Bottoms up!

Decades after their European competitors turned aerodynamics upside-down by employing “belly pans,” Big Three automakers admit they may have underestimated underbodies.

“If you look at their designs, if you look at the actions they have in place underneath the car, the management of airflow, it's obvious that, in the early phases of the program, it was an important design criteria,” says an engineer with one of the U.S. Big Three automakers.

So accustomed to being on top of the industry, Detroit apparently never looked down, concentrating on shapes that channeled airflow over the bodywork, instead of what lies beneath — a subject being pondered now as the specter of tighter fuel economy standards looms over Washington.

Meanwhile, automakers such as BMW AG were redefining airflow from the ground up. Underbodies have been a priority with the Munich-based automaker since 1980. That's when BMW acquired its first wind tunnel and began development of the 1986 7-Series — code-named E32.

Today, underbodies rank second — behind rear ends — on BMW's aerodynamics checklist, says company spokesman Wieland Bruch. Wheel housings and cooling airflow are third and fourth, respectively.

Such structured protocol helps explain why BMW's X5 sport/utility vehicle boasts a segment-leading drag coefficient of 0.35 — just 0.03 more blunt than the lower, sleeker Lincoln LS.

And, Mr. Bruch warns, expect an ill wind to blow on BMW's competition this month in Frankfurt. The automaker is unveiling “a huge step forward” in underbody technology with the debut of its new 7-Series.

“It's an orchestrated/comprehensive approach to optimize underbody airflow, wheel housing resistance, downforce on front and rear,” Mr. Bruch claims, withholding all details except the new 7's 0.29 drag coefficient.

With a nod to BMW, many other automakers clearly understand the importance of underbody design. For Lexus, it seems to be a breeze because the drag coefficients of its vehicles rank at or near the top of every segment in which it competes.

(However, success may have gone to the heads of its marketing department. A source tells WAW there were plans to claim the new LS 430 bests all production passenger cars with its 0.26 drag coefficient. Lexus lawyers correctly advised restraint — GM's EV1 boasts a 0.20.)

So why does Detroit seem to spit into the wind when it comes to underbody treatments as seemingly simple as belly pans? It all comes down to economics, says Roger Schulze, leader of DaimlerChrysler's newly christened aerothermal center of competence:

“This technique has been known in the industry at least 20 years,” he says. “But we've not been able to make a business case for the cost and weight.”

Don't rule out seeing belly pans on a General Motors product, says Max Schenkel, the No. 1 automaker's technology integration engineer. But don't hold your breath, either.

“You revisit each case,” he says. “Some vehicles, like a luxury vehicle, you may choose to go that route. … Where there's absolutely no attention paid to that in the underbody area, you can get about 5% to 6% improvement in drag by treating the underbody.”

According to the industry's rough measure, that's a performance increase of 2 mpg. But belly pans are not a panacea, he warns, because underbody component alignment must happen first.

“You also want to get the airflow to meet up with the airflow that's coming over the top of the vehicle,” he says.

The bottom line: “If you go the full belly pan treatment, you really must do the entire platform first.”

A quicker fix is a front air dam, which shields many of the underbody's uneven surfaces, Mr. Schenkel says.

DaimlerChrysler is trying hard not to underwhelm with its underbodies.

“What we're doing now is we're using some computer modeling to look at what we can do,” says Mr. Schulze. “With some ingenuity we might be able to do some things for free. If we make a fuel tank more aerodynamic, maybe that's a free improvement.”

Meanwhile, Ford is striving for process improvement. One goal is to formalize the sometimes tense relationship between engineers and designers, says Hau Thai-Tang, the No. 2 automaker's chief engineer of North America Car.

But it seems the winds of change are blowing particularly strong in another area. “We also look at clever things like A-pillar treatment … and how we manage airflow underneath the car,” he says.