Measure the drag coefficient of your car
 

iwilltry has write a tool, for measure the drag coefficient of cars.
available at http://www.instructables.com/id/Meas...t-of-your-car/

After to have watched this one, I have modified it in order to greater precision.
The change, it is in the air density parameter: in origin, this value was fixed to 1.22 Kg/m^3, now, it is calculated in order of air density based on the climatic condition, and isnt' required to adjust it for altitude

for example, now, in Rome, with 15°C temperature, 48% of humidity and 1012mBar pressure, the air density is of 1.1735Kg/m^3.

New version, is avaible :http://forum.ecomodder.com/attachmen...1&d=1206974721

no comment? :(

I am thinking: but the inertia moment of the wheels?
and the gearbox (or only the differential) losses?
The inertia wheels, isnt' a big problem: it is possible to add an ipotetical additional mass at the car, this system, translate up the total curve of deceleration.
But the losses from gearbox?
I have serch in the internet for to look the course of curve of loss for to know if it is liear or exponential.

ah, after indagation, I discovered that the rolling resistence isnt' constant, but in functio of speed:
Rr= F+K*Speed
F= Crr*Mass
K=???

I've seen this spreadsheet before, but haven't used it.

I think AndrewJ used it to estimate the change in Cd of his Civic.

Yep, I used it to calculate my Cd a few times.

I thought about changing the air density value, but I looked it up and it's based at sea level, Since I'm right at sea level, I didn't mess with it.

Not 100% sure, but air density values pulled out of books tend to be at STP. Standard temperature and pressure are not common values one would see at sea level. For example, I believe standard temperature is 0C...

I don't know how big a difference the true air density would make on Cd accuracy, but if the value was pulled out of a book it is probably off.

- LostCause

In the aviation world standard temperature and pressure is 29.92 @15 C at seal level.

Well, in my world...:p

You are right, but the idea of inaccuracy is still there (albeit, lessened...:o).

- LostCause

I imagine that's usually pretty close to sea level? :p

boys, I have big esteem for you, but if you answer therefore, I take care myself !

the file "stima Cx fabrio.zip" is a my modified version from original by Iwilltry.
Added new function for evaluating correct air density it is an improvement.
You could try sone variation in climatic condition for know itse impact.
This is, argument of my first post and you answers only after my second post when, I have exposed a new problem.
I must cry or laugh? :)

ah, Basjous has extimated drag coefficient of aerocivic using the original spreadsheet

I'll try both spreadsheets and see how they compare. I haven't calculated my new Cd yet, so I can do it with both spreadsheets.

Also, I'll try to use my local barometric pressure, etc. in my results for better accuracy.

It will probably be a few days until I can do it, though.

ok i'll try it. duely noted and saved. good work. i know from hot summer day to cold winter day it makes a big difference.

we were not ignoring you, we were sleeping.:)

At 50' elevation at Gainesville Raceway, I've seen swings in Density Altitude from +4050' to -550' in one season from summer to winter. That's huge on making HP, but also affects the amount of molecules you smash through at 135mph.

thanks to all for reports
Diesel John, I am sorry, I do not have remeber to use the smiles :(
Now, which councils in order to improve the precision of this tool?

I find this chart on the internet.

http://img529.imageshack.us/img529/3...tire020hy7.jpg
By fabrio

I is evidents, that for speed advanced of 80/90Km/h the rolling resistance, isnt' constant, and this, does not help us in the reserce of more precision for to calculate the Cd.
I think , that a bigger speed it is possible a more precision because smaller superimposition of Cd and RR is avaible.
But now on light of chart over, we are forced to remain under the 90Km/h

Guys,

If I may, I'd like to inject some numbers here so as to demonstrate the error that is involved.

As we know, the formula for drag force is F = .5 rho v^2 Cd A where rho is the density of air. If we do a coastdown test and measure the force used to overcome air drag at a known speed, we can rearrange the formula to calc Cd (assuming we know the area of our car):

Cd = 2 F / (rho V^2 A)

Suppose we do a coastdown test and determine that at 70mph (31 m/sec) we have an aerodynamic drag force of 78 lbf (347 N). Air density varies with temperature as follows: at 0C, density is 1.292 kg/m^3, at 10C it is 1.247kg/m^3, at 20C it is 1.204kg/m^3, and at 30C it is 1.165 kg/m^3.

If we do the calc using a density of 1.292 kg/m^3, and assuming an area of 20 sq ft (1.86 m^2), we get (if I haven't botched the arithmetic) a Cd of .3005; if use do the calc using 1.165 kg/m^3, we get a Cd of .333. That's a big enough difference to swamp the effect of a typical aero mod, so more accuracy in determining air density would seem to be in order (as would similar increases in the precision of measurement of pressure, temperature, and humidity).

As to the effect of altitude, rising from sea level to 5,000' (1524m) changes the density by about 16% which amounts to about .2 kg/m^3--in other words, more than a 30 degree (C) rise in temperature.

Water vapor also changes the density, but I don't have the numbers offhand.

--Steve

thank Steve.

about the effect of altitude on the air density, I woould say, that the case of excel attached, is not required adjustment.

STP varies based on the book. 0 C is only for chemists generally. In engineering it is often anywhere from 60 F-70 F. Most common in studies of fluids is 68 F or 20 C as they intercept exactly at the point and it's a pretty common air temperature. However, humidity will affect this greatly since usually they assume there is no vapor component.

Hi all. fabrio PMed me to come take a look at this thread. Sorry I don't pay more attention to the forums... too busy tinkering :).

Thanks, fabrio for improving my spreadsheet. I agree that changes in air density will have a significant effect on the Cd calculated so it's nice to be able to calculate air density based on values that are measurable (assuming you have a weather station handy).

The moment of inertial of the wheels can be considered too. The force required to decelerate the linear motion of the vehicle is F=MA where M is the mass of vehicle plus driver. The additional force required to slow the rotation of the wheels depends somewhat on wheel geometry but should be close to F=(m/2)A where m is the mass of all wheels. Thus the total force is F=(M+m/2)A. So simply adding half the mass of the wheels to the total mass of vehicle plus occupants may give more accurate results.

There is also the moment of inertia of the drive-train to consider but that is likely small do to smaller diameters of components.

There are also friction and viscous losses in the drive-train. The force of friction should be roughly constant and will result in over-estimating Crr. Viscous force (due to gear oil in diff and tranny) is proportional to velocity and will result in over-estimating Cd.

Note that ignoring friction and viscous losses results in over-estimating Cd and Crr while ignoring the angular inertia of wheels and drivetrain results in under-estimating Cd and Crr, so the two effects may partially cancel.

One thing everyone should keep in mind is that Cd and Crr are only useful for comparing one vehicle to another. If you want to evaluate the effectiveness of a mod, you are way better off simply comparing coast down times. For aero mods look at a high speed range (ex time to drop from 90 to 60 kph). For rolling resistance mods, look at a low speed range (ex time to drop from around 40 to 20 kph). Ideally, do A-B-A tests under identical conditions (preferably just minutes apart). Then air density and other variable factors become irrelevant.

Cheers.

If, for whatever reason, you want to measure Crr more accurately, without including drivetrain friction and viscous forces here is one method.

Use a spring scale to measure the force to pull your vehicle on flat ground at a constant speed (a walking pace). Call this force F1. Then jack up each wheel in succession, wrap a cord around the outside of it several times and use a spring scale to measure the force required to spin it at constant speed (walk away at the same walking pace as before). Add up the forces for all 4 wheels to get F2. Then Crr = (F1-F2)/Vehicle_Weight.

For even more accurate results, measure both drive wheels simultaneously (requires 2 spring scales and two people walking at the same pace).

I'd like to try this, but...
 

I would like a rough estimate using the modified spreadsheet, but I have 2 problems...

I'm sure most of the Kansas City folks around can attest:

1. The winds are RARELY calm, and 2
2. Finding a level road that's long enough, is rare

Does anyone else have the same problem?

Perhaps under calm winds, using the same road could produce a constant for testing changes, but not give a true Cd or Crr...

(obs: wind is the lowest in a long time SE @ 8 kts, but across town: SE @ 15 gusting to 22. Hrmf.)

RH77

Yep. I can definitely attest to having those problems.

My coastdown location goes around Humboldt bay, so it's exactly sealevel the whole way, but very windy.

As for any of you who are more inland, try finding a road through some river-bottom farmland. That's about as flat as you're likely to find.

How long of a road is needed (approximately)?

I'd say 1/4 to 1/2 mile stretch would be ideal since you want to be coasting for as long as possible at a high speed.

Has anyone else used Fabrio's modified sheet? I originally thought that temp (air density) would play a huge role and was wondering why it wasn't originally included in the spreadsheet. Where do we get that info? Anybody know any websites that publish something we can use for local areas?

Just wanted to point out that air density was included in the original spreadsheet as a constant that the user could change. Fabrio's modified sheet simply adds a calculation of the air density based on more measurable quantities (temp, pressure, humidity, etc). Your best bet is probably to measure these directly at the test site using a mini weather station rather than rely on information for your larger region.

...be sure to alternate DIRECTIONS when doing each coast-down test, and do so as soon as possible, so that wind speed and wind direction haven't changed, as they can do at morning and late afternoon.

...the idea is to do opposite-direction tests so that what you gain/loose going oneway, you loose/gain going the other way, so that the 'atmospherics' and 'road-grade' changes will effectively "cancel" each other out to yield a better (less influenced) answer.

Hi,

I've done a coast down test on my xA, and yes finding a flat straight road is the hard part. It does not have to be level, as long as it is a low slope -- it just can't have a dip or rise in it.

The other major piece of guesswork is the tire rolling resistance.

My best estimate based on 0.011 Crr (which is on the low end of "normal tires on smooth roads"), is 0.27Cd. (The stock rating is 0.31, though my car is a RS 2.0 that has a far more open grill than the standard xA.)

BTW, my average terminal speed after 70 seconds (starting at 70kph/43.5mph) is 27kph/16.7mph.

[Edit: At the time I did the coast down test I had the grill blocks, fog light covers, zigzag tape on A-pillars, top of the windshield, backs of mirrors, and trailing edge around the hatch, I had the rear wheel skirts, and the underpan under the hatch area, and the mini-Kamm panels.

Since doing the coast down test, I added the front wheel skirts, the video mirrors (which also reduces the frontal area), and I have lost the rear underpan. I hope to rerun the coast down test before I have to replace my tires...]

Great stuff Neil. Can you list your aeromods so we know what you've all done?

Hi,

Thanks Doax! I added the items I had modded to the post above.

How can one measure the rolling resistance of their tires as they are inflated on their car?

I don't see how to measure ONLY the rolling resistance, but you could measure the non-aerodynamic resistance just by using a force gauge to push or pull the car at VERY low speed (barely moving) in neutral. Aero drag would be negligible so if your car weighs, say, 2400 pounds and it takes 36 pounds to push it, your overall "mu" (Crr) would be 0.015. Obviously, this includes tires plus friction in everything behind the transmission. You'd want to get it to a steady (SLOW) speed so the kinetic energies (linear and rotational) are constant. Also, though a dimensional analysis shows that, to first order effects, tire rolling resistance depends only on down force (weight plus aerodynamic) and not obviously on speed, unfortunately aerodynamic down force depends on speed.

You can come reasonably close to the weight of the car with an accurate tire pressure gauge, two pieces of paper, and a machinist's scale. I think that's been covered in Ecomodder so, unless there's a demand, I won't waste time by repeating the method. The key here is the accuracy of the gauge - the typical pocket gauge may be off by a wide margin.

Hi this is my first time to post on ecomodder.

Is there anyway you could change the input method of the spreadsheet? I find it easier to take time measurements during coast down at specified velocities. For example I use the timer split function on my stop watch and split the lap times at 110, 100, 90, 80, and 70 kph. This is quite easy to do on the fly but I can't use this data in your spreadsheet with the current formating.

I'm surprised I didn't mention in my post (#26) above is that I video recorded each coast with the camera focused on the ScanGuage II, with the speed set to KPH. This makes the time *much* easier as you just stop the video at the moment you hit 70kph and note the time, and then let it play for 10s and then note the speed, and then play another 10s, etc.

You can do this far more accurately than doing manual data recording while driving, and it only takes one person rather than two.