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Old 07-30-2007, 05:41 AM   #1
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MEGA metro airdam

Check out some pics in my garage of the airdam I put on sunday. http://www.gassavers.org/garage/view/272
What do you think?
I have seen alote of talk on airdams lately, so i decided to give it a try. I was going to use lawn edging at first but decided to go with coroplast for a more rigid installation (plus I have plenty of coroplast lying around), and I could pick any height. I cut the 7" tall strip with my tablesaw, sure beats a razor cutter . It is about 3.25" off the ground. It rubbed once on the way to work this morning when i crossed railroad tracks too fast. I need to point it like a cow pusher only for roadkill .
I just hope it helps my aero instead of hurting it. I will attempt some coast down testing someday soon hopefully.
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Old 07-30-2007, 06:01 AM   #2
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I'll be interested to see the results. An air dam is a compromise, it's not going to help unless the increase in frontal area produces less drag than the underside of your car did. I don't know what the underside of your metro looks like, but I suspect that it's pretty draggy under there, being that you don't have a full undertray. I was looking into an air dam similar to yours for my civic, but I'm planning on a full undertray now because it's not going to be as prone to damage.
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Old 07-30-2007, 06:10 AM   #3
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Sweet.

Flat black paint is your friend...
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Old 07-30-2007, 06:11 AM   #4
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Yeah, a compromise it is . I think I would prefer an undertray but that would be harder for me to do. All the coroplast i have is in precut strips 17" x 74" so no large sheets. They are longer than the car is wide at least, so i could piece something together in strips.
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Old 07-30-2007, 06:24 AM   #5
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Nice, I used that house trim at HD. It was 5/16" thick and very rigid but it will snap before it bends too far. Requires a big radius bend.

Where abouts are you in Michigan?
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Old 07-30-2007, 06:30 AM   #6
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Quote:
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Sweet.

Flat black paint is your friend...
Thanks, i actually ran out of my normal black gloss paint part way thru so i finished over it with my hi temp stove paint. It matches the bumper plastic much better.

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Where abouts are you in Michigan?
I'm near Leslie (1/2 way between jackson and lansing).
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Old 08-01-2007, 12:56 PM   #7
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tested airdam

Well I did some coast down testing last night. I did them on a mile stretch of road near home. It is pretty flat in the middle where I was coasting, unfortunatly there were stop signs at each end so i had to accelerate pretty hard then end up braking at the other end. So the test was a bit wasteful on gas . I did bi-directional north/south runs so I could use the data for drag coefficient calculations and stuff. It wasn't A-B-A, just A-B unfortunately. I taped the speedo with a vid cam and watched it back w/ stopwatch. I timed speed between 55 and 45 mph (55 and 50 for Cd calcs). Conditions were 93 F (33.89 C), 30.03" hg pressure, 3 mph E NE winds, test mass estimated 1991 lbs (903.1 kg).

To find Cd I needed to remove the rolling resistance from the road load forces to single out the aero part. I didn't do a low speed coast down like I was suposed to because I wasn't sure I could rely on my speedo at those speeds. So instead I measured the force needed to roll the car at a fixed low speed on level concrete. I did bidirectional pulls that averaged out to 10.62 lbs. The comes out to .00586 rolling resistance (10.62lbs/1812lbs car weight without me in it). That seemed aweful low to me, but I did the test on smooth concrete not the road surface I did the other test on. I decided to double that # for the road surface to .0117 which seems more on par for LRR tires. I had to estimate frontal area too, the book I was using gave a formula to estimate it (.9 x height x width of the vehicle). Of course I wanted to try my own estimate by Analyzing a scaled down front view picture of the car and finding frontal area that way. With my frontal area estimate Cd came out to .327 without the airdam and .315 with it. Using the books area formula Cd came out to .260 for no air dam and .251 with it. Remember the rolling resistance is an estimate too so the relative results are all I'd go by not the actual values. Too many estimates = who cares.

Cut to results... Coast down time averaged 16.0 seconds with airdam and 16.3 seconds from 55- 45 mph without. In the 55-50 coast down range the airdam average acceleration was -.2846 m/s^2 and .2800 m/s^2 with out. So the airdam didn't help by a small margin. I probably made it too big. All else being equal, the drag coefficient did improve with the airdam by .01 ish but it did not make up for the added frontal area as seen with the acceleration rates.
I will probably shave off at least 2-3 inches and put it back on. It rubbed too much anyway, it plowed gravel pretty bad coming into my driveway .

Disclaimer... These are results from a newbe tester using Cd formulas out of a book called "propulsion systems for a hybrid vehicle", I probably did it the hard way
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Old 08-01-2007, 01:05 PM   #8
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(shameless self-promotion here)

See my recent thread on coroplast construction - especially re. attaching a belly pan to the coroplast air dam. Mine is really holding up nicely now.
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Old 08-01-2007, 01:20 PM   #9
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Yes, nice idea with the corner bead .

A much smaller better shaped air dam or just good undercarriage smoothing is probably in my future.
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Old 08-01-2007, 01:46 PM   #10
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Quote:
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I taped the speedo with a vid cam and watched it back w/ stopwatch.
Nice work. On digital video if you put it into your computer you can use the time stamp for each frame as a way to calculate the time elapsed. If yours was digital... Just a thought.

I am hoping that the next generation of ScanGuage will have a way to store data. Then the coast-down data could be logged on that, and it would take the time measurement error out of the equation.

Anyway, good job, and thanks for posting.
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