Measure the drag coefficient of your car

[cbergeron]

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.

[diesel_john]

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.

[Harpo]

Originally Posted by diesel_john

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.

[fabrio.]

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.


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

[SteveP]

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

[fabrio.]

thank Steve.

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

[Gone4]

Originally Posted by LostCause

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

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.

[IWillTry]

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.

[IWillTry]

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).

[RH77]

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