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In Formula 1 they have the Drag Reduction System (DRS). This reduces the drag on the straights to optimise speed and increases the downforce in the corners to increase handling. This makes sense to me.

I sometimes find myself following a posh car, usually a porsche, that has a retractable spoiler. These generally seem to have the opposite action, in that when the car is cruising on the motorway the spoiler will be up, presumably increasing drag and reducing fuel efficiency, and when the car turns off the motorway, slows down and approaches a roundabout the spoiler drops, presumably reducing downforce and roadholding.

What is the logic behind these systems?

User65535
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In a way you have answered your own question.

The DRS wing is there to keep there vehicle planted on the road to allow the vehicle to maneuver bends at high speed. At certain straight 'safe' locations, the wing can be changed to lower the down force and increase speed. If the vehicle was travelling very slowly (for a race car) there would be no need for the wing, since it wouldn't need the extra traction.

The spoiler on a Porsche is not needed at slow speeds since the tyres are providing enough traction and there isn't a large amount of turbulence at the rear of the vehicle that can cause drag, so the spoiler can drop to keep the familiar shape of the Porsche (see wikipedia where the 911 is mentioned). When the vehicle is moving fast, turbulence increases, so the spoiler raises to remove some of turbulence, which decreases fuel consumption and increases stability.

The Porsche (for example) spoiler doesn't increase drag, it actually lowers drag by changing the way the air flows over the rear of the vehicle, so reducing turbulence and drag. Not all spoilers are functional however, many are just for cosmetics.

HandyHowie
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