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View Poll Results: Would you be interested in an Android App for measuring the Cd of your vehicle
Yes- as you've described it 13 54.17%
Yes- but some mild changes needed 2 8.33%
No- Major changes needed to interest me 0 0%
No- not interested 2 8.33%
Yes but owns a non Android phone (iPhone, windows, other) 7 29.17%
Voters: 24. You may not vote on this poll

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Old 03-03-2012, 01:59 PM   #1 (permalink)
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Any interest in Vehicle Drag Calculator? (Android App)

I'm doing a project for a comp sci class and am wondering if anyone would be interested in a program for their Android phone (or tablet) that would help calculate the drag coefficient for their vehicle? It would involve inputting the weight of the vehicle and running a couple tests on flat road. The app would check your location for weather (wind and pressure and such) and get your altitude. After running the test a few times (for more statistically significant values) the app would then calculate an approximation for the coefficient of drag for your vehicle based on the tests and weather information gathered earlier. It would be similar in nature to the Aerodynamic & rolling resistance calculator but in a mobile format that collects data for you. All you have to do is hit some buttons and drive around.

This would be a useful app for determining the effect of aerodynamic mods that you've done to your vehicle if you do tests before and after. For instance, I built an aero-shell for my Tacoma similar to the one built by Bondo, and would be interested in quantifying the aerodynamic benefits from putting this shell on.

I have been considering doing a "lite" and "pro" version of the app in which the lite version you would input the vehicle mass, do a weather check (just air pressure), do a (possibly limited) number of "high speed" tests, and get the estimated Cd for your vehicle. The pro version of the app would include the features from the lite version and add "low speed" tests to see the effects of rolling resistance, also take into effect wind, changes in elevation and possibly tire type/pressure for more accurate measurements.

If you have a different phone or have suggestions for changes please leave them in the comments below.

I'm open to questions, comments, suggestions, motivational pictures, et c.


Last edited by ajohn7211; 03-03-2012 at 10:43 PM..
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Old 03-03-2012, 03:18 PM   #2 (permalink)
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That's a great idea. I have a bb so it won't work for me but i'm sure lots of people would be interested since it would be very accurate when you conect it to gps
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Old 03-03-2012, 03:33 PM   #3 (permalink)
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Ditto,
I'm a blackberry guy but love the concept.
I did buy a sophisticated laptop obdII software but havent mastered it.....
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Old 03-03-2012, 07:32 PM   #4 (permalink)
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i have an iphone.

you need to be able to correct to STP or SAE conditions.

I'm sure you'll get class credit for any app, if you want an app that actually works, read SAE#810186. I discovered that the way people "calculate" Cd through coast tests is very flawed. I have this paper, and was preparing to make a thread about it soon.
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Old 03-03-2012, 07:44 PM   #5 (permalink)
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Quote:
Originally Posted by ERTW View Post
i have an iphone.

you need to be able to correct to STP or SAE conditions.

I'm sure you'll get class credit for any app, if you want an app that actually works, read SAE#810186. I discovered that the way people "calculate" Cd through coast tests is very flawed. I have this paper, and was preparing to make a thread about it soon.
I'm aware its flawed because at highway speeds drag can account for a wide range of the percent of overall drag experienced by the vehicle and accounting for that is tricky. Thank you for the paper. I'll take a look at it here soon and hopefully can get something figured out.


And could I possibly get access to that SAE paper? If you have a digital copy or something that would help.

And unfortunately I'm on Linux so I can't develop for iPhone just yet.

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Old 03-04-2012, 02:50 AM   #6 (permalink)
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[QUOTE=ajohn7211;290883]
Quote:
I'm doing a project for a comp sci class and am wondering if anyone would be interested in a program for their Android phone (or tablet) that would help calculate the drag coefficient for their vehicle?
Great idea. I've got an Android and would use one version or the other.
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Old 03-04-2012, 07:04 AM   #7 (permalink)
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I'm on Android ( using it now in fact ).
I'm really glad you are making an Android app, since a lot of cool apps are iphone only.

I would buy the Pro version in a flash.

A couple of questions though : would this require an ODB II or newer car to work ( 1996 - current year )

I'm assuming the Cd figure is of coarse going to be a rough guess.
As you know I'm sure, there are a lot of things that affect a coast down tests accuracy that just can't be simulated without real world data.
Wind speed, frontal area, rolling resistance, interference drag ( I think that's what it'd called ) a.k.a. the drag from overtaking vehicles, or vehicles in the lane beside you., barometric pressure, hill grade, and so on.

Of those things I mentioned above, I can see how that you could input an accurate measurement for frontal area, approximate hill grade ( using elevation data from Google maps ) and perhaps even a rough estimate of the barometric pressure in the testing area, but how do you plan to tackle the wind speed problem ?

A huge thank you for doing this !
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Old 03-04-2012, 07:18 AM   #8 (permalink)
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I'm assuming that all the data information will be customizeable by the user.
In other words, the user can input their own data such as wind speed, frontal area, barometric pressure, rolling resistance, weight, hill grade etc.
This would allow the user to input more accurate data rather than going off of information from , for example, the barometric pressure that is posted online at the time from an airport nearby.

I would now have an excuse to purchase the portable weather station I've been looking at for years for just this purpose !

Shop AcuRite Wireless Digital Weather Center Thermometer at Lowes.com
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Old 03-04-2012, 10:28 PM   #9 (permalink)
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I'll just post some information here, instead of creating a new thread.

Wind tunnel testing is steady state, whereas coast down tests are in transient conditions. There's a ~1% difference between acceleration/deceleration runs and constant speed runs. Wind tunnels have the problem of localised turbulence. Cd and friction are not constant, however, he assumes that they are quadratic. Drag force is quadratic when Re is above 5x10^6 (roughly 60 kph).

There are some big caveats. He coasts one kilometre with measurements taken every 100 m (double integrate a*dt using the smart phone accelerometer). He ran the cars up to 130 kph down to 50 kph. If there is a head wind over 0.5 m/s (1.8 kph), or a cross wind causes a yaw greater than 5* the data point is rejected. Everything must be up to operating temp, so that friction changes minimally with each run.

"coast down tests give reliable results if the number of tests is adequate for a statistical analysis." He did 35-45 runs. Morelli saw between 0.22 and 1.56% variation from wind tunnel Cd figures (44 and 35 runs, respectively). I can't imagine how accurate 3 runs would be...with a cross wind...on a grade

the basic governing equation is:

-m' dV/dt = A + BV^2

m = momentum
V = velocity
t = time
A = coefficient of the resistance - constant term
B = coefficient of the resistance - quadratic term

A = Wsin(g) + R

W = vehicle weight
g = the constant slope of the road
sin(g) accounts for the component of weight in the direction of motion. hilly terrain needs a d/dt
R = mean resistance: tires and driveline (found from analysis)

B = Cx'*1/2*rho*S
C'x = 2B/rho*S

rho = air density = sqrt(gRT) plus correction for relative humidity
S = frontal area

Cx' = Cx + Cr + L/ro Sum (i=1 to n) Cwi

Cx' = mean drag coefficient (found using statistical analysis)
Cx = drag coefficient (calculated)
Cr = rolling resistance coefficient, and includes wheel induced drag (measured with a special rig)
Cwi = ventilation resistance coefficient of the rotating parts (measured on a dyno)

The car needs to coast over a long enough time interval (about a minute) so that we can safely assume that most of these factors are independent of time.

the L/ro... term ranged from 0.01165-0.0119 and his cars had fairly closed wheels. Since this is the ventilation term, moon discs will assure consistent values from car to car - and let us use 0.01165 with small error.

His Cr ranged from 0.062-0.072 for the cars he tested. If we blindly use 0.08, it represents a 4-29% error in Cx (wrt to his circumstances). His baseline was Cx 0.457...so the more aerodynamic the car, the worse the assumption. However, since this method is very close to wind tunnel testing, we can do baseline testing with the published Cd and calculate Cr. It's a lot of work, and without Cr we cannot know Cx.

I hope this is enough to chew on until I post the heavy mathematics.
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Old 03-05-2012, 11:57 AM   #10 (permalink)
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Quote:
Originally Posted by ERTW View Post
I'll just post some information here, instead of creating a new thread.
I think the topic of limitations of DIY coastdown testing is certainly threadworthy on its own.

It's of interest to more than a few members, and I suspect unfortunately it won't get as much exposure inside this thread.

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