MrMiracle

Strangely enough much of the useful info I've learned about drag came from a book about electric cars. No.1 on the list of drag sources is the rear of the car. That big flat slab at the back of your car creates the most drag. Porches have probably the most aerodynamic rear ends, gradually tapering off to a point. The problem is that a narrowed rear end is considered fairly ugly, and unappealing to the average driver.

No.2 is the windscreen, but there isn't much you can do about that.

No.3 is the underside of the car. Automakers know that drivers don't spend much time looking at the bottom, so they don't bother to do much with it.

Cars are restricted in aerodynamics because of the fact that they have to carry passengers. Passengers need to see out in all directions and have some means to control the car. But what would be the fun in not being on the inside?

LTSpeed

Uh, I thought it was clear I was kidding...

v7guy

iTrader: (1)

There's been alot of trash (bickering) thrown in here since I last posted 14 pages ago.....alot of good stuff too. Only have two points to mention as it's the only pieces of info I currently have that I think would be beneficial to this thread.

1st.......The gaps in the body are a significant source of airflow into the body panels, I have pics of my car while doing some string tests(94 firebird @ 90-100mph) Where the bumper isn't shimmed up where it needs to be giving roughly a 1/4 to 1/2 inch gap. The yarn strings that are just above the gap are being pulled into it, the same thing is happening around the headlights. Those flip up headlights with the extra gaps in the front end aren't as great as one would think. I have no idea how it compares to the lines in the camaro for its headlights. If anyone is interested in it I can send them the pics. Any vent in the hood needs to be in the front third, after that the area appears to become higher in pressure. This is another observation from the previously mentioned yarn test. I'd like to do some oil testing but I'm not sure I'll get around to it.

2nd......The rear end of the car. Ever wonder how the top of your bumper gets so filthy? The air doesn't cleanly leave the car, it tends to try to follow the curved edges until it seperates(the previously mentioned coanda effect), creating alot of drag in the turbulence from where it does seperate. The air rushing back at the rear end causes more drag. Further compounding the problem is our rear bumper that hangs down catching the air under the car acting as a parachute. The rear end of every supercar and the vette is very sharp and squared off to help the air cleanly seperate. It still has more drag than other shapes, but it's the cleanest design while still keep the interior size typically needed of a car.

chuntington101

very good point there v7guy!

if you ever see simulated air after a F1 car has been through it you will see just hw messed up it is! again its a case of compromise! drag for down force.

oh and the influence for other F1 cars is about 10 car lengths i think! thats why its sooo damn hard to overtake! unless you have a VERY long straight to slingshot past you have no chance of getting close enough because you loose all your downforce!

thanks Chris.

HPP

Yes, but one that has been covered pages ago.

v7guy

iTrader: (1)

These things were already mentioned, thought I would bring it back up as the thread has steered a bit off track due to the hardheadedness of a few people, hoped it would get us some more info from the members that know more than the majority of us.

mld

Cd isn’t everything.

How aerodynamically a car performs is not simply a measure of it’s Cd but rather how well the car utilizes the aerodynamic forces that it encounters.

A car on a race track needs down force for the car has little traction at high speeds.

Those huge spoilers may cause drag but it is drag that is wanted that produces downforce on the rear end down.

You might improve the Cd of the Trans Am by removing the rear spoiler but it would reduce downforce in the process.

LTSpeed

Believe me, even those big spoilers produce drag that is unwanted. It's not the drag that produces the downforce--the drag is an unwanted result. The key to a spoiler is how to maximize downforce while adding minimal drag. A wickerbill on the rear edge of a wing is a good example. It might add no downforce at all, but it can reduce the wing's drag up to 25%.

mld

Correct. I should have not worded that the way I did. The downforce is wanted. The drag is not.

chuntington101

here are just a couple of examples of great arodynamics!

the first is design for max downforce and is road legal (and a pretty hot car!!!!!!!)! this looks very extreame and it is! the whole thing is designed to make downforce! and it will make equal to its own weight (500kG) at 150mph! it uses techniques gained from F1 (designer was a former F1 designer!)!!!!

www.caparo-t1.com

this one is desinged to take the diesel land speed redord! its designed to have a very low CD (about 0.19!!!!) and to produce no downforce at all and no lift.

www.autocar.co.uk/news_article.asp?na_id=219580

no if i could have one it would be the one on hte top!

thanks Chris.

njn63

Kinda late but here is some more backing on the "tear drop shape" being the best:
http://www.insightcentral.net/encyclopedia/enaero.html

Yeah yeah, it's a Honda, but it also has a .25 cd

MadBill

The site talks of "the lowest Cd (0.25) of any mass produced car", however with more than 1,000 built I would think the GM EV1 electric car, with a Cd of 0.19, almost 25% better, would hold that record..

njn63

yeah, i saw that. Do you have any more info about the EV1? I honestly knew nothing about it until i read this post but i might of seen it when i was little somewhere.

My guess is they misquoted a Honda press release. Honda usually words stuff very specifically and often people misquote it and leave out key words when reposting it on the internet. Either that or they just made it up.

MadBill

I just Googled "GM EV 1" and got a ton of hits...

Hoss Ghoul

iTrader: (2)

The thing about the GM EV1 was, like most early electric cars, it wasn't actually available to buy. They were lease only, for about 3 years IIRC, and then GM took them back...basically a rolling test bed/PR deal.

MadBill

With not many more than 1,000 put in service, just taking into account the engineering and development expenses (reportedly between 1.0 & 1.5 billion dollars) and even ignoring the manufacturing costs, they were also the priciest personal vehicles 99.9% of the leasers every drove, at close to $1,500,000 each...

x phantom x

iTrader: (14)

OMG, just please, STOP ARGUING!!!! I have read 7-8 pages of your drivel. You have absolutely no clue what you are talking about, and are just tossing out random incoherent thoughts and attempting to pass them off as fact. You state something as if you know what the hell you’re talking about … it is proven incorrect a post or two later, then you attempt to act like “oh, yeah, that’s what I meant.”

And no, closing the underside of the car up is not going to produce the 3,500 lbs of lift it would require to lift a 3,500lb car into the air .....

mld

.... wonder why GM is having money probelms

of course their is always the what if factor.

Just imagine if the EV1 had worked. If it had a good range to it. I think I might well have brought(leased as the case may be). They even had fairly good acceleration, if I remember correctly.

If it had worked... you could simply pull in your drive at night and just plug in!!! Gas prices... you wouldn't even care.

JoshuaGrooms83

iTrader: (2)

I just joined this thread to learn about underbody panels. One thing i dont understand is why anyone would CARE about aero dynamics of a car IN REVERSE!!! if you are driving a car 150mph in reverse... congratulations, you just won at the special oylmpics. no point.
Now back to actual TECH talk.

Does anyone have photos of belly pans on 4gens? I know where you can get a rear Defuser for Firebirds and it looks pretty cool, 6LE makes em, and they seem like they would do a decent job, but alot of underbody panels need to be use to make it more effective. Im setting up my car to take advantage of the Nurnburgring so any kind of aero advise would be cool. Im considering some kind of front splitter as well. just not sure which kind yet.

white2001s10

iTrader: (4)

HPP is correct about the aerodynamics.
The shape of the rear of a car is more important in reducing total drag than the front of the car.
A vehicle with a higher CD can in fact have lower total drag than a car with a lower CD, due to difference in frontal area.

All things being equal, less frontal area means less total drag.

As far as lift vs down-force, for best efficiency (speed vs power) it is best to be as close to neutral as safety will allow. For drag racing, this inevitably means taking steps to reduce lift, preferably with the least drag penalty.

As far as the car going in reverse example, the principle is valid, though it is counter-intuitive due to the commonly observed shapes of things like automotive side-mirrors, rockets, and bullets. In each of those examples the leading edge is tapered and the trailing edge is blunt. If you think about the design of most of these everyday examples you realize that the blunt trailing edge is designed out of practical necessity. You have to be able to see properly in your side-mirror, the bullet requires a thrust surface for force to be applied evenly, etc ...
The working principle can be effectively used to make improvements to aerodynamics, though these changes often come with compromise in other areas.
Practical aerodynamics is studied in detail as it relates to improved fuel efficiency and extended coast-down times at sites such as:
www.ecomodder.com

Measurable gains at street speeds are usually hard to come by. Many street/strip drag cars can reach speeds between 40 MPH and 60 MPH by the 60 foot mark, and so aerodynamics can and do come into play even in the quarter mile.

I personally have not put much effort into aerodynamics on my street/strip project cars, and admit that many of my modifications, while improving strip performance, also hurt the aerodynamics, and total drag of the car.
I do have one instance (anecdotal) of improved performance due to an aero change on my 3rd-gen f-body.

My 3rd-gen was running some larger stickies on the rear for traction (27" diameter), which made the rear wheel-well around 2" higher than the front wheel-well, and gave the car what I would describe as a mild rake.
Even a mild rake causes a significant increase in frontal area, as I found out, because it also lowers the nose at the same time as raising the rear. Whatever other aero problems my car had were apparently amplified by this slight change in attitude.

I made an adjustment to the rear to bring it down to a point where the car was completely level, -everything else remaining the same including the tires.
The car picked up just under 0.15 seconds, and just over 2.0 MPH in the quarter mile.
At the time of this change my car was running around 11.0 @ 122 MPH.
Aside from the test equipment at the track (GIR), I was using a VC2000 accelerometer and a Davis weather computer to record test data.

With the accelerometer data this car was running consistently to within 0.004 seconds, and 0.15 MPH.

Track numbers are far less consistent due to the rollout factor, and were usually within +or- 0.05 seconds, and 0.15 MPH.

I'm basically saying that the car was very consistent, so I trust the test results as not being a fluke.

I performed A,A, B,B testing and did not go back and reverse the modification.