Coastdown test for an aeromodded 1998 Ford Escort ZX2: Determining Cd & Crr

[BabyDiesel]

Originally Posted by dirtydave

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?

My brother rode along and took time for me. He started the timer @ 55 mph and would say 8...9...10. At the 10 I would call out my speed and he would record it.

I definitely think a full boat tail will help. 0.269 Cd is still not considered low drag to me, even with my numbers approaching Prius territory. I want <.20 Cd and I am discussing with Aerohead about getting the ZX2 to 0.15.

A lot of my work has been quick and sloppy, honestly. I want to go back and refine all I can with better materials and with greater precision. My rear belly pan does not have the correct angle for the lowest drag and I need a middle section pan.

My biggest gains are going to come from the engine and transmission. The ZX2 is too deeply geared IMO to maximize the potential of a low Cd. The load on the motor will have to increase in order to reach a maximum BSFC, and I have three ideas regarding how to get there: Higher gears, lean burn w/ turbocharging and having an late intake opening (think Atkinson cycle).

[BabyDiesel]

Originally Posted by freebeard

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.

You are correct freebeard, the difference in A and B is the direction of travel. I will comment on the 7th run in a new post. My brother did the data keeping and timing

[BabyDiesel]

Originally Posted by NeilBlanchard

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.

No bias with my brother. He kept an eye on my speed and did the timing. I have no way of knowing the coefficient of drag of my tires. I have searched Google and nothing turned up The are Chinese made, regular car tires, so I knew the Crr would be between 0.01 & 0.015 from researching typical Crr's Mine came out on the low side.

[BabyDiesel]

Originally Posted by aerohead

Don't forget
*Passenger side mirror delete
*Coroplast wheel covers

Thanks for catching that aerohead! I did not have the coroplast wheel covers on during the testing though. I may have had a crack at breaking 0.26 if they were on

[Cd]

How long did the tests take ? I would think several hours.

I'm also curious about what people were thinking. Not that it matters, but can you imagine the comments ? :-)
" Oh look - he forgot something again. "
" What did he forget this time ? "
" He must have dropped something valuable out the window there - he keeps going back and forth ! "

[BabyDiesel]

Originally Posted by Cd

How long did the tests take ? I would think several hours.

I'm also curious about what people were thinking. Not that it matters, but can you imagine the comments ? :-)
" Oh look - he forgot something again. "
" What did he forget this time ? "
" He must have dropped something valuable out the window there - he keeps going back and forth ! "

Here is the time it took Cd - Time: 10:44 a.m. - 11:17 a.m. It was hidden in the 1st post. Shockingly short huh? I wasted no time!

Now if I add the time to compile the data into the spreadsheet and zero out the graph, then it took over 3 hours to get the finished results

Haha! Truthfully, I was wondering if anybody was watching and their thoughts. The chances were high for the police to get called on me! I drove past the same houses 7 times, and people start getting spooked when someone does that.

Suspicious scientist is suspicious

[BabyDiesel]

I have talked of the 7th run and not said anything about it. This run was to test both driver and passenger windows down halfway and their effect on the overall Cd. I run without A/C, so it is common for me to crack the window a little bit so I don't melt

However, what if I took it to an extreme for me? It irks me to have one window down halfway when I am up to speed. But am I just psyching myself out?

Let's see the data!

Run #7 Averages of the previous 6 runs
t=0 - 55 \ 55
t=1 - 48 \ 49.5
t=2 - 44 \ 44.5
t=3 - 39 \ 40
t=4 - 35 \ 36
t=5 - 31 \ 32.5
t=6 - 28 \ 29

All bias aside. Having both the windows down had an overall negative impact on coasting. However, the impact was extremely small! The speed of coasting with both windows up was equal to the coasting speed when both windows were down halfway, on occasion. The #7 run averages 1.25 mph at all data collection points during the test in comparison to the average of the previous 6 test.

What you can take away from this - as long as you are not a hardcore hypermiling stickler such as me, pushing limits and trying to squeeze every mist of fuel as efficiently as possible, then I doubt you will notice a difference in the real world having your windows down partially

Edit - There is no graph for the #7 test. I would need more runs to get an honest representation of the effect, and I only did one. Sorry y'all!

[Cd]

I'm not good at understanding math and such. I'm much more visual.
Even something as simple as your charts makes no sense to me.... but i wish i understood.
One thing i would like to know is if you started the test by adding in the factory quoted Cd figure.
If so, how do people know what variable to use when they don't know their cars original Cd ?
It would be interesting to see if a test would show the car as having the factory quoted Cd if you removed all your mods and retested.

[freebeard]

Since you are thinking about a boattail, take a look at http://ecomodder.com/forum/showthrea...tml#post481536. He's shedding parts on the highway. BamZipPow has also run aground lately. I'm not a big fan of coroplast and duct tape.

Look for a build quality that would pass tech inspection at a race track. I just got a good part for an airdam on my car. It's the 'towel rack' bumper overrider from the American spec 67-down Beetle front bumper (cut down), and it will hang upside-down under the later bumper with tabs on the bumper brackets. I've got steel sheet for the diverter.

My goal is to get Prius-like mpg, starting with a 0.40 Cd. But first it's off to the Woodburn Drag Strip for the NW Bugrun on Saturday. Oops, that's today.

[BabyDiesel]

Originally Posted by Cd

I'm not good at understanding math and such. I'm much more visual.
Even something as simple as your charts makes no sense to me.... but i wish i understood.
One thing i would like to know is if you started the test by adding in the factory quoted Cd figure.
If so, how do people know what variable to use when they don't know their cars original Cd ?
It would be interesting to see if a test would show the car as having the factory quoted Cd if you removed all your mods and retested.

I'm sorry that you are having a difficult time understanding the data, Cd. Is there anything I can do to help? I know you are not the only one who is having difficulty, you're just the only vocal one. I don't have an Education degree for nothing

I input my original Cd figure of 0.33 into the input location and it was waaayyy off. So I started going down: 0.3, 0.29, 0.28 and so on. I would watch the Sum of Errors^2. This showed the total deviation of the pink line in comparison to my data points, squared. When this was minimized as far as I could get it (0.005 something!), then I knew I was pretty spot on

I can used my points that were plotted from my coast-down test and input the old Cd number back in to show the difference. The pink line would show the 0.33 Cd deceleration compared to the 0.269 Cd plot points of now.