Front air dam / valance questions

[Geo]

Dave, that under tray is critical for cooling. It increases the pressure differential between the front and back of the radiator increasing flow through the radiator. Without it, air moves from under the car up behind it decreasing the pressure differential.


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George Roffe
Houston, TX
84 944 ITS car under construction
92 ITS Sentra SE-R occasionally borrowed
http://www.nissport.com

[lateapex911]

Yes, mine was mangled, and it didn't make it back on the car one time. I was reminded that it is needed to be legal. Ooopps. What a pain it was finding another one, and IT is a piece o crap!

Your wife is cool, watch the obsession!

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Jake Gulick
CarriageHouse Motorsports
ITA 57 RX-7
New England Region
[email protected]

[timrogers]

Originally posted by GregAmy:
Tim, good points on all that nasty stuff hanging down. I'm confident that an airdam cleans that up. What I'm really curious about, though, is how much drag is that versus the additional frontal area of an airdam? If you look at the dam I fabricated last year (http://www.gatm.com/cars/nx2000/spoiler.html) you'll see that I added at least 4 inches to the front of the car (it's the amount of the pink part which sticks down from the factory spoiler). That didn't even maximize the rules; I think I could have gone another inch.
GregA[/B]

Greg, the only real way to find out how much air dam is too much is through testing with an adjustable air dam and splitter combination. If you were to put vertical slots in your current air dam on which you could have the fasteners of a lower piece of the same shape slide up and down on,(with an adjustable splitter attached to the bottom, of course) you would have a great testable contraption.
Now, here is where the fun begins. I guess you could test for top speed first while taking it easy through the corners, to get a handle on your overall drag. The reason for this is that with a splitter, you can really start to make some significant downforce on the front of the car and make it want to swap ends in high speed turns. This has been a proven fact with the SE-R Cup guys out in CA.
Once you have a baseline for which airdam depths and splitter settings have a given relationship with top speed, then start tuning the airdam in concert with suspension settings to avoid getting a panoramic view of the high speed turns at your testing track while searching for faster lap times.
The problem with this method is that you need a track with a pretty long straight so that your corner exit speed does not affect your speed at the end of the straight.
For giggles, you can go to http://www.getfaster.com/
and click on "Road Racing" then "Tech Tips" then "Physics of Racing Series" to get to the "Speed and Horsepower" section. It shows some good calculations and knowing that the NX2000 has a factory quoted Cd of .32 and I estimate the frontal area at about 20 square feet, you can do the calculations and then wad them up and throw them in the trash as they do not take into account that we race with the windows down.

Actually, once you find out your top speed, you can look at your dyno chart and compute what your racing Cd is.

[Tom Blaney]

Being that I make the spoilers, and I have a pretty reliable and consistant car/driver package (except for the occasional drama queen episodes) I have modified the size of the splitter lip to see what the effect is (also that's why one or two of them were left on the track or dragging under the car while the race progressed ) And as noted I am a fan of rules compliance, the splitter lip can't protrude past the front bumper I found that a consistant 1" lip all the way around seems to work the best. I did try a 3" lip once and it got pulled off of the car within a few laps indicating that there was a lot more downforce being generated and perhaps too much since all it will do is create drag. Keep in mind that the splitter is doing double duty (not including grass trimming) it is reducing frontal drag and increasing downforce on the front end, too much downforce may seem good for a F1 car but were talking about < 150hp in our cars. Bigger is not always better ..

This is a photo of the airdam
http://www.sbmsinc.com/photos/airdam_0.JPG

This should take you to my site
http://www.sbmsinc.com/race_shop.html

[This message has been edited by Tom Blaney (edited November 25, 2004).]

[This message has been edited by Tom Blaney (edited November 25, 2004).]

[jlucas]

[quote]Originally posted by timrogers:

knowing that the NX2000 has a factory quoted Cd of .32

"Factory" posted Cd number are... shall we say... optimistic.



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Jeremy Lucas
Team Honda Research
Kumho - Cobalt - Comptech

[Bill Miller]

Greg,

I lost the time w/ the airdam. And it was a 1st gen. (AW11) MR2. Be a great ITB car. All four corners are totally adjustable for camber/caster/toe, not to mention that everything mounts on subframes. I used to have a full set of corners that I'd take to the track. I missed out on the last race at Bridgehampton because I broke an inner tie rod, and couldn't get a replacement. After that, my moto was, if I had it, I brought it!

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MARRS #25 ITB Rabbit GTI (sold) | MARRS #25 HProd Rabbit
SCCA 279608

[Knestis]

A coast-down test is a great, repeatable way to determine the drag hp of an entire car's package. {It's a negative number, in vehicle dynamics terms.)

It doesn't require the kind of space that a top-speed test does and can even be done on public roads: Another argument for street-legal IT cars.

K

[gran racing]

That's interesting Bill. MR2 Jake recently put an air dam on the car and got his best time at Lime Rock. There are a lot of factors that were not taken into consideration and the air dam may not have contributed to this. (Other factors like me being in front of him and he just can't have that.)

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Dave Gran
NER ITB #13
'87 Honda Prelude si

[Bill Miller]

Dave,

I don't think you're getting to the same terminal velocity at LRP as on the long course at the Glen. I really seemed to notice it at the end of the back stretch, going into the bus stop. Not to mention that lap times at the Glen are ~2.5 times what they are at LRP. With the longer distance, even small reductions in velocity will have a big impact on lap times.

But again, the air dam design certainly wasn't optimized, or even modeled.

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MARRS #25 ITB Rabbit GTI (sold) | MARRS #25 HProd Rabbit
SCCA 279608

[ITANorm]

Originally posted by Bill Miller:
Greg,

I fooled around w/ an airdam on my old AW11 MR2. Didn't have any fancy data aquisition, but did find that I lost almost 2 seconds on the long course at Watkins Glen. I didn't play around w/ multiple configurations, but couldn't really see how any design was going to get back those 2 seconds and then gain me time.

Bill,

Not to mention that if you start monkeying about with the airflow under an AW11 (at least in IT-legal configurations), you will start having issues with engine bay temps. (Which will lead to clutch hydraulic vapor-lock, etc.)

[dyoungre]

Just a few 'theoritical' points:

Frontal Area: take a picture of the car directly from the front - it is the 2 dimensional projection that you see. Going lower witht the airdam may increase the literal area, however:

Airflow over versus under the car: Think of what the big deal in F1 wind tunnels is with the 'moving ground plane'. As an analogy, think of a main bearing - with the crankshaft surface doing 70 mph. The oil right at the crank surface is doing 70 mph, and the oil at the block is doing 0 mph. In between, the oil is shearing, which is drag. Same hold true for the air under the car - the air trapped in the exhaust tunnel, around the gas tank, anywhere on the underside surface is going 70 mph, while the air at the ground is going 0 mph. That difference in speed occurs in a much shorter distance than if the ground weren't there (or if it were moving at the same speed as the car, like in a wind tunnel without a moving ground plane), and so the shear effect, or drag, is much greater.
As someone said before, it the pressure (or looked at another way, the density) of the air under the car that can be reduced by an air dam by getting the air to flow over (creating downforce, or eliminating lift) or around your car.

On the 79-80 RX7, there is a lot of lift, even at 70 mph, because a lot of air is forced UNDER the bumper instead of over it; Mazda reshaped it for '81. My airdam changes the high speed handling enough to cause me to adjust my sway bar setting - equal to removing the stock rear sway bar. My experience agrees with what others stated - the biggest difference I saw with/without the airdam (after I lost it in turn 11 at NHIS) was about 60 deg F in radiator cooling. For your Honda, that could mean the difference between your fan being on or not (alternator current draw), or you could potentially reduce the size of the opening, which should reduce drag. The devil may be in the detail, but I think that it is worth trying, even without an F1 wind tunnel.

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Dave Youngren
NER ITA RX7 #61

[Nigel Stu]

There are some interesting articles in the last 2 issues of "RACECAR ENGINEERING" (at least that's what I think its called... someone please correct me if I'm wrong) that talks about airdams and splitters. Good reading if you can find it in your area (UK print, I have been able to find this mag at B&N and Borders in the Detroit area)

[Racerlinn]

Back to Dave's original question.
I have had good luck working with vinyl house soffet. Has a good smooth surface and is easy to work with, both cutting and with a heat gun to make bends. Just don't over-use the heat gun, the vinyl will shrink and yellow. Practice makes perfect. Plus it's cheap, less than $10 for a 12 foor piece. But it does not take paint well, so you better like the color white... (see below)

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Steve
'92 ITA Sentra SE-R
www.indyscca.org

[gran racing]

Easy to work with and cheap work for me!

Where could I find this? Would a home depot or Lowes carry this stuff?

I can't tell from the picture - do you have a splitter as well?

Thanks.

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Dave Gran
NER ITB #13
'87 Honda Prelude si

[Racerlinn]

I've not been bold enough to try a splitter yet, I have enough on my plate right now making sure the car will be ready for spring. Here's a good sample of a low budget one (that I hope to someday boldly copy on my own): http://www.nissport.com/spoiler/default.asp
Thanks Geo and Grover!

As far as soffet (or soffett?) you will find it at most home improvement "speed shops". I use my unofficial "sponsor" Lowes. It's back with the flashing and gutters. Generally it has some ribs similar to vinyl siding, but the flat sections are about 4" wide. I use the heat gun to bend a nice 90 degree flange to use to bolt it up to the bottom of them bumper cover. I put a small, tight 180 degree curve on the bottom edge for rigidity. The hardest part is to conform to the curves at the side of the bumper, wrapping around to the fenders. You have to notch out the top flange in multiple places to get the vinyl to properly curve. Again, practice makes perfect.

My unsubstantiated, underengineered, set of the pants opinion: I think the front of car feels better at highway speeds. More drag? Maybe. But I agree with others like Tim that I would rather have the clean front edge rather than the air hitting the bottom of the tranny, motor, etc.
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Steve
'92 ITA Sentra SE-R
www.indyscca.org


[This message has been edited by Racerlinn (edited November 30, 2004).]

[bill f]

Boy, I hate to admit to an advanced age, especially one that would allow me to remember this!

In 1975, during the infamous "gas Crisis", Car and Driver did some early testing of wind cheating devices to see what would enhance gas mileage. Their base test sled was a '75 Pinto 2300cc engine, with automatic transmission (more were sold than 5 speeds at the time, I believe). It was done on an instrumented car, at 75MPH, as I recall, on one of the southern Super Speedways...Talledega, perhaps?

Besides using headlight covers, blocking off the grill, then varying detrees of open, using an add-on rear spoiler, they also fabricated an air dam with wrap around edges, which extended down from the already vertical bumper.

Relative to the front air dam, the conclusions ran along these lines:
The LOWEST, deepest dam was the most effective. They began with a dam which was almost touching the pavement, and had to eventually trim the dam to allow for driveway clearance, only. Trimming was proved to diminish the effectiveness of the dam. I repeat: Lowest was the best.

Their explaination (theory, at the time?) was that the air drag from the additional frontal area was still less than the under-chassis drag from the "dirty" underside of the car. Cleaning up the underside (blocking off the air source) reduced the drag beyond the addition of drag from the increased frontal area.

Incidentally, the combination of devices they worked with (basic materials, with hand tools) gave and additional 25% boost in gas mileage.

I kept a copy of that article for years, as I was building a B Sedan Pinto, and copied their work. They then went on to evaluate various devices on a few other cars...I remember the Datsun Z car with commercial air dams used.

Not promoting any position here, only reporting what I remember of the article.

Good racing.

Bill

[Knestis]

I'm reviving my old rallying tradition of celebratory motorsports limmericks, for your listening pleasure...

A budget-wise racer named Moffet,
built airdam and splitter from soffit.
Took weeks, the poor dope,
so he'd better hope
that he don't hit a curb hard and off it.

K

[lateapex911]

touche'

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Jake Gulick
CarriageHouse Motorsports
ITA 57 RX-7
New England Region
[email protected]

[Racerlinn]

Originally posted by Knestis:

A budget-wise racer named Moffet,
built airdam and splitter from soffit.
Took weeks, the poor dope,
so he'd better hope
that he don't hit a curb hard and off it.
K

Yep, that's me!! I've already gone thru two version in the last few years wacking cones. But I can make one in a couple hours now.


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Steve
'92 ITA Sentra SE-R
www.indyscca.org

[Geo]

Originally posted by Racerlinn:
I've not been bold enough to try a splitter yet, I have enough on my plate right now making sure the car will be ready for spring. Here's a good sample of a low budget one (that I hope to someday boldly copy on my own): http://www.nissport.com/spoiler/default.asp
Thanks Geo and Grover!

This one is actually quite strong.


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George Roffe
Houston, TX
84 944 ITS car under construction
92 ITS Sentra SE-R occasionally borrowed
http://www.nissport.com