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Old 04-24-2011, 05:02 PM   #1 (permalink)
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Calculating Drag Coeffiecients Formula

I'm very impressed by the lists I've been able to obtain regarding factory vehicles. But what is the formula that is used/measurements that are taken? I'd like to calculate my car's drag coefficient POST-modification.

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Old 04-24-2011, 06:26 PM   #2 (permalink)
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What lists are those?
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Old 04-24-2011, 09:29 PM   #3 (permalink)
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This list:Vehicle Coefficient of Drag List - EcoModder
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Old 04-24-2011, 10:36 PM   #4 (permalink)
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You will have to do a coast down test. Here's the Instructables page and spreadsheet I used:

Measure the drag coefficient of your car

I found it much easier to set my Scan Gauge II to KPH and video record it during the coast down -- when it hit 70kphm pause, it look at the time, and let it play for 10s and pause it, look at the speed, etc.
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Old 04-25-2011, 07:10 AM   #5 (permalink)
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Good tips

Quote:
Originally Posted by NeilBlanchard View Post
You will have to do a coast down test. Here's the Instructables page and spreadsheet I used:

Measure the drag coefficient of your car

I found it much easier to set my Scan Gauge II to KPH and video record it during the coast down -- when it hit 70kphm pause, it look at the time, and let it play for 10s and pause it, look at the speed, etc.
Thanks for those tips, Neil. I'm planning to do the video thing as I get around to field testing. With $4/gal gas I don't want to burn any more fuel than necessary so wringing the best data I can from each test is a goal. Amplified readings in km/hr never occurred to me but now that you mention it the added "sensitivity" seems value added for test purposes.
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Old 04-25-2011, 09:23 AM   #6 (permalink)
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Thanks Neil,

I'll attempt this sooner than later.
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Old 04-25-2011, 01:03 PM   #7 (permalink)
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The spreadsheet provided on that Instructables page is set up for KPH, so I just changed the ScanGauge II units for the coast down tests, so I didn't have to change the spreadsheet or do the math to convert anything.

I taped my camera in place to view the ScanGauge so I only had to start recording after getting up above 70kph and keep recording until the car slowed down below the minimum speed. If you have sound on the video, just say the particulars like "Run 3 going west" etc.

Also, you should do a set of runs *before* any mods, so you can calibrate the tire rolling resistance -- assuming you know the "factory" Cd for your car. Then when you do the runs with your aero mods, just keep the tire rolling resistance that worked best for the stock runs.
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Old 04-25-2011, 05:29 PM   #8 (permalink)
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sheesh, this seems like a lot of work.
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Old 04-25-2011, 06:09 PM   #9 (permalink)
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yeah, and you really need to do it both before and after to get anything even slightly meaningful. i have done it, but it was a lot of work, like you said.
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Old 05-04-2011, 03:22 PM   #10 (permalink)
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Cd

If you will establish the baseline mpg performance of your car at a constant velocity,say 55-mph,after you mod the car you can deduce your new drag coefficient from your new mpg figures.
A 10% drag reduction will net a 5% improvement in mpg at a constant 55 mph.
The only variable you can have is the shape of your car.

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