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Old 06-11-2008, 06:23 PM   #1 (permalink)
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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.

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Last edited by 88CRX; 06-11-2008 at 06:31 PM.. Reason: revise wording to clarify
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Old 06-12-2008, 03:17 PM   #2 (permalink)
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coastdowns

I won't even try 'em anymore.The wind hasn't stopped blowing in Texas since Hurricane Katrina.I think some of the members have access to a software program to reduce the data.You need a very flat stretch of road,with no traffic,fully-warmed-up-car,and do a minimum of twenty runs,ten in each direction,to cancel out any effects grades might present.There cannot be any wind.NONE!!!!!!!! The gas tank ( diesel ) has to be topped-off before every run,as mass and inertia have tremendous impacts on the coast-downs.You must have complete weather data for the entire test period,as air density and its effect on air drag can vary during the test.Dry-bulb temp,wet-bulb temp,and barometric pressure must be in the bag.You can use a psychrometric chart from an air conditioning company to determine your local air density ( in slugs for US units) by dividing your air's specific weight by the gravitational constant ( 32.2 Ft per second per second ).Test speeds are typically run,coasting down from 70-mph,to 20-mph.You can use a cassette-recorder,calibrated to a stop-watch to record velocities,at one-mph-intervals.Later,you can replay the tape,listen to yourself yelling speeds at the recorder and construct your velocity/time curves on graph-paper.How the vehicle coasts at the lower velocities, establishes your rolling resistance force coefficient,which is fairly constant from 20-to-70,necessary to deduct from your power-curve,to reveal your aero drag component.Newton's F=MA is basically at work here,as it requires so much power( force per unit time ) to alter the momentum of the vehicle from any point to another point.By knowing the mass of the vehicle,air density ,and frontal area ( can be estimated ) ,your drag coefficient will fall out of the equations after the rolling resistance is factored out.I never did it with software,as P.C.s were pretty rare back then.I paid CAR and DRIVER Magazine engineers to test my CRX at the Chrysler Proving Grounds and they kept track of all the worms.It was a bargain! Good luck on that one!
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Old 06-13-2008, 02:33 AM   #3 (permalink)
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Quote:
Originally Posted by aerohead View Post
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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Old 06-14-2008, 02:32 PM   #4 (permalink)
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Chrysler Proving Grounds

Quote:
Originally Posted by .Cd View Post
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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Old 06-26-2008, 01:22 AM   #5 (permalink)
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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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Old 06-26-2008, 01:45 AM   #6 (permalink)
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Thanks for the info.

do you have the equations and constants you used?

What was done to your car?
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Old 09-15-2008, 02:09 PM   #7 (permalink)
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Smile

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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Old 09-15-2008, 05:34 PM   #8 (permalink)
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attention

Quote:
Originally Posted by Cd View Post
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 ?
Cd,sorry for the late response.Ummmmmmmmm your question about the media is a good one,I'm not really sure how to respond.I can't take much credit,as many came before me and have been lost to antiquity.As Sir Issac Newton explained once,"I have merely stood on the shoulders of giants".Or something like that.That's how I feel.------------------- Also,I think the media gets hungry for stories and looks for human-interest stuff with wide appeal(lots of people purchasing fuel!).Some of the journalists that did pieces,I knew through friends,aquaintances,or associates,so it's not like Walter Cronkite chased me down for a story.--------------------- I've felt for many decades now, that everyone should be introduced to these concepts,as,I don't know how we'll ever dig ourselves out of the present energy situation without it.----------------------- I was fortunate to meet and visit with Alex Tremulis and wife at his home,the year before his passing.I had just come from Bonneville,and even though Alex' eyesight was failing,he could see what I'd done to the CRX and gave me the big thumbs up! It meant alot, as you should have seen some of the models laying around his home that he'd proposed.He had achieved 100-mpg with a Subaru-powered streamliner,and had been working towards 200-mpg.Alex had a very fertile mind and was a super-advocate of aero efficiency.--------------------------- We're all indebted to both John Gilkison and Jessica Savage who made the original EV World article possible,which has made a connection to,and seems to continue to bring people into the modding network.My work was simple compared to theirs.
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Old 09-15-2008, 05:51 PM   #9 (permalink)
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equations

Quote:
Originally Posted by 88CRX View Post
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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Old 09-15-2008, 06:00 PM   #10 (permalink)
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what was done

Quote:
Originally Posted by 88CRX View Post
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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