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Old 05-28-2015, 01:39 AM   #1 (permalink)
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Thumbs up Coastdown test for an aeromodded 1998 Ford Escort ZX2: Determining Cd & Crr

*Test performed on 05/27/2015*

I was able to get the ZX2 out to do some coastdown testing yesterday morning! This test was to determine the drag coefficient (Cd) and the coefficient of rolling resistance (Crr) of the Ecoscort, a 1998 Ford Escort ZX2. I am using the "measure your drag coefficient" at Instructables.com.

The conditions are as follows: Start/End

Time: 10:44 a.m. - 11:17 a.m.
Temperature: 77*/79*
Dewpoint: 72*
Relative Humidity: 83%/79%
Winds: WSW @ 3/WSW @ 9
Elevation: 197 feet
Air Density (rho): 1.156 kg/m^3
Engine Coolant Temperatures: 205* - 220*
Tire psi: 70 front, 60 rear
Weight: ~2500 pounds

The car has the following aerodynamic modifications:

85% grill block
Covered foglight holes
Windshield wiper cover
front & rear belly pans
Smooth 14x4 space saver wheels
Rear wheel skirts
Driver side and passenger side mirror delete

Stock Cd for an Escort ZX2 is 0.33, stock frontal area is 20.8 square feet, and the stock CdA is 6.9 square feet.

The test was a series of 6 runs in ABABAB fashion. A 7th run was done to test a common effect on aerodynamics(will say more at the end). The road I was testing on was surrounded by trees, thereby negating much of the effects of the light winds. All test were started at 55 m.p.h. and coasted in Neutral for 60 seconds, recording the speed every 10 seconds.

t=0 is when time was started. It is always at 55 m.p.h. t=1 is 10 seconds, t=2 is 20 seconds in, and so forth.

Run #1
t=0 - 55
t=1 - 49
t=2 - 44
t=3 - 40
t=4 - 36
t=5 - 33
t=6 - 28

Run #2
t=0 - 55
t=1 - 50
t=2 - 45
t=3 - 40
t=4 - 36
t=5 - 32
t=6 - 30

Run #3
t=0 - 55
t=1 - 49
t=2 - 44
t=3 - 39
t=4 - 36
t=5 - 33
t=6 - 30

Run #4
t=0 - 55
t=1 - 50
t=2 - 45
t=3 - 41
t=4 - 36
t=5 - 33
t=6 - 29

Run #5
t=0 - 55
t=1 - 49
t=2 - 44
t=3 - 40
t=4 - 36
t=5 - 32
t=6 - 29

Run #6
t=0 - 55
t=1 - 50
t=2 - 45
t=3 - 40
t=4 - 36
t=5 - 33
t=6 - 29

Very consistent!

On the Instructables website, an Excel document is given. The top part is simply plug in your values.



The bottom part is where things get fun! You are in charge of figuring out your Cd and Crr. The objective is to get the line as close to the dots on the graph as possible. The Sum of Error^2 will show how close you are to matching. The lower, the better.



According to my calculations, my new Cd is an amazing 0.269! while my Crr comes in at exactly 0.01. The Crr number is a typical value for a non-LRR car tire's rolling resistance. My coefficient of drag has dropped over six hundredths with only the few mods listed, and I have not yet anything to the rear of the car except for the rear belly pan.

To put it in perspective, a 2nd-gen Toyota Prius has a Cd of 0.26 and the 1st-gen Honda Insight has a Cd of 0.25

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Past threads:
ZX2 modding thread
Ecomodder's Top 10: How they do it!
ZX2 Aerodynamics: Shooting for 0.15 Cd
ZX2 coast-down testing for Cd & Crr

Last edited by BabyDiesel; 05-29-2015 at 05:13 PM..
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Old 05-28-2015, 10:49 PM   #2 (permalink)
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Here are pictures of the modifications done to achieve the lower drag:

Wheels & Tires



Wiper Cover



Rear Wheel Skirts



Rear Belly Pan



Front Belly Pan



Grill Block



Foglight Covers

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ZX2 modding thread
Ecomodder's Top 10: How they do it!
ZX2 Aerodynamics: Shooting for 0.15 Cd
ZX2 coast-down testing for Cd & Crr
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Old 05-29-2015, 12:08 AM   #3 (permalink)
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Quote:
Originally Posted by BabyDiesel View Post

The bottom part is where things get fun! You are in charge of figuring out your Cd and Crr. The objective is to get the line as close to the dots on the graph as possible. The Sum of Error^2 will show how close you are to matching. The lower, the better.
Even after all these years, I still haven't figured out how to use that Instructibles Excel sheet. I'll come back to this thread for questions
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Old 05-29-2015, 12:44 AM   #4 (permalink)
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90 day: 14.97 mpg (US)

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I'll be more than glad to help you out jedi-sol!
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Past threads:
ZX2 modding thread
Ecomodder's Top 10: How they do it!
ZX2 Aerodynamics: Shooting for 0.15 Cd
ZX2 coast-down testing for Cd & Crr
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Old 05-29-2015, 05:23 AM   #5 (permalink)
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For a moment there, I thought you left the front wheels un-covered ( that first pic you posted )

Congratulations !

So what has been done about the rear of the car - mainly the edges of the boot/trunk ?
Stock, that car has a very rounded rear*
Did you get around to installing a trip strip or a small spoiler ?

Also, was the front air dam installed at the time of the test ?

( *Where else but EM can you talk about 'rounded rears' with a straight face ! )
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Old 05-29-2015, 07:17 AM   #6 (permalink)
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Quote:
Originally Posted by Cd View Post
For a moment there, I thought you left the front wheels un-covered ( that first pic you posted )

Congratulations !

So what has been done about the rear of the car - mainly the edges of the boot/trunk ?
Stock, that car has a very rounded rear*
Did you get around to installing a trip strip or a small spoiler ?

Also, was the front air dam installed at the time of the test ?

( *Where else but EM can you talk about 'rounded rears' with a straight face ! )
Bahahaha!!! Only at Ecomodder could we talk about these things that would make old ladies faint if they walked in at the wrong time

Uhhhhhh..... That is because they were uncovered I did not like the small gap in between the cover and the wheel as the covers cannot lay flat against these wheels. So I left them off. In order to avoid any further ridicule, they will be going back on today

I have done nothing to the rear, sadly. I have not had the time since I have been trying to figure out front wheel skirts. I am wanting to install a Kammback and sharpen the edges of the bumper to promote cleaner separation. I am going to try a trip strip on the A-pillars soon and see what that is all about. Eddie25 posted something about one recently and said something happened, I can't remember.

And the air dam was not installed either! I tested the car with it looking like this:



The rear hatch was down BTW
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Past threads:
ZX2 modding thread
Ecomodder's Top 10: How they do it!
ZX2 Aerodynamics: Shooting for 0.15 Cd
ZX2 coast-down testing for Cd & Crr
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Old 05-29-2015, 09:16 AM   #7 (permalink)
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did you have a friend ride along?

do you think a boat tail will do much,for you even with the low speeds you drive and the Aero already in place?
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Old 05-29-2015, 10:59 AM   #8 (permalink)
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Quote:
The test was a series of 6 runs in ABABAB fashion. A 7th run was done to test a common effect on aerodynamics(will say more at the end).
What is the difference between A and B? The direction of travel? When will we find out about the 'one weird trick' on run #7?

I'm with dirtydave, having different persons driving and timing would eliminate a source of bias.
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Old 05-29-2015, 12:28 PM   #9 (permalink)
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I did a coastdown test on my Scion, and my method of data collection worked well, I think: I video recorded the SG II set to KPH. I paused the video at the instant the car reached the 70KPH, noted the time, moved 10s forward, and noted the speed.

There is no bias.

The challenge with this is to "know" the Crr in order to get the estimate for the Cd.
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Old 05-29-2015, 05:02 PM   #10 (permalink)
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Don't forget
*Passenger side mirror delete
*Coroplast wheel covers

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