I've noticed some posts about some front spoilers with "canards" incorporated therein, which add downforce to the front at speed. Also, there are of course rear spoilers that add downforce to the rear at speed.

Here's my question. Why does the car need this downforce at speed? Somebody told me that, at speed, the air pressure under the car is greater than that in the ambient atmosphere, thereby "lifting" the car. Is this true? If so, I'd like to know. If not, I'm curious about why our cars need more downforce at speed.

Thanks,
Richard

 

technically, a car is much like an airplane wing. Flat on the bottom, rounded on the top. It takes the same amount of air longer to travel over the top of the car than it does under it. According to the equalization of gas principle, the air that goes over the car must travel a longer distance in the same amount of time (thus making it go faster). This creates a low pressure area over the car, and a high pressure under the car. Canards and rear diffusers are meant to "diffuse" the low pressure area and to produce downforce, which cancels the low pressure.

Google "Bernoulli's Principle." (I think that is how you spell it). It is the same principle that keeps a plane in the air.

John

 

John:

Good explanation!

Now a second question. I understand that the airflow over the curved surface on the top side of the car produces "lift" as in an airplane wing. But does also the air pressure beneath the car increase over ambient air pressure at speed?

Thanks,
Richard

 

It you have a front diffuser, no. it diverts air from UNDER the car, thus creating sort of a vacuum effect. This is why the Ford GT has a COMPLETELY smooth under carriage, with front AND rear diffusers, the engineers wanted to get the air to go around the car, instead of under it. The more air under the car, the greater the pressure. Ford wanted to keep air from going under the car. This is why the Saleen S7 can, technically, drive upside down at speeds greater than 150.....it creates more downforce that the car weighs.

John

 

indy cars have an inverted bubble underneath their body, creating a vacuum. i once read at 60 mph an indy car could technically drive upside down.

 

That sounds about right.....there was a HUGE controversy in INDY in the 50's because (I think it was) Lotus who entered a car that had 2 engines....1 for driving the car, and 1 to power a fan under the car that "sucked" the car to the ground. Won by a TON!

John

 

Yes, all that air that goes through your radiator (that's a really big hole)? Guess where it goes? It goes under the car. Corvettes have put a gill on each side of the car just behind the front wheels to let some of that air out rather than letting most of it go under and out the sides or all the way to the back.
Of course, if you have a big gap in our hood edges or a poor fitting hood, some of the air (and the pressure) will go there.

 

I am not sure about Lotus doing this. I do know that Chaparral was the first to try this technique. They used a snowmobile engine to suck the car down...hence the nickname "sucker-car".

Chaparral

Indy cars use an underbody venturi, as do formula cars and all other modern race cars. This venturi speeds up the air flow under the car, creating a low pressure region. Essentially, when air is forced to travel faster (over a car, through a venturi, etc.) the pressure drops, creating a low pressure region on the respective surface. Indy cars cannot drive upside down at 60 mph. Drag, lift, down forces do not really come into play until 80 or 90 mph. These forces are not linearly proportional to speed either. Pressure forces triple with speed. there is three times as much drag/lift/downforce on a car at 100 mph compared to 50 mph.

The smooth underbody is to keep the air moving as fast as possible. As soon as the air slows down, the pressure increases. The front diffuser, among other things, is mainly to reduce the amount of air that travels under the car. The rear diffuser, among other things, is mainly to provide the air with a clean exit from under the car, minimizing drag. This slows the air down and reduces turbulence-induced drag. You could try to drive the Saleen upside down at 150 mph, but you couldn't pay me enough money to stand underneath you. The race version has SIGNIFICANTLY more downforce through aero work than the street version. The street version will never achieve 100 percent downforce (and I doubt the race version comes close as well).

Not trying to pick you guys apart, but I do this for a living


Bernoulli is the man.

 

Um, most every drag formula I've ever seen is Cd*v^2 (times the air density, if you want force). I.e, drag is proportional to the square of the speed. So there's four times the drag at 100 compared to 50 (and whether it's miles or kilometers doesn't matter).

Now, this is force, not energy & power. The power required to overcome the force and keep a vehicle moving at speed is proportional to the cube of the velocity (~v^3). This is why those NASCAR things run >600HP but only go 200 or so. HPH

 

No they don't. They go with the square of speed. And the horsepower required to push the car through the air goes as drag times speed, (ie. the cube of speed).