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Old 07-20-2007, 09:02 PM   #1
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Coast-down Test

I have had may air dam on for a while now and never knew if it was helping my mileage or not. So today I did an A-B test on it. I did multiple coast-down tests on a nearly flat loop of highway frontage roads. Due to the speed limit, I ran the trials from 50 mph to 45 mph.

With air dam
13 trials
average time 8.87 seconds
std dev .74 sec

Without air dam (15 minutes later)
15 trials
average time 8.30 seconds
std dev .55 seconds

Student's 2-sample t-test says that the difference is statistically significant.

So it turns out that the air dam DOES help. Which is nice.

I didn't A-B-A because it takes a lot longer to put the dam back on and I didn't feel like doing it in the parking lot next to the highway. With A and B being 15 minutes apart, I feel okay that I wasn't being tricked by any changes in temperature, wind (there was none), barometric pressure, etc. The car was warmed up from my drive home. I would have liked to have a stretch of perfectly level road with an 80 mph speed limit, but it would be too far of a drive from my house. :-)

To time the coast downs, I pointed my video camera (on a tripod) at my speedo, and then downloaded the video to my computer to look at the timestamps when the needle went past 50 and then past 45. This method worked well and seems safer and more reliable than trying to watch the needle and work a stopwatch.

What I really really want now is a spreadsheet or online calculator where I can input the start/end speeds, the time, estimated Crr, vehicle weight, etc, and from all that get a CdA. Does anyone have one or know of one? If the data is to be believed, then I think the dam is worth a 10% reduction in CdA, which seems like too much.

Ask questions if you have any. And if you know an online coastdown/CdA calculator, please let me know.

Respectfully submitted for your approval,
Bill
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Old 07-21-2007, 01:58 AM   #2
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Bill, you have reached the same conclusion on your air dam that I did with mine. I have recently added another 3 inches to the bottom of my air dam which contributed even more. Good job of testing.

I think maybe MetroMPG.com has a calculator on his site.
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Old 07-22-2007, 02:56 PM   #3
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Finally found it!
http://web.archive.org/web/200408030...DownCalcs.html

My CdA is 0.957 with the air dam (1.07 without the air dam)
My Crr is 0.010

So the air dam gives me something like a 10% improvement in my Cd. That makes me happy. :-)

I put a belly pan on yesterday. Need to do the rear pan in the next couple of days.

Keywords for next time I need this:
Coast down
coast-down
coastdown
CdA
Crr
mph
speed
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Old 07-22-2007, 06:33 PM   #4
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Bill - see also http://metrompg.com/tool-aero-rr.htm if you like.

The extra 'A' would have made the results beyond reproach, but based on the rest of your methodology, I'd be comfortable drawing conclusions based on this. Nice work!
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Old 07-22-2007, 06:44 PM   #5
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Originally Posted by MetroMPG View Post
Bill - see also http://metrompg.com/tool-aero-rr.htm if you like.

The extra 'A' would have made the results beyond reproach, but based on the rest of your methodology, I'd be comfortable drawing conclusions based on this. Nice work!
Hi Metro, how you calculate the CRR?
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Old 07-22-2007, 07:10 PM   #6
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Originally Posted by MetroMPG View Post
Bill - see also http://metrompg.com/tool-aero-rr.htm if you like.

The extra 'A' would have made the results beyond reproach, but based on the rest of your methodology, I'd be comfortable drawing conclusions based on this. Nice work!
Thanks! What do you mean by "extra A"?

It occurs to me that I need to check and see what temperature and pressure the calculator assumes. I am at sea level, but it was about 85 degrees... And I also want to get a good estimate of area so I can calculate the Cd.
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Old 07-23-2007, 06:32 AM   #7
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Quote:
Originally Posted by Bill in Houston View Post
Finally found it!
http://web.archive.org/web/200408030...DownCalcs.html

My CdA is 0.957 with the air dam (1.07 without the air dam)
My Crr is 0.010

So the air dam gives me something like a 10% improvement in my Cd. That makes me happy. :-)

I put a belly pan on yesterday. Need to do the rear pan in the next couple of days.

Keywords for next time I need this:
Coast down
coast-down
coastdown
CdA
Crr
mph
speed
Bill, in the calculator you found, do you think the low-speed coastdown is intended to quantify the rolling resistance? At very low speeds the aero resistance would be practically nil.

I tried to just enter your 50-40 speeds into the calculator and got no result. Evidently a low-speed coastdown parameter is needed to complete the calculation. What did you enter to get an answer with this calculator?
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Old 07-23-2007, 07:14 AM   #8
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Yes, I think that you are correct. The low speed is used to help get Crr. I'm not sure the exact math, but I remember reading in my searching that if you plot, uhh, something, the slope is the CDA and the intercept is the Crr. It makes sense to me that if you plot the "slowing down force" vs speed, then at 0 mph some terms would drop out of the equation, and you would be left with a term with the Crr and some constants.

So anyway, I did it two ways. I felt like, from stuff I had read, that my car would have a Crr around .010. So at first I just monkeyed with the low speed numbers until I got a Crr of 0.010. Then my conscience bothered me, and I did a coast down from 15 to 10 mph, and put in those results, and it really did give me a Crr of 0.010.

What I entered:
50 mph
45 mph
8.87 seconds
15 mph
10 mph
21 seconds
3640 lbs
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Old 07-23-2007, 07:27 AM   #9
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These are two terrific tools to guestimate what results to expect from specific changes. It would be convenient to have an option to work the MetroMPG calculator backwards from reported fuel economy of the Scanguage, but a little trial and error will allow nailing down the CRR and Cd figures for a vehicle if you know what the average consumption is at specific high and low speeds.

Thanks for taking the time to create it!
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Old 07-23-2007, 08:42 AM   #10
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Originally Posted by CO ZX2 View Post
Bill, in the calculator you found, do you think the low-speed coastdown is intended to quantify the rolling resistance? At very low speeds the aero resistance would be practically nil.
yes CO ZX2.

to high speed, it is possible to calculate only the total resistance ( aero drag+ CRR).

accelerationHS=V2-V1
Rt= mass*accelarationHS

accelerationLS=v2-V1
Rcrr=mass* accelerationLS

Raerodynamic= Rt-Rcrr



Cd= (2*Raerodynamic)/ro*A*V^2)

A= frontal area of car
V= speed (m/sec)
R= resistance (Newton)
ro= air density (1,19 kg/m^3)
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