabbianelli says the robust structure of a full perimeter frame resists flexing and is ideal for a pickup's heavy “box” loads and high towing demands.

SUVs, particularly small and midsize, are another matter. The full frame is much less vital unless the vehicle is used for heavy towing.

But even most light-duty pickups rarely haul big loads and are used as “lifestyle” vehicles the majority of the time, says Swamy Kotagiri, Cosma's executive vice president-engineering.

How light pickups have been used for the last 10 years or so has diverged from the original intent. Pickups have “become more of a means of transportation than a work vehicle,” he says. “If that is the usage, you don't see the advantage of a frame.”

Both engineers say declining volumes in the pickup segment signal a trend back toward pickups primarily as work vehicles. That creates the opportunity to offer the lifestyle-truck customer a vehicle that is less rugged, less work-oriented — a “pickup” without the heavy frame, perhaps.

“We tried to cater to every different need of the customer by building all these different (pickup) variations,” says Kotagiri. Today's long pickups, with limousine-like cab volumes, seem to prove that for many customers it's no longer about what can be carried in the bed or towed.

Harvey Bell, GM North America's executive director-advanced vehicle development, agrees, saying high fuel prices practically guarantee lifestyle customers will begin to be “far more discriminating” about buying a lot of pickup or SUV capability they don't intend to use.

Bell says the transformation of many of today's body-on-frame SUVs to unibody architectures is inevitable.

“I would suggest that's already happened,” he says, pointing to the rapidly expanding cross/utility vehicle segments, most of which employ unibody structures and are gradually nudging out full-frame SUVs.

But less certain is the potential to switch mainstream pickups to unibody structures. “That's far more problematic,” Bell says. “The pickup buyer is pretty traditional. That's going to be a different ballgame altogether.”

Engineering Provides Answers, But What About Towing?

A change in customer behavior is one thing, but the industry needs weight savings now.

So how much really can be saved by ditching a frame? In general terms, something on the order of 200 lbs. (91 kg), sources say.

That doesn't seem like much, considering an all-wheel-drive V-6 Ford Explorer weighs 4,628 lbs. (2,100 kg). Getting rid of the frame in favor of a unitized body shell wouldn't even net a 5% savings.

Bell says pickup/SUV frames have enjoyed a lot of weight optimization in recent years. Thanks to metal-bending advances such as hydroforming, coupled with computer-assisted design, today's frames aren't the lardy targets of old. Hydroforming, alone, cuts as much as 100 lbs. (45 kg) from some fullsize-vehicle frames.

“Do I think there's potential to do more than that (cut 200 lbs.)?” asks Bell. “Yes.”

Cosma's engineers also say completely removing the frame, which isn't feasible, probably won't deliver out-of-the-ballpark weight savings.

“You'll have to add mass to the body,” to achieve the kind of safety, crash and duty-cycle performance expected from the body-on-frame vehicle, Kotagiri says.

“If everything stays the same, I don't think you'd gain much,” adds Gabbianelli.

Magna's Cosma unit doesn't have much to lose either way. It's the industry's largest frame supplier and a major cradle supplier, and the company's engineers say most of its existing tooling and metal-forming equipment could be used to make more cradles and subframes for unibodies, if need be.

Kimberly Rodriguez, principal and practice leader-automotive services group at consultancy Grant Thornton LLP, fears suppliers will be hurt more by the market's significant, fuel-price-driven shift from trucks and SUVs to cars.

Loss of that volume, she says, will threaten many frame-component suppliers, particularly Tier 2 players.

“The makeup of the market is going to be dramatically different by the time (a shift from body-on-frame to unibody construction) happens,” Rodriguez says. “Some suppliers won't even be around to have to adjust.”

And some long-held totems about customer requirements for trucks, such as towing needs, will have to fall.

“The first thing you've got to look at in this decision: Are you going to do a lot of heavy towing,” says Bell. He says big towing demands will be difficult to meet with front-wheel-drive architectures, the likes of which typically underpin unibody structures.

Bell says FWD vehicles aren't good for towing much more than about 5,500 lbs. (2,496 kg). “We could design (FWD) vehicles to tow maybe 8,000 lbs. (3,630 kg),” he adds. But then plenty of other components, including the body, would have to be beefed-up to handle that kind of duty.

“It's proven that a unitized construction can probably do what a frame does in the medium (SUV) segment,” says Cosma's Kotagiri. But a unibody pickup still would require some type of extra rear substructure to handle reasonable towing loads.

The Cosma engineers essentially agree that somewhere around 8,000 lbs. of towing capacity would be the theoretical limit for any unibody pickup or SUV. Above that, a full frame is required to handle the braking forces and load transfers. The most powerful light-duty V-8 body-on-frame Chevrolet Silverado pickup, for instance, can tow 10,700 lbs. (4,853 kg).

How many truck buyers need to tow that much? Reliable figures about the number of customers who tow are notoriously difficult to pin down. Bell thinks it's less than 20%.

Most who tow do so with fairly light items: personal watercraft or all-terrain vehicles, items light enough to tow behind just about any unibody CUV.

However, Bell doesn't rule out future unibody structures, even FWD layouts, able to handle the extreme towing of today's fullsize pickups or SUVs. He's seen plenty of innovation in his day.

“I would never say ‘never’ about something like this,” he admits, adding those with extreme towing demands always will have a body-on-frame truck available to them.

“How many people really need to tow 8,000 lbs.?”

Probably No Moves Until Next Generation

Is saving a couple hundred pounds really worth it for vehicles that already weigh well north of 2 tons (1.8 t)?

“It depends on what else is going on at the time,” says Bell. For existing vehicles, he adds, “You can probably do other things and save as much. You wouldn't just do it (change to a unibody design) to save 200 lbs.”

But, he says, when it's time to do an entire vehicle redesign, such as the case suggested by Ford's Explorer America concept, the engineering brief could be directed toward the unibody architecture.

It's well known most manufacturers “have programs to do body-frame integral” SUVs, Bell says.

And almost any such program likely will draw on engineering initiatives that already have addressed the need to adapt traditional pickups and SUVs to unibody designs.

Last year, Chrysler LLC and Daimler AG undertook just such a project.

Partnering with the American Iron and Steel Institute, the companies combined intensely weight-optimized advanced steel alloys with a new computer-aided engineering tool — topology optimization — to develop vastly stronger body-in-white structures that might be ideal to take over the duties of a perimeter-frame structure for midsize pickups and SUVs.

Topology optimization, says AISI's Ronald Krupitzer, vice president-automotive applications and market development, essentially mimics the way nature has honed certain animals' physiques for their surroundings. Bird bones have evolved to be mostly hollow, for instance.

AISI's goal was to achieve design and material cooperation to handle loads most efficiently at the lowest weight, says Krupitzer.

Along with Mercedes Group Research and MB Tech, the topology optimization tool determines ideal material thickness for specific load paths and durability requirements, matching them to the unique properties of the new high-strength steels.

The result: a unibody structure that reduces overall vehicle weight 13%. Though not particularly slanted toward towing, Krupitzer says it could be accommodated. “You can build that in with underbody modules,” he says.

It just so happens Chrysler is retooling its Jefferson North assembly plant in Detroit (current home of the Jeep Grand Cherokee SUV) “to produce a future generation of vehicles more efficiently, with world-class quality and an improved environmental footprint,” says Chrysler Vice Chairman and President Tom LaSorda.

That likely means a new-age, weight-optimized Grand Cherokee with a true unibody architecture, rather than the body-in-white stiffened by a frame used today. The tough structure could assure Jeep's rep for durability while markedly reducing weight. A next-generation Dodge Durango could piggyback on the investment.

For years, the industry has been toying with ways to design lightweight SUVs with unibody layouts.

In 1997, the AISI teamed with Porsche Engineering Services in the Light Truck Structure study. Although the pickup developed through the LTS used a fairly conventional frame, the SUV, which used many of the same weight-optimized components, attached front and rear subframes to the body-in-white to cut the number of parts in the integrated structure 32% and chop weight 19%.

This is the kind of learning likely to be applied when — and if — some current body-on-frame SUVs are converted to unibody designs. A potential boost: A bipartisan group of 10 senators recently proposed an $84 billion federal-energy package that includes $500 million in auto-industry funding for light materials.

Ford apparently is serious about the Explorer America, and one industry source insists Toyota is ready to green-light a production version of the A-BAT as a trial balloon.

But the notion of widespread conversion of body-on-frame pickups and SUVs still is no slam dunk. GM's Bell says scarce investment dollars and corporate resistance to radical change mean there might not be a way, even if there's a will.

“If we're going to have vehicles of that size, they're going to have to get better fuel economy,” Bell says. “This would be one path that helps.”

But when asked when we'll see the first high-volume “traditional” SUV or pickup moved to a unibody platform, Bell is circumspect.

“I wouldn't want to hazard a guess,” he says. “There are going to be corporate leaders who are gun shy” in terms of directing investment to such a comparative leap of faith.

Bell fears “another, more critical use for the capital” could, for many auto makers, derail a full-scale frame-to-unibody initiative, despite the fuel-economy pressures coming in the near future.

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