Front air dam / valance questions

[gran racing]

How much does adding a front air dam / valance help? Especially on ITB, C and A cars, I've always viewed this as an item more for show.

Now that it is winter and I'll have some time on my hands, maybe it would be worth spending some time to add one. What cheap and fairly easy methods have you used or seen that work well. (I'm basically looking for a Home Depot solution. I do not weld nor want to pay someone to weld one up for me.) I've seen some people use the bed landscaping plastic stuff, but am not so sure about that. I'm also planning on adding brake ducts and would most likely cut holes in the air dam for this purpose.

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Dave Gran
NER ITB #13
'87 Honda Prelude si

[Greg Amy]

Funny you should ask that, Dave. I've been kinda wondering that myself before I bothered to go about fabbing one.

Most folks figure the front air dam is there to clean up the airflow, primarily underneath the car. The thought is that if you move the air aside, rather than letting it go underneath the dirty side of the car, it's less drag. Of course, parasitic drag is a direct result of frontal area, and it seems obvious that adding an air dam increases the frontal area, right?

I got to thinking about this more based on what we learned as a result of data aquisition (using an Action Digital DL-1) at the 13-hour VIR race last month. I've reviewed the data, comparing my daytime stint to my evening stint, the same car, same track, same day but with the addition of our nighttime driving lights (http://it2.evaluand.com/gti/enduro04.php). I've picked four of my fastest laps from each stint, and I found where I *really* hit the slow (2nd gear) Oak Tree perfectly and at the same exit speeds.

You know what? Those lights dropped my lap times approximately 1/2 second per lap and 3 mph top speed *just* on the back straight from Oak Tree to Roller Coaster. Not surprising, right? OK, so why is it that we expect that adding frontal area with driving lights (especially *those*) INCREASES drag, while we accept as gospel that additional frontal area from a spoiler or air dam DECREASES drag? What is it about that under-car flow that's so important?

Granted, we're talking apples to oranges with the driving lights comparison. The lights were pure drag, whereas an airdam may cause additional drag while decreasing under-chassis drag, yes? Further, that straight is a very long one and partially uphill, so it's an extreme example. However, I still think it's something to think about. I'd love to learn more about the core reasons for front spoilers and airdams, and why is it we accept the additional frontal area of them without question.

Thoughts?

Greg

[Knestis]

<hijack> Hey - I want MY data. </hijack>

The other component, at least theoretically, is increased downforce due to decreased air pressure under the car.

The concept goes that by closing off the front of the car, you create a low(er) pressure area that migrates some distance back under the car. Of course, with an IT car, the leakage is pretty massive so the effect - if there is any measurable - will be biased toward the front pretty significantly.

Low pressure under the engine compartment should have the effect of improving flow through the radiators, too.

Now, whether any of this translates into improved lap times is another question. It is pretty impressive how much decreased drag DOES improve performance.

I used to do a science demo with a simple LEGO car on an inclined plane: It starts with a finite amount of potential energy at the top, and by rearranging the same pieces (same mass, right?) so that a 3x7" plat sits at 90* to the direction of travel, I could get repeatable tests that showed 1/3 or more of the energy got used up shoving air around, compared to having the plate stuck down flat on the chassis.

The only way we'll ever really know is by testing.

Building something is going to be a pain in the butt since there are all kinds of wacky contours on the front bumper of a typical IT car now...

K

[Greg Amy]

Ah, yes, professor. But, if we *do* increase downforce, now we're increasing *induced* drag as well. And, if we increase cooling flow, we're increasing parasitic drag through there as well (the amount of cooling drag is incredible)!

I really think our low-power, relatively low-speed cars will benefit much more from decreased parasitic drag than we ever can from any negligible downforce or cooling improvements. But you're right: it's all about testing. Everything else is theory and posturing...

I think you will find that most of the airdams do very little in the way of downforce and do increase the drag on the car.
The main benefit is reducing lift on the front of the cars. If you look at a majority of the cars in IT in stock form, most will have a valence that drops from the bumper area downward at a 30 to 45 degree angle.
A good example would be a SpecRX-7 compared to an ITA first gen with a good airdam.


Another benefit would be to have a place to mount brake duct openings in the air dam in a high pressure area for better airflow to the brakes.

[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.

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

[JeffYoung]

Silly question perhaps from a non-technical person, but are we 100% sure that decreased frontal area always results in decreased drag?

In other words, is a properly shaped, but somewhat larger, frontal area that breaks and directs air in the most efficient manner better than a smaller but more blockish one?

Back when I still had some math skills and thought possibly about studying aerodynamics and aerospace engineering, I recall seeing diagrams and studies that showed a properly smoothed and contoured, but larger, frontal area resulted in a faster flow around it than a blockish one.

The blockish one created disturbances and eddies at the corners, slowing the air down at those points, causing air to pile up in front of the spoiler and result in more overall drag.

Any merit to that?

[lateapex911]

The way I see it, and air dam can never INCREASE frontal area on an IT car. The rules prevent that.

So....no drag increase can be attributed to an increase in frontal area.



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

[timrogers]

Originally posted by GregAmy:
OK, so why is it that we expect that adding frontal area with driving lights (especially *those*) INCREASES drag, while we accept as gospel that additional frontal area from a spoiler or air dam DECREASES drag? What is it about that under-car flow that's so important?

The lights were pure drag, whereas an airdam may cause additional drag while decreasing under-chassis drag, yes? I'd love to learn more about the core reasons for front spoilers and airdams, and why is it we accept the additional frontal area of them without question.

Alright Greg, lets get back to the core basics of how aerodynamic drag is calculated. Drag force = frontal area * drag coefficient. So, you need to look at your car as one complete unit or system to truely ascertain the total amount of drag that it is producing. I just went down to look at my 2 garage queen NX2Ks which have nothing but the factory air dams on them. As a WAG, I think we can put an IT legal extention on the air dam of about 2". Look at all of the junk that hangs down in that 2" zone: oil pan, suspension arms, exhaust, etc. None of that stuff is very aerodynamically "clean" and its counting as frontal area anyway, so why should we let a 120 MPH blast of air hit it? Remember also that the drag force goes up (is it logarithmicly?) You double the speed and the drag force goes up by a factor of 4 because you are not only hitting the molecules of air twice as hard, you are also hitting twice as many molecules per given unit of time.
Sure, we can't do a full underbody tray, but if we can significantly reduce the speed of the resulting airsteam that is hiting the "dirty" parts of the underbody, we end up with a net reduction in drag force. Of course at the same time we are hoping to reduce front end lift, so why not try to kill two birds with the same stone? Also, depending on the design of the airdam, (remember that with our enclosed bumpers, the airdam can start just below the nose of the car)you can try to persuade more of the air to dump to the sides or over the hood. Which way would you want the air to go? Greg, you're the pilot with data acquisition & access to pitot tubes and pressure transducers... Go to town my man! :-)

Tim

[lateapex911]

I'd like to blame the computer for this...

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

[This message has been edited by lateapex911 (edited November 24, 2004).]

[This message has been edited by lateapex911 (edited November 24, 2004).]

[joeg]

Air Dams work on my car.

Back to back testing many many years ago on the QEW in Canada.

Dam adds at least 7 MPH to top speed. Probably a bit less dramatic on a race course because you do not have straightaways exceeding 2 miles in length.

Cheers.

[spnkzss]

GregAmy wrote :

<font face=\"Verdana, Arial\" size=\"2\">You know what? Those lights dropped my lap times approximately 1/2 second per lap and 3 mph top speed *just* on the back straight from Oak Tree to Roller Coaster. Not surprising, right? OK, so why is it that we expect that adding frontal area with driving lights (especially *those*) INCREASES drag, while we accept as gospel that additional frontal area from a spoiler or air dam DECREASES drag? What is it about that under-car flow that's so important?</font>

When we ran the 4 hour Enduro this last weekend at Summit, we got out for practice and ran some times with the lights on and lights off. The lights on laps where consitantly 2 seconds slower than the lights on times. Luckily (or unluckily in a way) it rained so we never noticed the 2 second lap drops. What we attributed the drop in time to was the draw on the alt. not drag. Obviously there is some addition to drag, but drag is not the reason you would loose .5 sec is it?

Spanky
ITC #73 '90 Honda Civic WDCR-SCCA

[Tom Blaney]

Talking from experience, I strongly believe that an airdam makes a big difference in handling especially on tracks with long sweepers (like Pocono,Summit & the Glen). Properly designed with a splitter, it improves airflow under the car, and increases front bite. I have run with and without and I always notice the difference. Plus it will give you a good place to hang the brake duct hoses that are also a good thing to have.

Considering the fact that most pro teams spend a gagillion dollars developing them (and the associated weight) they must do something.

[JohnRW]

Originally posted by joeg:

Back to back testing many many years ago on the QEW in Canada.

Probably a bit less dramatic on a race course because you do not have straightaways exceeding 2 miles in length.

Cheers.

Har. The QEW is Canada's 'Bonneville Salt Flats', although the stretch of 405 past Pierson Airport (NW of Toronto) could be a runner up - 10 lanes each way, 90+mph in the dually, pulling an enclosed 24', keeping to the far right so as not to get run over by the traffic. Man...I miss Mosport...

[JeffYoung]

Tom, thanks for the info, and a follow up question.

I am fabricating a spoiler/splitter for my car over the holidays. The spoiler is a rally spoiler developed for the car back in the late 70s (its a TR8), so I assume some develpment went into and I do admit it looks fairly "slick" rounded edges, etc.

However, it has no splitter or undertay, so I've taped on some fiber board and started laying fiberglass on top of it.

What's the best design for a splitter/undertay generally, or is it car specific? Right now, my splitter extends out about 7-8 inches on the edges of the spoiler (which is in the shape a gentle parabola), tapering to 1-2 inches in the middle where the parabola is most convex.

On the undertray side, the undertray extends and covers the entire bottom of the soiler (and therefore the car) for about 12 inches.

I could go out a lot further in the front, and a lot further in the back under the rules (my bumpers stick out a LONG ways). Is there any advantage to that?

Should I track the shape of the parabolic spoiler, or should the splitter taper as I presently have it configured?

Any advice you could give would be appreciated.

Thanks.

Jeff

[Tom Blaney]

The splitter that I use (and sell) for the CRX is made from plastic sheeting (used by the dirt trackers for fender wells) and 040 aluminum sheet. The design has changed a little from time to time to simplify mounting strength. I believe that the undertray is very important because there is a lot of turbulance caused within the engine bay so the undertray entends (within the rules) back to create a smooth area under the motor/trans area. I played with the size of the splitter and the angle and find that about 1" and level still seems to be the most effective. If you look on my site you will see the example of the unit.

To add, I did find that although my CRX is normally very neutral, when the splitter was "missing due to an off" that I developed a push on turnin that normally was never there, definatly more noticable in fast sweepers. And being the calm and collected driver that I am, I would always back out and accept my fate

www.sbmsinc.com/race_shop.html


[This message has been edited by Tom Blaney (edited November 24, 2004).]

[evanwebb]

Hey all, I have an interesting back to back with/without air dam story. Several years ago as I was just getting into racing I took a ride with an instructor at Summit Point in his car, an early Scirocco. He was driving and things were going well on a slightly damp day until he amde a slight mistake at the exit of turn 5 and drove all four wheels off into the dirt. He corrected and came back on with no apparent problem or damage but when we came around onto the front straight again he noted that the car seemed a little slow and there was a bit more buffeting for some reason. When we got back to turn 5 we noticed his air dam laying in the dirt off to the side. So it definitely seemed to have somewhat of an effect...

[gsbaker]

I can't offer any advice based on personal experience, but it is probably safe to say that, in general, any basic dam with a splitter will help more than it will hurt.

It's too bad there aren't more rules of thumb racers can use. A good friend is a consulting mechanical engineer who does a lot of computational fluid dynamics (CFD/computer simulations of gas flow). Unfortunately, that stuff gets very complicated.

Gregg

[Greg Amy]

Good info, all. Don't know if you've noticed it over the years, but I'm not one to accept "common knowledge" as The One Truth (no duh, they say.)

Bill, I don't quite understand if you lost that 2 seconds WITH or WITHOUT an airdam on your MR-2? (Man, what I wouldn't give to drive a second-gen MR-2 in ITA. I've always been a big fan of that car...)

JeffY, parasitic drag is definitely a function of frontal area, so decreased frontal area will result in less drag, all other things being equal.

Jake, you most certainly CAN increase frontal area with an air dam; the rules allow it to go as low as the bottom of the wheel, and that is most definitely lower than any production car part. Imagine going over a speed bump if your undercarriage was only 4 inches off the ground (225mm times .45)!! It can even go down another 1/2 inch or more, given we can all run 15" wheels.

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.

Tom and Evan, your experience is very valuable. Thanks. Tom, where's your web site? I'd like to see some photos.

(On a side note, I remember the first time I saw Tom's spoiler/undertray. It was at the 2002 OMP Challenge, I think, and the assembly was in the cabin of the car while it was on the trailer in the infield. I saw the undertray part and thought, "that CHEATIN' b*****d." Then I re-read the rules and thought to myself, "that CLEVER b******d.")

I am confident that a front spoiler, especially when coupled to a partial undertray, is likely an advantage. I'm just not accepting that as gospel; I really want some proof...

GregA

[gran racing]

I too am one of those who question things. What I was thinking about tonight is if there is a point (car & speed) when the front air dam has a negative effect. Someone mentioned that fact that pros do it and spend tons of money developing it. I agree that based on this it seems like it would make sense for us too. BUT there has to be a point where the added weight to the front, drag, ect. out weight the benefit gained. I guess that's why I specified A, B & C cars. I know the line isn't so clear.

What is kinda funny (or maybe not) is that right next to my computer I have the 2003 GCR. On the back cover, they have pics of cars. It is interesting to see the AS not have an air dam. I know, that means nothing...

Tom - thanks for sharing the information. I went to your site but only found your professional info. not race info.

The undertray is interesting. A while back when I was first starting to build my car, it came with an undertray. I didn't give it much thought and yanked it out. Besides, it made working on the car a pain in the arse. But maybe it wasn't such a silly thing. Hmmm.

(I really need to stop thinking about racing so much. And my wife thinks I'm obsessed?)

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Dave Gran
NER ITB #13
'87 Honda Prelude si