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
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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
. 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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