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I've been researching the physics involved with spoilers and I've just been turning up the same information about spoilers giving the back wheels of a car more traction by acting as an up side down airfoil.

So my question is, what exactly causes the car to apparently gain lift while moving quickly, so as to merit the use of a spoiler, and is this entirely due to the aerodynamics of the vehicle in question?

Edit: Clarity of question.

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I'm just winging it (so to speak), but such cars have front skirts to prevent air squeezing in under the car. The inverted wings press downward. They don't have to be counteracting any lift - the downward force is welcome even if there is no lift to be counteracted, as it gives the wheels more grip on the road. –  Mike Dunlavey Dec 21 '12 at 20:51
    
This all stems from an argument I'm been having with a friend, he believes that the vertical, downward effects of gravity lessen when an object is going at speed, so essentially he believes the car would loose weight. Does this sound as crazy to you as it does me? –  Matthew Morgan Dec 21 '12 at 20:58
    
@MatthewMorgan Yes, that sounds crazy. There are more upward forces apparently, but the gravity for on the car itself does not change. –  Bernhard Dec 21 '12 at 21:11
    
I see, I thought I must have been going mad with the ferocious way he's been defending his position, sadly I can't get him to budge an inch as he just gets remarkably angry that I would double his knowledge. What exactly would then cause these upward forces? –  Matthew Morgan Dec 21 '12 at 21:18
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Mathew, was this the question that you meant when you asked Speed and loss of weight? If so, then I made a mistake in closing it and owe you an appology. For future reference, you can tell ask us to change our minds: if you had written me a comment stating that you question was about cars we could have figured out the nature of the misunderstanding, editted the earlier version into a shape and re-opened it. –  dmckee Dec 22 '12 at 3:08

2 Answers 2

up vote 3 down vote accepted

It's because the shape of the car uses Bernoulli's principle: the difference of pressure between the air above and below the car causes a net upwards vertical force which increases when the car goes faster.

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this is partially correct. if you were to flip the car upside down and make it go at high enough speed it will still experience lift. the pressure on the bottom side is higher because the air flow is bounded by the road, so the velocity of tends to be lower. –  mythealias Dec 21 '12 at 22:52
    
Thanks you both very much for explaining this to me, I'll look into Bernoulli's principle further. –  Matthew Morgan Dec 21 '12 at 23:09
    
@mythealias The flow being bounded by the road, and viscous, actually can suck the car to the ground. It's not at all accurate to say that lift is generated or that the pressure under is higher than the pressure over the top. Remember: air accelerates as it is confined (Venturi effect), and pressure drops as it accelerates (Bernoulli). –  tpg2114 Dec 27 '12 at 7:20

The design of the car decides many things. For example, it is entirely possible (and desirable) to actually generate downforce just by going faster. This is called ground effect and in essence works by making the car low enough that air goes faster under the car and not over it, so Bernoulli's principle actually sucks the car to the ground rather than lifts it up. Some race cars in the 1960's and 70's actually added fans to the underside of the car to increase this effect. My personal favorite is the Chaparral 2J which had two engines, one to power the car and the other to power the fan. It's unfortunate they had a tendency to catch fire...

The overarching goal from generating downforce is to increase grip on the road. Of course, this comes at the cost of increased friction and aerodynamic drag. In fact, to improve overtaking in Formula 1 races, they introduced a rule in 2011 that allowed an adjustable rear wing so on straights the driver could lower the downforce generated by the rear wing to improve speed. But in turns, the wing would generate more downforce to keep the car stable. Also noted in the article is the balance in tire wear by adding force to the front or rear by a spoiler.

Another reason downforce is desirable is car stability. For a front engine car at low speeds, it is unlikely to exceed gripping strength of the tires. But at high speed, a front engine car will tend to over-steer because the back wheels will have less weight on them and tend to slip before the front. So in that case, a spoiler adds downward force on the rear tires and changes the steering characteristics. The same is true for front spoilers on mid- or rear-engine cars.

So, it is unlikely that cars actually generate lift when going faster. Very few will actually do that, and high speed cars actually tend to generate downforce just by the act of going faster. Spoilers, or wings, are added to improve certain characteristics of the vehicle, namely: tire wear, cornering and top speed.

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The reason cars in general don't generate lift is actually the result of careful design. Certain cars - I believe the original Ford Capri was one - were notorious for becoming light at the front when driven fast. This was due to the symmetrically tapered, somewhat bullet-shaped front of the car forcing air underneath; later models featured a rudimentary air-dam which prevented this effect. –  user20843 Feb 11 '13 at 18:07

protected by Qmechanic Feb 11 '13 at 19:48

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