EcoModder Forum How do you perform a coast down test

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 06-11-2008, 06:23 PM #1 (permalink) EcoModding Apprentice   Join Date: Jun 2008 Location: Waikele, Hawaii Posts: 108 88CRX - '88 Honda CRX Standard DX 90 day: 32.74 mpg (US) IS250 - '07 Lexus IS250 90 day: 25.79 mpg (US) RX300 - '00 Lexus RX300 90 day: 18.38 mpg (US) Thanks: 0 Thanked 5 Times in 5 Posts How do you perform a coast down test How do you a coast down test to find out Aerodynamic Cd? I'll guess the following: 1) Car in Neutral 2) Performed on a flat road, so weight will not factor in (on an incline, if you have a heavy car it will not slow down as fast as a light car). You can technically do it in an incline but the angle must be factored in. What speed do I start and end the clock? It should be faster speed, because at low speeds, the percentage of mechanical energy loss will be a larger, because the affects of Aerodynamic Cd is less at lower speed. So I'm guessing start at 80mph down to 60mph. Given the mass of car, speed, and time to Aerodynamic Cd, what equation do i use to convert to Cd. I'm guessing there will be a factor to compensate for Mechanical losses. __________________ Last edited by 88CRX; 06-11-2008 at 06:31 PM.. Reason: revise wording to clarify
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Quote:
 Originally Posted by aerohead I paid CAR and DRIVER Magazine engineers to test my CRX at the Chrysler Proving Grounds and they kept track of all the worms.
I've wondered what your Cd was on that car.

So you paid to use the Chrysler Proving Grounds ??

Big ... big bucks !
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Last edited by Cd; 09-17-2008 at 09:30 PM..

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Chrysler Proving Grounds

Quote:
 Originally Posted by .Cd I've wondered what your .Cd was on that car. So you paid to use the Chrysler Proving Grounds ?? Big ... big bucks !
After Bonneville,in 1991,I wanted some independent verification for the drag on the CRX. Glen Scharf,at GM Aero Lab,told me over the phone,that if my highway mileage had gone from 52mpg,to 60.6mpg,and nothing had changed on the car,but the shape,that basically I was looking at Cd 0.23.
The top-speed figures gave me Cd 0.24.Pretty close agreement. I called up to CAR and DRIVER in Ann Arbor,Michigan,and they told me that if I could get the car up to them,that one of their guys could run it through the ringer for me.We agreed to a date,and in October,1991, I drove it up there to Hogback Road. CAR and DRIVER has access to the Chrysler Proving Grounds,at East Chelsea,Michigan,for testing,and we took the car out there and did top-speed and coastdown tests.C and D ran the data-reduction for me,and mailed me a report with the results. I'm going to post all the data as a "case-study",showing the before and after numbers.Looking over the data set,I found something that I wanted to bounce off CAR and DRIVER,have emailed them with the data,and am waiting to hear back. They're awfully busy getting their publication out each month,so I must wait 'til it is convenient for them.I'll post when I can.Hope that answers your question.
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 06-26-2008, 01:22 AM #5 (permalink) Ultimate Fail     Join Date: Feb 2008 Location: Austin,Texas Posts: 3,037 Thanks: 2,261 Thanked 759 Times in 441 Posts Thanks Phil. So how do you feel about being one of the first people out there to Ecomod your car and have it get media attention ? __________________ "You don't fail until you give up. You learn, and move on." - Christ " Hypermiling is not going slow, it's driving with more intelligence." - UFO
 06-26-2008, 01:45 AM #6 (permalink) EcoModding Apprentice   Join Date: Jun 2008 Location: Waikele, Hawaii Posts: 108 88CRX - '88 Honda CRX Standard DX 90 day: 32.74 mpg (US) IS250 - '07 Lexus IS250 90 day: 25.79 mpg (US) RX300 - '00 Lexus RX300 90 day: 18.38 mpg (US) Thanks: 0 Thanked 5 Times in 5 Posts Thanks for the info. do you have the equations and constants you used? What was done to your car? __________________
 09-15-2008, 02:09 PM #7 (permalink) EcoModding Apprentice     Join Date: May 2008 Location: North Texas Posts: 112 Thanks: 0 Thanked 2 Times in 2 Posts Coastdown calculator for CD. You will need to know your vehicle weight, frontal area plus temperature & baro pressure. To negate winds and road grade, suggest you test twice going opposite directions. coastdown.xls
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attention

Quote:
 Originally Posted by Cd Thanks Phil. So how do you feel about being one of the first people out there to Ecomod your car and have it get media attention ?
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equations

Quote:
 Originally Posted by 88CRX Thanks for the info. do you have the equations and constants you used? What was done to your car?
I do have an SAE paper which contains the formula they used for some coastdowns conducted near South Base,Edward's Air Force Base,California.in the 1970s.They may cite an earlier investigation,which has become the standard protocol.---------------- I believe there has been reference made to some on-line programs,where you can plug in your scatter-plot of velocity/time data points and other test data,and it will spit out your car's Cd.I've never used it,and the Lake Ray Robert's Dam HWY-455 that I've used for past coastdowns,now has so much traffic,I'm reluctant to even consider future coastdowns.I need an abandoned runway as SAE used at Edward's.------------- Also,there is published, a" Bosch Manual",which may contain all the dynamic test procedures.I've never seen the thing.It's about \$100.00(US) I understand it to be a fabulous reference.----------------------------- If you like,I'll dig out what I have and make you a copy and mail it.Perhaps you could post the portions that might be of benefit to members.This is the part thats difficult for me.Let me know.
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what was done

Quote:
 Originally Posted by 88CRX Thanks for the info. do you have the equations and constants you used? What was done to your car?
The mods to the CRX was: 6-inch streamlined over-fascia with integral grille-block,front chin spoiler(modified from a VOLVO airdam),front wheel-well gap-fillers,rocker panel extensions,hatch-wing extension,12-inch boattail,full rear wheel skirts, MOON wheelcovers,and segmented bellypan.------------- There are a couple of photos of the car in the phil knox aerodynamic photos album.

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