Coast down test methods and determining cD?

[puddleglum]

Does air density also change with humidity like it does with temp. or is it not an issue. E.G. would air be substaintially denser in fog than a dry sunny day?
Is there any rule of thumb for the change in rolling resistance relating to temp. or is it dependant on the vehicle?

[Ernie Rogers]

Originally Posted by puddleglum

Does air density also change with humidity like it does with temp. or is it not an issue. E.G. would air be substaintially denser in fog than a dry sunny day?
Is there any rule of thumb for the change in rolling resistance relating to temp. or is it dependant on the vehicle?

Hello, Puddlegum,

I am checking in a handbook for you (Handbook of Chemistry and Physics, CRC Press). At 95 deg F, here are the air densities at sea level--

Dry air at 95 F.........1.146 kg /cu.m.
100% humid air........1.122 kg /cu.m.

That's a density decrease of two percent. Suppose you are driving at a speed where 70% of the energy is used to oppose air drag. Then, a 50 mpg in dry air becomes---

50 x (0.7/0.98 + 0.3) = 50.7 mpg in 100% humidity and 95 F

At 70 deg F, the moisture content in humid air has less effect--

Dry air at 70 F.........1.201 kg /cu.m.
100% humid air........1.190 kg /cu.m. This is 1% less

Effect of greater density of cool air--

50 x 1.146 /1.201 = 47.7 mpg.

Effect of raising humidity from zero to 100%--

47.7 x (0.7/0.99 + .3) = 48.0 at 100% humidity and 70 F

If you lower the temperature another 25 degrees to 45 F, the effect of humidity drops to about 1/2%. (Air density rises to 1.226, a bigger effect.)

The effect of fog is to lower fuel economy, partially cancelling the effect of high humidity. The fog effect is smaller, so forget it.

I hope that puts your mind at ease.

Ernie Rogers

[Vekke]

I did today some coasting testing.
Temperature was +17 celsius so RHO~1,2176

120-80km/h north avg time 36,92s south 36,11s--> total avg 36,52s
40-20 km/h north avg time 65,18 south 75,68 --> total avg 70,43s
Frontal area is 1,8772 m2
weight 890 kg

Based on ernies formula

Cd=((M*a-crr*M*g)*2)/A*rho*V^2

What speed should I use or have I made a mistake in the formula?

Someone with computer skills should make similar exel sheet like this:
Drag Coefficient Calculation

[NeilBlanchard]

This is a better spreadsheet:

Measure the drag coefficient of your car

http://www.iwilltry.org/b/wp-content...oefficient.xls

[Vekke]

Yes that is good sheet but I should have looked that before made my measurements... Now I have to make new tests as I did not measure 70 seconds of data.

Also I have to try find better place to measure. its difficult in Finland to find level road that is long enough for this purpose.

[gone-ot]

To find air density (SI or IP), try this applet: Linric Company PsyCalc®

[Vekke]

Cd 0.221 and Crr 0.0079 So I was able to reach same drag coffefficient as MB CLA blue efficiency model which has needed numerous hours in CFD, wind tunnel and big bugs. So I reached best production car Cd up to date with internet, books and tuft testing and virtually zero budget. I wonder what those figures would have been if I would have put all my coming innovations to this car?

http://www.youtube.com/watch?v=4nkdG...ature=youtu.be

[NeilBlanchard]

Originally Posted by Vekke

Yes that is good sheet but I should have looked that before made my measurements... Now I have to make new tests as I did not measure 70 seconds of data.

Also I have to try find better place to measure. its difficult in Finland to find level road that is long enough for this purpose.

That is the challenge I had hear in New England, as well - straight, flat and level roads with little traffic where you can turn around at either end to make 3 runs in each direction, are very hard to find. Do it at night would help on the traffic front, at least.

And video recording the ScanGauge set to KPH makes the data collection relatively easy - just watch the video and start timing as you hit the target speed (70kph?) and then pause it every 10s and note the speed.

[Vekke]

Yes that was method I was using. I put the speed to 125 km/h and started coasting. put the zero mark to 120 km/h and then just pause every ten seconds. I yelled the speeds as I had previous day problems to read the numbers from ultragauge screen due to sunlight. Also in the last test other direction speeds were little hard to see.