imported_MY SiR

Hey,

I was thinking of posting some possibly interesting information. Mods, if this should be in a different section, feel free to move it.

I am thinking of writing on a series of things related to the automobile. This time, I figure I will discuss aerodynamics in general.

If anyone has any inputs/additions/corrections, feel free to add them to this thread.

To start: (Simplified) Background to Aerodynamics.

What is aerodynamics? It is the ease of which fluid (air is a gas, which is considered a 'fluid' in Engineering terms). passes over something.

It is usually measured in Cd (Co-efficient of drag). The smaller the Cd, the better the aerodynamics.

Many have heard of Cd but that is not the complete measurement of how smooth a vehicle cuts the air.

Frontal Area is also a factor and is not part of the Cd number.

So, an aerodynamically shaped SUV actually could be subjected to more air drag than a brick shaped compact car.

Significance of aerodynamics to fuel economy:
At highway speeds (80+ kph), it is more efficient to drive with your A/C on than with your windows opened. Opening your windows has a significant effect to aerodynamics.

Again, at highway speeds, the aerodynamics of the vehicle is more of a factor than weight. So, a Ford Crown Victoria is more fuel efficient than many Minivans and SUVs (even some 4 cylinder ones) at highway speed.

At high speeds, the aerodynamic drag exponentially increases so, if you are comparing speeds of say, 80 kph and 160 kph, at 160 kph, your aerodynamic drag is 4 times what it was at 80. Your fuel economy will also drastically be worse (but there are other factors affecting fuel economy as well).

Wider tires makes aerodynamics worse. If you ever see top speed cars, their always have skinny tires. Why do sports cars have wide tires? Cornering is improved.

F-1 cars have actually 'bad' aerodynamics. The 'wings' on them are used to increase downforce and actually increase Cd and frontal area but it increases cornering speeds, not everything is straight line speeds. If you look at cars in IRL at Indy, they have really small wings...

I think this is it for now...

shlammed

Nice info Mark. I guess this can fall under general automotive talk so we can leave it here.

bbarbulo

iTrader: (2)

air management is very important, but unfortunately, besides leaving the body stock, there isn't much we can do ourselves to improve aerodynamics. Actually when I put my OEM front lip on, I definitely noticed a great increase in high speed stability... having less air go under the body reverses the airplane wing effect, and should create a vaccum under the car to make it more stable. or something like that... I forget exactly how it goes hopefully Mark will correct me if I'm wrong.

imported_kane2k

A few things:

Aerodynamics is the study of the motion of gas, usually air. Fluid dynamics is the study of the motion of fluids. The aerodynamics that you're talking about is drag reduction.

Cd is a coefficient in an equation and is a useless number without knowing what that equation is. The reason being that the drag and lift equation is dependant on a reference length. What's a reference length you ask, it's whatever the person working with the equation wants it to be. You could come up with a Cd of .6 or .2 for a Porsche depending on how you defined your equation so that number is insignificant unless you know the methodology behind obtaining it. Also, you can make frontal area part of the Cd if you make the reference length a function of frontal area, so again, this is completely arbitrary stuff without knowing the methodology.

Opening your windows and killing your drag numbers is completely dependant on the specific case. If you're travelling at 140 km/h and you have the window open the air may not be able to turn the corner of your windshield fast enough for it to come into contact with the open window and so, aerodynamically speaking, it wouldn't matter. The same would be true if the car was designed to deflect air away from the opening. There are also boundary layer effects at higher speeds which would also affect this.

The word that you're looking for in your relation of speed and drag is exponentially, not logarithmically. Drag is proportional to the square of speed.

Nova_Dust

This is the type of information we should have more on this board. Good stuff.

I for one am debating between speed and cornering. While speed is nice to have, cornering is important on circuit and track (and also the weight of one single wheel + air mass + tyre). Which is why, I am sort of deabting between 15x6.5 or 15x7 wheels.

As for the front downforce, will canards actually increase it significantly or it is just a bling factor?

Notorious

Great information!!

The Prophet

excellent info mark

bbarbulo

iTrader: (2)

for track 15x7 wheels Charles
as for canards... bl|ng I mean, how do you figure that about 5 sq inches of carbon fiber bolted to your bumper can help?? If anything, the amount of flex in your bumper alone would cause that thing to flap in the wind at high speeds, rendering the canard useless. at least so I think

imported_kane2k

If you routinely hit 200 km/h coming into a corner that you're going to take at 160 and your car has a slight front end instability, then canards will provide "significant" downforce. Other than that they just look silly. As bbarbulo pointed out too, you need to mount them firmly to take advantage of them, which most people don't do.

Also, wheel aerodynamics is important for open wheel race cars, not full bodied cars. The rolling resistance difference and the weight effects are far more pronounced than the drag produced by a wider wheel. Even at high speeds, i.e. higher than a Civic is going to hit on a track with turns, the overall drag will not change much.

Nova_Dust

Ya, I suspect I will need 15x7 wheels Bruno. I am pretty firm on that now. I mean I was, but I think little extra weight can be ignored.

Kane, how do you firmly attach the canards? On most cars, all I see are 3 L shape brackets. Unless they fabricate the bumper with canards on them already?

imported_MY SiR

As I wrote, gas is considered fluid. If I am wrong then they must have been teaching me the wrong thing in 3rd year fluid dynamics.

I am curious, what is the equation for Cd? Can you also quote your sources? I always thought Cd and frontal area were independent of each other and they were both needed to calculate total drag. At least that's what I remember but university was so long ago...

If the window opening was small, I would agree however, it is relatively large so, air would eventually find a way in (at least at speeds cars are capable at). On top of that, the air inside a vehicle is at a lower pressure so, air would also be 'sucked' in. With a deflector, that would cause more drag to start and then behind it, you would have vortecies that could possibly cause air not to enter the cabin but that deflector would have to be awfully big.

This I am in complete agreement with you. Exponentially is the word . My bad.

About wider tires, yes, they have far more impact on open wheel racers however, I don't think you can claim it is insignificant on regular vehicles. If it didn't manufacturers wouldn't go through the trouble of adding aerodynamic aids just in front of the wheels on regular passenger cars. (next time, crouch down and take a look in front of the front wheel on almost any new Honda/Acura and you will see what I am talking about).

BB: basically what you wrote is correct.

Nova: I would think canards can have an effect even at 100 kph. Will they help in cornering for auto crossing? Likely not.

Another piece of information.

Rear 'spoilers' are used in race cars to improve downforce over the rear driven wheels. On a Honda, they don't help if there is any downforce dialed in. It will just make the front end lighter and that is not what you want on a FWD car. All genuine Honda Accessory spoiler are designed to have zero lift and zero downforce. It is there strictly for looks.

zeeman

iTrader: (1)

I agree with Nova Dust we need more technical discussions on here. This is interesting stuff....keep it up guys.

imported_gatherer

on a side note since autocross was mentioned... there are very few autocross cars that have to consider downforce. I know only of one guy thats thinking about it .. but thats because he's done everything else... as for Carnards useless even at the track go to a solo 1 and see how many guys are using them.

for the most part the A mod cars in the States are the ones that concern themselves with downforce.

here's an article on the unbeatable Phantom car used in A mod in the USA nationals

the downforce this car creates is astronomical

http://www.napylon.com/Phantom.htm

imported_kane2k

Gas is a subset of fluids, just as aerodynamics is a subset of fluid dynamics, which is what my masters degree is in.

There is no equation for Cd, it is obtained experimentally or through numerical analysis. The defining formula is:

force = C * density * (V^2)/2 * reference area

You measure the force applied, the density, and the velocity and pick a reference area and then you swap the equation around and solve for C(lift or drag). The reference area can be the frontal area, but it doesn't have to be.


The air has no reason to seek the inside of the car. It gets pushed away from the side of the car by the presence of the windshield and has no reason to turn back. The air inside the vehicle is at a higher pressure than the air outside the window. The deflector would not necessarily increase drag nor would it have to be that big.

What we're talking about here is the effect of the tire size vs. the effect of the tire. Yes, the tire creates drag. No, a 7 inch wide tire does not create significantly more drag than a 6.5 inch tires. If you go back to the drag equation and use the frontal area of the tire, the reference area would grow by a factor of 7/6.5 = ~1.1, so the drag created by the tires would increase by a factor of 1.1. The thing is that the tires on a passenger car make up a small fraction of total drag. So you have a big number + a small number *1.1 ... which is not significantly different from just the big number + small number.

imported_kane2k

If they were connected solidly enough to the bumper and the bumper was fairly rigidly mounted to the frame then that would be good enough. You'd be looking, off the top of my head, at about 15lbs max of force each side so whatever would hold that weight dynamically would do.

Brett

I've been interested in building panals to fit under my car for some time now. My thinking is that if i smoothed out the area around the front suspension the air would flow better under the car. If you've seen the bottom of a Ferrari 360 then you know what im talking about. ..

Nova_Dust

Thanks Kane

Brett, you are talking about diffuser right? Flat panel below the body will make the car more stable and stay at the ground at higher speed. It can also help out the bottom of the chassis, instead of using a cross member, a whole sheet metal welded in will have a stronger effect. Depends on how thick the metal is.

Brett

yeah some kind of diffuser. I have Wings West bumoper on the front and the plastic covers for underneath don't come up to the edge of the bumper like stock. So i thought that if i could build a piece that went from the bumper to somewhere around the front suspension it would have a dramatic effect on the airflow under the car.

Nova_Dust

I think if you can tackle the back bumper first, that would be a good area to build a diffuser. The bottom of the car in general is pretty flat, not smooth though.

Create a diffuser that connects to the base of subframe (where the subframe meets the body) and tie to the bumper. It will be interesting I think.

shlammed

If wind has no reason to turn into a cars window when it's open, why does my head get pounded by it? and my hair flies everywhere?