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Old 08-20-2019, 09:11 PM   #1 (permalink)
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Coast down testing

Is there a spreadsheet on the forum somewhere that has calculations for coast down testing / rolling resistance?

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Last edited by modified metro; 08-21-2019 at 10:09 AM..
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Old 08-21-2019, 08:26 AM   #2 (permalink)
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Yes - this was posted here on EcoModder a long time ago, and I have used it, and I think others have to.

https://www.instructables.com/id/Mea...t-of-your-car/

I used my phone to video tape the speedometer in KPH, and did the timing by watching the video later. This is much easier and safer and more accurate, than trying to do it while in the car.
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Old 08-21-2019, 10:08 AM   #3 (permalink)
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Thank you for the response.

I had a freind that helped me with coast down testing once. I did the test runs in each direction, just as you suggested. Recording it and then reviewing it later. He had a speadsheet and did the calcuations. I don't have that option at this time.

I have just started to try some basic aero mods to help improve my mpg and want to get a base line. Recently put a air dam on the front of my metro.
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Old 08-21-2019, 11:16 AM   #4 (permalink)
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coastdown

The official protocols are found in the Society of Automotive Engineer's (SAE) Handbook.
The mathematics runs to about 2-1/2 pages.
The allowed tolerances for the test track,actual test procedures,data recording resolution,continuous weather monitoring,and data reduction,I believe,is beyond the scope of the 'hobbyist,'
I baled out and hired CAR and DRIVER to do it for me,at the Chrysler Proving Grounds.They have a dead-flat straightaway track, which is dedicated for this sort of testing,hidden inside a forested area,protected from wind.There's no traffic,as it's a controlled area.
They have the the 4-wheel scale weighing system required.
Car and DRIVER provided test driver,the optical 5th-wheel data acquisition hardware,data storage/retrieval,and data reduction computer program,plus an onsite weather station.
To accurately test a vehicle would require the disassembly and independent measurement of every rotating component in the vehicle for polar moment of inertia.A daunting proposition.
Some years,there can be as little as 7-days/year, when the weather is agreeable enough to do the test.
You can do some things to experience 'trends',but even the automakers have abandoned coastdowns for the wind tunnel or CFD,where they can completely control the environment.
Just sayin'
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Last edited by aerohead; 08-21-2019 at 11:17 AM.. Reason: spell correct
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Old 08-21-2019, 08:00 PM   #5 (permalink)
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That^.

Unfortunately, despite its complication coast down testing is still probably the most viable means of aerodynamic testing for enthusiasts like us. The instructable is a very approximated version, so it will only give you a general idea of drag coefficient and rolling resistance coefficient. Some things you could do to improve accuracy:
-following SAE J1263, do it on a day with wind speeds <10 mph and 10 runs total (5 in each direction) for any one configuration
-take a photo of your car from a distance and get an accurate cross-sectional area measurement rather than estimating
-weigh it on a truck scale and get a reasonably accurate curb weight rather than estimating
-use as flat a road as you can find, throw out any runs with traffic interference and redo them
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Old 08-21-2019, 11:35 PM   #6 (permalink)
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Quote:
Originally Posted by Vman455 View Post
That^.

Unfortunately, despite its complication coast down testing is still probably the most viable means of aerodynamic testing for enthusiasts like us. The instructable is a very approximated version, so it will only give you a general idea of drag coefficient and rolling resistance coefficient. Some things you could do to improve accuracy:
-following SAE J1263, do it on a day with wind speeds <10 mph and 10 runs total (5 in each direction) for any one configuration
-take a photo of your car from a distance and get an accurate cross-sectional area measurement rather than estimating
-weigh it on a truck scale and get a reasonably accurate curb weight rather than estimating
-use as flat a road as you can find, throw out any runs with traffic interference and redo them
That is what I did the last time I did a coast down test, except I only did 3 runs in each direction.

I use the national weather service to find a day when winds are calm.

We have a truck scale at work tha I can use to get the weight to within 20 lbs.

I know of a pretty flat road to do the coast down test on.

The cross sectional measurement I can probably get from the last coast down test I did.

This is a copy of the last coast down test I did.

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Old 08-22-2019, 02:16 AM   #7 (permalink)
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Quote:
Originally Posted by aerohead
at the Chrysler Proving Grounds.They have a dead-flat straightaway track, which is dedicated for this sort of testing,hidden inside a forested area,protected from wind.
Quote:
I know of a pretty flat road to do the coast down test on.
What is the environment? Wind conditions can vary minute by minute. Try just before dawn.
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Old 08-22-2019, 09:54 AM   #8 (permalink)
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Quote:
Originally Posted by freebeard View Post
What is the environment? Wind conditions can vary minute by minute. Try just before dawn.
It is a small 2 lane road on the out skirts of town. It is a industrial area, with a few buildings and in some areas some woods.

I try to do it after work, since there is less traffic. I work 2nd shift and it is kinda not a option to do it at dawn.
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Old 08-26-2019, 01:44 PM   #9 (permalink)
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Saturday after work it was a calm night. So I went out to the road that I do coast down testing on. There was little to no traffic on the road while doing the runs.

I did 3 each direction, and recorded it on my phone.

I filled out the info on the spreadsheet and I'm not sure of the results if I may have not done some thing coreect or what. If any of you have used it before and see something I am not aware of please let me know.

The results came out of a CD of .368 which is higher than what the factory figure was on .34.

I did try adjusting the CD and CRR numbers to make sure the line on the graph corresponded to the dots on the graph, that part looks correct

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Old 08-26-2019, 05:13 PM   #10 (permalink)
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The factory wind tunnels vary between themselves by a similar amount.

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