airflow question

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

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.

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.

http://static.flickr.com/24/43613074_5632c1fc88_m.jpg

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?

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. :lol:


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.

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.

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

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.

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_%28force%29#Common_misconceptions

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 :)

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

http://en.wikipedia.org/wiki/Lift_%28force%29#Common_misconceptions

Please don't quote wikipedia as a technical reference.:banghead:

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.

Please don't quote wikipedia as a technical reference.:banghead:

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.

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.

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.

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.

The similarities between a car and an aerofoil include a curved top surface and a large flat area underneath.
This is consistent with both the traditional explanation of lift (faster air = lower pressure) and the alternative lift mechanism described in your links (reaction of downward accelerating airstream).

A flat plate can creat lift, but in doing so it creates a large amount of turbulence. This leads to a large amount of drag and makes it a particularly inefficient device for creating lift (or downforce).

Design features used in the automotive world to reduce lift (and create downforce) include air dams to restrict airflow under the car, external aerofoils (upside down wings) and rear diffusers (which are again an upside down wing).

This is consistent with both the traditional explanation of lift (faster air = lower pressure) and the alternative lift mechanism described in your links (reaction of downward accelerating airstream).

Just for clarification since it sounds like you took the time to read the links.

The alternative explanation(s) involves the downward acceleration of air, and thus respects Newton’s Third Law. It also still respects Bernoulli’s principle with one subtle but key difference. The change in air velocity doesn’t change the static pressure; the change in static pressure changes the air velocity.


A flat plate can creat lift, but in doing so it creates a large amount of turbulence. This leads to a large amount of drag and makes it a particularly inefficient device for creating lift (or downforce).

I agree, and the same is true for most ground vehicles. This is why I think it often a misnomer to say a car is wing shaped. Because people are thinking since it has a curved upper surface and flat lower surface it behaves similarly to a airfoil. However, an airfoil is a highly specialized device that maximizes lift and minimizes drag.

Just for clarification since it sounds like you took the time to read the links.
The alternative explanation(s) involves the downward acceleration of air, and thus respects Newton’s Third Law. It also still respects Bernoulli’s principle with one subtle but key difference. The change in air velocity doesn’t change the static pressure; the change in static pressure changes the air velocity.


I'm having trouble agreeing with that theory.

Applying it to an accelerating flow can make sense, as you can visualise the higher pressure upstream causing the acceleration. But applying it across a diffuser doesn't seem logical.

not sure if this is dead or not yet, but the only way a flat plane will create lift is if it is put at an angle to the direction of the air rushing past it.
its the same effect as when you stick your hand out the window of a moving car.
the lift created is because there is a massive low pressure area right about where your knuckles are (on your hand that is) and a high pressure area underneath your knuckles.
this design works well at low speed, but due to the ammount of turbulence created, an airfoil is required at any higher speed than a paper airplane would travel.

if you look at an airfoil, the bottom is identical to the flat plane. the top, however, is specifically designed to minmixe turbulence, which creates drag.


i think that even if there were wheels that were designed as an impeller to suck air from beneath the car, the brake disks get in the way of any serious airflow, and the sheer ammount of air coming in from other places negates any effect the wheel would have anyway.

the only way it could have much of an effect is on oa car with low ground clearance, tight side skirts, a massive front spoiler, and inboard brakes.
basically, a GTR racer.

the Mercedes benz CLK DTM AMG is a good example of this on a road car.

well this is turning into an argument but thanks every body for helping

haha, sorry