airflow question

[AMGalltheway]

hi everybody i am new but anywho on to my qeustion would it be possible to have rims that act as fans and suck air out from under the car and through the engine bay thus cooling engine and brakes or is this been done is impossible or what any help is appreciated thanks

[curtis73]

Many wheels create a gentle suction but even if you had wheels that were set up to efficiently move air they probably wouldn't counteract the aerodynamic forces already at work. Underneath the car is an area of lower pressure. How strong depends entirely on the car. I don't think even really big wheels would counteract the tendency for air to rush in.

[2.2 Straight six]

Originally Posted by AMGalltheway

hi everybody i am new but anywho on to my qeustion would it be possible to have rims that act as fans and suck air out from under the car and through the engine bay thus cooling engine and brakes or is this been done is impossible or what any help is appreciated thanks

yes, the McClaren SLR uses wheels that flow air over the brakes to aid their cooling. they look like fans.

[AMGalltheway]

doesnt the shape of the car , like a wing , encourage high pressure under the car? and then the wheels i propose , pulling air away from the under body, only help encourage high to low pressure air flow?

[Alastor187]

Originally Posted by 2.2 Straight six

yes, the McClaren SLR uses wheels that flow air over the brakes to aid their cooling. they look like fans.

The webbing on that wheel is about as useful a fan, as some panties on Brittney Spears’ closet floor.

Originally Posted by AMGalltheway

doesnt the shape of the car , like a wing , encourage high pressure under the car? and then the wheels i propose , pulling air away from the under body, only help encourage high to low pressure air flow?

Semitics, but typically the shape of the car encourages a low pressure over the car. The net result is a higher pressure under the car with respect to the top.

It is definitely possible to create air flow with the wheels, but I do not see it being very effective. Both in terms of performance and cost, suspension and body modifications will provide much better results for cooling and downforce.

[curtis73]

We're backwards... cars naturally create low pressure on top since the air going over it has to go faster than the air underneath. Wings redirect air upward at the rear creating downforce for traction. The same thing works for front air dams. By foiling the air it helps create lower pressure under the car which further aids in high speed stability.

[AMGalltheway]

wait so air effects such as spoilers and front air dams create low pressure? im confused???

[curtis73]

Yes... UNDER the car. They create high pressure elsewhere.

If you think about Bernouli's principle, the faster moving air is lower pressure. So, just like an airplane wing, a car has a greater distance to travel on top of the car than the bottom. It tends to act like a wing which creates lift in the upward direction; lower pressure on top, higher pressure underneath the car. On cheaper cars (like my wife's Tercel) you can feel it at about 90 mph. The steering gets really light and floaty. So, the purpose of things like spoiliers is to alter the airflow and "spoil" that effect.

Front air dams limit the flow of air under the car forcing it around which lowers pressure under the car. Therefore as you accelerate, there is more suction under the car helping stability. But, just like in an airplane, more lift (either up or down) means more drag which limits top speed. On high performance cars, the secret is a balance: enough downforce to maintain stability, but not so much that it limits top speed too much. Pretty much any car above a Tercel (picking on my wife's car ) will have spent considerable amount of time in a wind tunnel and the balance will have already been struck between body mods for downforce and slippery aerodynamics for efficiency.

its also important to mention that all of these body modifications only start taking effect at about 80-90 mph. Up to any normal sane speeds you won't notice the difference.

Did you see the movie "driven" about Indy racing? the scene where they are on the streets driving an indy car, the car drives over a manhole cover and lifts it sending it flying. While the scene is totally implausible at the speeds they were going, at 200 MPH, an indy car is making enough suction under the car to probably do that.

[Alastor187]

Originally Posted by curtis73

If you think about Bernouli's principle, the faster moving air is lower pressure. So, just like an airplane wing, a car has a greater distance to travel on top of the car than the bottom.

This explanation of lift is falling out of favor, when you have some time you should read this:

http://en.wikipedia.org/wiki/Lift_%2...misconceptions

[AMGalltheway]

okay so a proper balance of "aero mods" would prety much negate any effect the "fan rims" would create well that pretty much answeres it 4 me thanks for all your help

[KiwiBacon]

Originally Posted by Alastor187

This explanation of lift is falling out of favor, when you have some time you should read this:

http://en.wikipedia.org/wiki/Lift_%2...misconceptions

Please don't quote wikipedia as a technical reference.

[KiwiBacon]

Originally Posted by AMGalltheway

okay so a proper balance of "aero mods" would prety much negate any effect the "fan rims" would create well that pretty much answeres it 4 me thanks for all your help

There was a very good thread on the eng-tips website about fitting fans to wheels for cooling brakes.
The upshot (opinions from engineers on race teams) was that it works very well but often isn't necessary.

[Alastor187]

Originally Posted by KiwiBacon

Please don't quote wikipedia as a technical reference.

I never stated it was a technical website, not that it would matter one should be critical of any source. Nevertheless, one should also be open to new information and should evaluate the validity based on content not source.

If you don't want to read about an alternative explanation on Wikipedia then try one of these:

http://amasci.com/wing/airfoil.html
http://www.av8n.com/how/htm/airfoils.html
http://www.regenpress.com/

Or Google until you find an acceptable source.

[KiwiBacon]

Originally Posted by Alastor187

I never stated it was a technical website, not that it would matter one should be critical of any source. Nevertheless, one should also be open to new information and should evaluate the validity based on content not source.

If you don't want to read about an alternative explanation on Wikipedia then try one of these:

http://amasci.com/wing/airfoil.html
http://www.av8n.com/how/htm/airfoils.html
http://www.regenpress.com/

Or Google until you find an acceptable source.

Those articles are all discussing the fundamentals of how an aerofoil works. None of them are arguing the fact that conventional wing shapes work well at creating lift.

The problem with cars is they are shaped like a wing. Without design features to counteract it, they produce lift at high speeds.

[Alastor187]

Originally Posted by KiwiBacon

The problem with cars is they are shaped like a wing. Without design features to counteract it, they produce lift at high speeds.

The only similarity between a car and airfoil, as suggested in post 8, is that both have longer 'path' lengths along the upper surface.

My point was that this is not a criterion characteristic for developing lift, despite the fact the most common explanation of airfoils suggests the opposite.

A flat plate for example can create a significant amount of lift even though the top and bottoms surface path lengths are equal.

To digress even more off-topic, a car really isn’t shaped like a wing it is shaped like a car. Not to say the shape won’t create lift, but in my opinion it is often a misnomer to call it a wing just because it creates lift and has the airfoil similarity already mentioned.