Maximizing Boxy Flex Aerodynamics Challenge to Ford Designers, Engineers

ALLEN PARK, MI – A little known battle at Ford Motor Co.’s leased wind-tunnel facility took place here not long ago, as designers of the new ’09 Flex fullsize cross/utility vehicle went head-to-head with aerodynamics engineers.

At issue was how to keep the boxy shape that made the concept version, dubbed the Fairlane and a big hit on the auto-show circuit, while also improving the production version’s aerodynamics to boost fuel economy.

“(The Flex) presented a big challenge,” Wayne Koester, aerodynamics engineer, says. “Everyone liked the style, the look of it. From an aerodynamic standpoint, that really limited what we could do. So through the whole development process, (we looked) at how we could improve the details of the basic shape.”

Ultimately, the engineers decided to alter the vehicle by “getting rid of some of the sweep of the front bumper,” he says, noting the rear corners around the roof header and also taillamps were “finessed” and the headlights squared.

The tweaks resulted in a more boxy design than the concept vehicle, but with an improved aerodynamic performance.

The Flex’s width and low-riding stance gave the team some room to play, as did its ride height, which is a major factor in a vehicle’s aerodynamic performance, Koester says. “The underbody creates a lot of drag, so the closer the body is to the ground, the more airflow goes over the top, creating less drag.”

Ford says Flex most aerodynamic vehicle in CUV lineup.

The team lowered the ride height nearly 1 in. (2.5 cm) for a ground clearance of 5 ins. (12.7 cm), which is the standard for most cars.

Flex chief designer Rich Gresens, who was laid off last week due to ongoing cost-cutting measures, says shaping the production version required a lot of give-and-take between the design and aerodynamics teams. “It’s that synergy between the two groups that makes things work.”

Nevertheless, making styling modifications often can be a bitter pill to swallow, especially when such changes threaten to significantly alter the fundamentals of a design.

“There are times when it’s discouraging,” Gresens says. “Everyone wants a good-looking car that also has the best drag coefficient. (Some changes) work fine with the aesthetics. (Other) times, I may have something that might look really wild, and they’ll say, ‘That’s going to take us down a mile a gallon.’”

Gresens refers to the Flex’s 2-box design as a “professional square,” a style of vehicle that is becoming more prevalent after being made popular by the Scion xB.

While its design appears to be the very opposite of an aerodynamic shape, Koester says the Flex is the most aerodynamic vehicle in Ford’s CUV lineup, besting the Edge and Escape. It also boasts better aerodynamics than most of its competitors, he adds.

“The coefficient of drag for this vehicle is 0.335, which is very good relative to the competition,” he says. “A competitive vehicle in this segment is around 0.375. That 0.040 difference equates to (better than) 1 mpg (0.4 km/L) at highway speeds, with everything being equal.”

Crafting an aerodynamic vehicle is not as simple as it might seem, Koester says. “You really have to look at the details and how the air is flowing around (the vehicle).

“Some Ferraris and Lamborghinis are not that aerodynamic. Even the Ford GT was not a low-drag car,” he says, noting exotics often are designed with less-than-ideal aerodynamics in order to increase the amount of downforce, which keeps high-speed vehicles from becoming airborne.

Gresens says design compromises are necessary in today’s fuel-economy minded market. “You can get the design you want, but you have to do a bit more, have to go that extra mile to maintain what you want aesthetically,” he says.

“There are things you have to change that you may not have changed in the past. But that’s where I look at it as a challenge. We have a different set of criteria now, and you have to get the most out of it.”

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