Unibody vs. body-on-frame - sport-utility frames - includes related articles on low-cost sport utility vehicles and frame styles

Automotive Industries, Jan, 1998 by Lindsay Brooke

You've just been handed a billion dollars to fund the major new sport-utility vehicle program you're assigned to manage. In 30 months, your team and a cast of hundreds will develop an SUV that stuns its competitors. It will meet or exceed countless cost, timing and engineering bogeys set by your boss. Then the rave reviews will pour in. Congratulations, bub -- you've created a new benchmark.

Now, how are you going to do it? If your experience is like Gerhard Fritz's, brace yourself for a battle over the vehicle's basic architecture. Fritz, the chief engineer of Mercedes-Benz's remarkable new M-Class SUV, recalls the high emotion and heated debate during the early going.

"We had very strong discussions about whether to utilize a unibody construction, or a separate frame," he explains. "Our new vehicle had to be a real sport-utility, equally good on- and off-road. It also had to be a Mercedes in every way, yet it had to do it very cost-competitively. These discussions lasted for three months," he says, with a hint of glee that the haggling is long kaput.

Mercedes has deep experience with both types of structure, and could have gone either way with the M-Class. But it chose body-on-frame (BOF). So did Chrysler, deriving its new Dodge Durango straight from the capable Dakota pickup, in a blazing 23 months. Ditto for the popular, high-profit SUVs of Ford and GM. Range Rover and Land Cruiser remain BOF stalwarts, with no sign of retreat. And as the chart on page 35 shows, BOF will be the preferred method of building most of the SUVs sold in North America over the next five years. Among the big-volume players, only Jeep's Cherokee and Grand Cherokee are certain to continue with unidbody, the Cherokee having pioneered it for SUVs in 1984.

Where unibody use is expanding is in the so-called "hybrids," the mainly compact Japanese car/truck SUVs derived from passenger car platforms. Recent entries like the Honda CR-V, Toyota RAV4 and Subaru Forester are redefining "sport" and "utility," and in the case of Lexus' RX300, luxury (See p. 38), generally with less mass than comparative BOF vehicles. "Unibody is the wave of the future for SUVs," asserts Mike Robinet, an auto analyst with CSM Corp. in Farmington Hills, Mich. "Weight is everything to automakers today and unibody vehicles usually are more mass-efficient, with potential for higher fuel economy."

While they may eventually rule the world, unibody SUVs now face a new generation of competitors that are exploding old myths about "truckish" and "primitive" BOF vehicle dynamics. Mercedes' ML320 is already widely regarded as the benchmark in its segment. Magazine road tests report the surprising Durango betters the Grand Cherokee in ride quality and steering, and it's quieter at speed than Nissan's unibody Pathfinder. Durango proves that a well-designed pickup truck chassis also can be an excellent SUV platform. Expect the next wave of pickup-based SUVs from Nissan and Toyota to follow suit.

"Body-on-frame gives more latitude to isolate the chassis, which improves ride quality," explains Jim Hall, an industry analyst with Auto Pacific in Southfield, Mich. Adds manufacturing expert Jim Harbour, of Harbour and Assoc. in Troy, Mich.: "If you don't have low NVH in a sport-utility today, you're not competitive. Until recently, light truck frames haven't been very stiff, and NVH has suffered terribly compared with passenger cars."

That's changing quickly. Recent SUV programs have elevated frame rigidity to a top priority. The hydroformed frame on GM's '99 C/K series (See Decemder '97 AI, p. 40) is so stiff that weld joints on proto-type cabs cracked like peanut shells as frame stress loads shifted into the body structure. Stouter frames allow more precise suspension tuning, for better handling.

"In our unibody studies, we found that the rubber elements in the control arms needed to be softer, to insulate the suspension. That would have compromised on-road handling," says Mercedes' Fritz.

To achieve a given structural stiffness or crashworthiness, unibody construction offers, on average, 10% to 15% less mass per vehicle than BOF, says Jeff Dieffenbach of IBIS Assoc., a technology consultancy in Wellesley, Mass.

Fritz agrees that extra weight is a BOF penalty; the M-Class is perhaps 50 to 90 pounds heftier than it might have been as a unibody, he says. "But that's insignificant compared to the frame's advantages" -- durability, NVH isolation, incorporating the front and rear crossmembers into the bumper and tow hook system, and using crossmembers as subframes for the independent suspension.

Compare the curb weights of four 1998 SUVs:

Body/Vehicle            Chassis       Curb Weight
                                         (lb.)
Ford Explorer           BOF              4,146
Nissan Pathfinder       Unibody          3,945
Mercedes ML20           BOF              4,200
Lexus RX300             Unibody          4,037

And like all Toyota vehicles, the RAV is saddled with the automaker's traditionally high vehicle development costs, calculated by Harbour and Associates to be over $1,250 more per vehicle than Chrysler's overall.