By on January 8, 2009

Life’s a drag. If you’re an automobile, the faster you go, the more of a drag it becomes. As early as the 1920’s, engineers realized that a car’s shape was no less important than an airplane’s; it determined the the automobile’s aerodynamic efficiency, which has a major impact on its fuel efficiency. (Cars may not have been born from jets, but the same rules apply.) For mass motoring, decades of cheap gas made automotive aerodynamics more of an optional art class than a required science. Now, with government regulators demanding maximum fuel efficiency, aerodynamics are back in play, headed for the mainstream. Active aerodynamics are taking center stage.

Active aerodynamics uses moving surfaces or parts to change the aerodynamic behaviour of a vehicle. The most obvious example: the retractable spoilers found in a number of sports cars (e.g. Audi TT). Another recent, less famous but equally important application: BMW cars equipped with the Efficient Dynamics package. Air flaps at the front of the vehicle regulate air flow to the engine. If the ECU determines that engine cooling is not needed, the flaps close to reduce drag.

Other manufacturers are sure to copy Bimmer’s lead. At the same time, various carmakers are developing active aerodynamics that reduce drag at higher speeds. Saab has shown a proposal (don’t they always?) to improve a hatchback’s drag by using an extending rear parcel. Although the feature is an aesthetic nightmare,  the principle is sound.

This little black deflector on a Citroen C4 Picasso may also become an active surface. Reports indicate that the feature could be part of a rear light module including the mechanism for surface actuation. At the same time, automakers are experimenting with downforce– aerodynamic aids that “press” the car towards the pavement to increase road holding. High end automobile manufacturers have spent considerable effort modifying the underside of their vehicles to improve handling (e.g. the Ferrari F430). Actively manipulating these surfaces may yield some incremental benefits for overall fuel efficiency.

But incremental aerodynamic and, thus, fuel efficiency gains are all the rage. There’s no question that the quest for better high speed mileage will lead to more vehicles with active surfaces.

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20 Comments on “Auto-Future: Active Aerodynamics...”

  • avatar

    That is very interesting. I recently parked a Jeep Wrangler as a daily driver for a new Civic. It is amazing that these two vehicles weigh the same, and the Jeep gets 16 mpg (or as low as 14 in a strong headwind) and the other gets 33 mpg no matter if the wind is blowing or not.

    Well I just hope that all of those extra moving parts are going to be reliable as professional service sure is expensive these days. I could see a dash warning light coming on that the anti-drag modulator is not responding (lol). And to think we complain that today’s cars are harder to work on by home mechanics!

  • avatar

    I like technology like this. It’s like idle-stop: simple, easy to adopt to any number of vehicles and not terribly obtrusive.

    Another hope I have is that they’ll start lowering the floor of vehicles. There’s no reason for the majority of cars out there to have six, seven or eight inches of ground clearance when the most arduous terrain they’ll traverse is a speed bump. It cuts into passenger space and kills mileage—have a look at the useful interior space of low-floorers like the Mazda5 or Rondo versus the high-floor crossovers. Variable-height suspension (Veyron, Touareg) might be another trick in this arsenal: something that can hunker down at highway speed would probably do some good, while still allowing the “on stilts” feel in the city.

    So would covered wheel-wells. I miss those, too.

    Saab has shown a proposal (don’t they always?) to improve a hatchback’s drag by using an extending rear parcel. Although the feature is an aesthetic nightmare, the principle is sound.

    Hah! That was the beauty of the original Saab hatchbach. It was ugly, but it was functionally ugly. I was sad to see that design go away in the 2003 9-3. Incidentally, there’s a lot of the same aerodynamics in the Prius and new Insight as in the old 99/900.

    This is ugly, too, but if any can (or could, before GM drove it’s fanbase away) get away with it, Saab could.

  • avatar

    IMO all vehicles should have active suspension to lower the car waaaay down when the road surface is favorable. I think this is a big factor in drag reduction.

  • avatar

    The VW Corrado had an active spoiler that was pretty cool until it broke. Actually that’s true for the Corrado in general.

    I seem to remember the Lincoln MKVIII lowering itself at speed to reduce drag. There was a commercial demonstrating it as well.

  • avatar

    Doesn’t the Bugatti Veyron also have diffuser flaps that close along with a retractable spoiler? And isn’t it actuated by hydraulics that require their own radiator?

  • avatar

    Some guy named Jim Hall built Chaparral race cars in the 60’s that had active spoilers connected to the rear suspension, and even a quickly-banned wind machine that generated downforce by sucking air from under the car. More recently, Lamborghini put movable cooling air scoops on the Murcielago that extend at high speed, but also retract at extreme velocity to avoid being ripped off the car.

  • avatar
    Ralph SS

    Disclaimer: I am not an aerodynamcic engineer, so, correct if I’m wrong. =O)

    I seem to recall reading back in the day when we were discovering all this stuff during the first two gas crisis’.

    Aerodynamcis, as they apply to an automobile’s fuel efficiency are on a vector, relatively flat until you get up around 50 MPH. From that point on it becomes a more significant factor – less than that, not so much. So, that being said and all else being equal, aerodynamics will have little effect on city mileage. Perhaps a little more in rural (50mph) driving, but not enough to warrant active bits changing air flow. Which, again, correct me if I’m wrong makes this technology a non-player in the vast majority of miles racked up on the consumer owned automobile.


  • avatar

    I think alot can be done my simpling lower ride height and doing some under-tray work. I was sitting behind a Mazada CX-7 today in traffic and thought ‘man this vehicle would be 10 times better if it was 4″ closer to the ground’. I’d bet such a drop in ride height along with a smooth under-tray would yield another 2 mpg on the highway, especially for these brick-like trucks/SUVs. Activate suspension tied to the 4WD mode of such vehicles would be perfect.

    My wife’s Passat gets really good highway mileage and sure enough the quick-lube guys hate it because of all the under-tray plastic bits keeping the air flow clean and smooth. Looking under a modern vehicle, especially an SUV, its clearly an aerodynamic nightmare.

  • avatar
    Rev Junkie

    Active aero, like all other automotive innovations, will eventually trickle down to the most basic economy cars in about 10-15 years, less if the fuel effeciency gains are worth the extra cost. The first car that I recall had active aero was the Mitsubishi 3000GT, but it really came into play with the 253mph Veyron, and is trickling down to the next Porsche 911, and will hopefully be on entry-level sports cars like the Z4, Boxster, TT, etc. The BMW 5-series already has a flat belly pan, it may get active aero in its next iteration as well.

  • avatar

    The early 3000GT VR-4 had quite a few active aero features, but as far as I’m aware they were failure-prone and more of a gimmick than a useful enhancement to aerodynamics.

  • avatar

    Aerodynamcis, as they apply to an automobile’s fuel efficiency are on a vector, relatively flat until you get up around 50 MPH

    Try this little experiment:
    1. Bicycle as fast as you can, hunched over the bars, Tour de France style, wearing a good helmet and a close-fitting bike suit
    2. Sit up straight and try the same thing, wearing street clothes
    3. Strap a big piece of cardboard to the front of the bike, flat-side facing forward.

    Aerodynamics matter at any speed.

  • avatar

    I don’t see how this stuff makes a lot of sense at current fuel prices, so I suspect it will be a niche thing for a while.

  • avatar

    This article fails to mention the fact that active aerodynamics have been a niche thing that have been on some cars for many years. The Mitsubishi 3000GT had them, so did some Chapparel race cars and the Volkswagen Corrado.

  • avatar

    This is old hat. I remember seeing an old Gran Torino with rusty quarters (is there any other kind?). When it got up to speed, the front quarters, freed from their lower moorings, flapped like wings. I am sure it helped. Should’ve seen those suckers go!

  • avatar

    You all missed the point! It’s like Knight Rider (the original) where KITT goes into “super pursuit mode.” The future is gonna be sweet! 200 mph cars for everyone!

  • avatar

    Another recent, less famous but equally important application: BMW cars equipped with the Efficient Dynamics package. Air flaps at the front of the vehicle regulate air flow to the engine. If the ECU determines that engine cooling is not needed, the flaps close to reduce drag.

    Not terribly recent, and (apparently) even less famous…

    The same feature was on the 87-90 928. It was there to reduce drag. Kind of a PITA to keep it working, but it does work.

    Bimmer’s lead? Not to take away from the Roundel folks, but they were hardly first on that one.

  • avatar
    Greg Locock

    “aerodynamic efficiency, which has a major impact on its fuel efficiency.”

    Not at speeds that most (>50%) people drive at. Do the maths. If by ‘major’ you mean what most people would understand by that innaccurate term.

  • avatar

    Lots of people spend lots of time at 75mph. Plenty of drag at those speeds. Most of the engine’s horsepower are going towards countering drag even at lower speeds, so a reduction in either factor (coefficient of drag, or frontal area) is definitely significant. And in some cases it can lead to cooler-looking cars – the Civic Si coupe’s wing reduces drag by 3% even as it reduces lift.

  • avatar
    Greg Locock

    Well, let’s do the maths. Let’s say you improve your car from a very respectable 0.30 to a best in the world 0.26, based on a frontal area of 3 square metres.

    You will save roughly 3 hp at 75 mph.

    Does that sound like it would lead to a “major impact on fuel efficiency” ? No.

  • avatar


    Being an owner of an ’87 928, I can speak directly about this. It wasn’t so much they were a PITA to keep working, it’s that typically the fix was simple when they did break, but not really worth the money. The claimed improvement in aero was to bring the car’s cD down to .34 from, I believe .38. But nobody could ever really provide any proof that having the operational flaps would improve fuel economy. Plus, if they failed in the closed position, you really increased the odds of destroying the engine – cooling was not one of the highlights of the 928.

    So most people disabled them in the open position, and Porsche eliminated them in ’91 after a spate of customer cars came back with closed flaps and ruined engine blocks.

    Mine? The flap motor is dead, and one of the wires going back to the cooling ECU is shorted. So it’s permanently open.

    It seems recently more cars have these, typically some type of rear spoiler, and I think they have improved reliability. But I suspect a more effective item would be a full undertray. The 928 S4 and later had a partial undertray as well. It *was* a PITA to take off an on, and added, at least for me, an hour to any work under the engine. So it’s off now, and I haven’t noticed a change in fuel economy. Some say it’s mainly do direct cooling air to the hydraulic motor mounts, another item that tended to crap out. But I think a well designed full undertray could accomdate routine maintenance without adding significantly to billable work hours, really improve fuel economy, and not be susceptible to failure like active aerodynamics.

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