Automotive designers face three certainties in life: death, taxes and physics. The latter may be why more and more vehicles on the road today bear a striking resemblance to one another.
Designers can dream big, but when it comes time for a splashy concept car to transition to production, “we all have to abide by the same laws of physics. It doesn’t matter if we don’t like them,” says Nina Tortosa, General Motors aerodynamicist for the auto maker’s Voltec/E-Flex programs.
Aerodynamics long has played a role in increasing vehicle performance and fuel efficiency. But its role has taken on new importance as looming federal mileage requirements push auto makers to find more ways to stretch a gallon of gasoline further.
Most auto makers set a coefficient of drag (Cd) target early in a product-development program to guide engineers and designers toward that goal.
Princeton University defines aerodynamics as “the ratio of the drag on a body moving through air to the product of the velocity and the surface area of the body.” The lower the better.
The Chevrolet Volt extended-range electric vehicle serves as a prime example of how fluid dynamics ultimately determine the finished shape of a vehicle. With a Cd of 0.28, the Volt’s design offers less air resistance than any sedan in Chevrolet’s history, GM says.
But the car now plying U.S. roads looks remarkably different than the concept that bowed to great fanfare at the 2007 North American International Auto Show in Detroit. When it came time to ready Volt for production, Tortosa says many changes had to be made, noting the concept was just that.
“The initial changes from concept to production were matters of the platform,” she says. “The concept looked rear-wheel drive and the production version is front-wheel drive, so the proportions changed.”
The biggest design adjustment was made to the back of the car. The concept had a rounded rear end with a short overhang – not an ideal aerodynamic shape. Sharper edges were added to the production version, which helped control air separation points at the back of the vehicle.
“If you can control separation, you can minimize drag,” she says.
While Tortosa didn’t borrow many cues from GM’s groundbreaking EV1 electric vehicle, which was built from 1996-1999, she did sign out one of the cars for the night just for inspiration. The EV1 boasts a staggering 0.195 Cd rating.
“There are some aero enablers on the EV1, but that was a 2-seater and (the) Volt seats four, so the proportions were going to be different,” she says.
Tortosa is quick to point out designers and aerodynamicists do not have adversarial relationships. “We allow them as much flexibility as the program team allows.”
However, meeting Cd targets is crucial, especially on vehicles such as the Volt, whose air-flow design goes a long way in extending its electric-only range, she says.
But the relationships between designers and aerodynamicists have not always been rosy.
Joe Dehner, chief of Dodge and Ram Design at Chrysler, says there was a point during his 22-year career tempers ran high when it came time to altering a design to satisfy aerodynamic requirements.
“We, as designers in the late ’80s and early ’90s, were in an organic phase, but (aerodynamicists) didn’t want organic lines,” he says. “We would take it to the wind tunnel and they would put corners on (it), and (we) would say, ‘You’re ruining my design.’”
Dehner’s view of aerodynamicists and their work today is decidedly different. In fact, he says designers who argue aero requirements stifle their creativity “aren’t trying hard enough.”
He cites the ’11 Dodge Charger and Chrysler 300 as examples of how good design and aerodynamics can go hand in hand. Although both models share Chrysler’s LX platform, they boast similar aero numbers yet are styled differently to reflect their respective brands.
Achieving today’s aerodynamic requirements is more difficult than in the past, Dehner admits. “The envelope we work within is very small.” Where once it was inches, “today, it’s millimeters.”
The best way to reach stated goals is to work cooperatively with aerodynamicists, he says. For example, early designs of the Charger fell just shy of meeting its aero target in early testing. Aerodynamicists said the rear tires were exposed to too much air and suggested pulling out the rear sill to shield the area.
Dehner was reluctant to make the changes. But once he did, the car not only hit its Cd goal but the styling also was better.
The design change gave “a Coke-bottle concave shape to the side of the car,” he says. “Now we’re promoting that Coke-bottle theme, and it wasn’t originally intended.”
Still, it’s important designers don’t kowtow too much to aerodynamicists, some say.
Melvin Betancourt, Ford’s exterior-design manager, says if vehicle designs religiously adhere to the laws of physics “then you will get cars that start looking all the same.
“If it hinders design, I say absolutely not,” he says. In the case of the ’11 Explorer, “we kept working with aerodynamicists to see what (options) were readily available to us and what quick changes we could make to bring the drag down.”
Reducing drag on larger vehicles is difficult because there is more surface area, designers say. In the case of the ’11 Explorer, meeting its Cd target came down to a minor addition that just happened to make the difference in achieving the desired airflow.
“If you look at the spoiler, there’s a little lip you would almost say is nonexistent,” Betancourt says. “But it took air that would normally coil over and pushed it away from the rear of the vehicle to take the drag out.”
The trick has been used before on SUVs, he admits, noting that’s the reason why the rear glass on utility vehicles accumulates dirt. But it’s a small price to pay to achieve aero goals while maintaining the integrity of the original design.
Aerodynamics has become so integral to vehicle design because it is the most cost-effective solution to improving fuel economy and mitigating wind noise.
But it’s not just sheetmetal that contributes to air flow. Other methods that affect fluid dynamics resistance include the addition of underbody “belly” shields to deflect air below the vehicle, and active-grille shutters that block air from entering the engine bay under certain driving conditions.
Tying all these approaches into the most cost-efficient solution is the job of the performance integration engineer.
Larry Laws serves in that role at GM and partnered with Tortosa on the Volt project. His job was to ensure the car achieved the required 40-mile (64-km) electric-only range. He had a laundry list of areas that stood between him and his goal because they create friction, sap energy and reduce mileage.
“You have tire, brake and transmission losses; motor losses; aerodynamic and parasitic losses, such as headlights and radio,” he says. “You have to set targets for all of those parameters.” It took 29 months to get the Volt on the road, “and we met nearly every day with all the different functional areas.”
A quick cruise through automotive history proves aerodynamic vehicles can be produced without the high-tech tools used by today’s aerodynamicists. Examples include the ’75 AMC Pacer with a Cd rating of 0.32 and the ’47 Saab 92 with a Cd of 0.30.
Larry Erickson, chairman of the Detroit-based College for Creative Studies Transportation Design Dept. and a former Ford designer, says he stresses to students that design needn’t be sacrificed in the name of aerodynamics.
Erickson describes the optimal aerodynamic design as defined by physics as resembling a Toyota Prius, with wheels that are flush with the body and a hard vertical edge at the rear. Sometimes, one or two such aerodynamic elements can be blended in a way that results in a truly unique style, but he admits designs that stay true to physics tend to resemble one another.
Erickson cites the Opel Calibra, built from 1989-1997, as an example of a vehicle that blended physics and a sound design philosophy into a striking car. “The Cd on it is awesome, in the high 0.20 range, while most cars on the road today are around 0.30,” he says.
“That car had a killer drag number and looked great.”
Erickson’s real-world design background, combined with his tenure at CCS, provides him with deep insight into the issues designers face when confronted with aerodynamics requirements.
His advice to students: “Good teams work together to come up with a solution that looks good and is aerodynamic.”