Coast down test methods and determining cD?

[wwkayaker]

I am going to be doing some coast downs for my car in an A-B-A fashion. Here are my questions .

1) How do I determine cD off of a coast down test? Sometimes, I find it hard to believe that my car starts with a cD of 0.30. Especially, since the same year Eclipse starts with a cD of .34. that is horrible for a sporty car!

2) How far? Even in central Indiana it can be hard to find a paved side road that doesn't have a stop sign every mile.

3) Exactly, step by step, how is a coast down conducted? Should I use cruise and step on the clutch at the same spot?

4) Can I accurately do this without a SGII or other device? My speedo shows every 5mph. I don't have a measuring wheel either.

I already know to use weather dot com and record all of the weather conditions for the nearest town to the test site. Also, I have a few good flat roads to use that are miles long with a couple stop signs.

I think I have all of my questions in here.

[NeilBlanchard]

Hi,

Here's the "Instructables" page that I used:

Equipment

I highly recommend a point-and-shoot digital camera that can do video for about 1 1/2 minutes or more -- go a little above 70kph (or convert the spreadsheet to mph) and start the video, put it in nuetral and coast for 70 seconds after you hit 70kph. Do three passes in each direction, and then watch the videos, and note the time you hit 70kph, then pause the video every 10 seconds after that, and enter the speed in the spreadsheet.

The trick with this is to get a relatively accurate frontal area, and you'll still have to make your best guesstimate of the tire rolling resistance -- this is really tricky, because a very small change in RR makes a rather large difference in the Cd.

[NachtRitter]

I was curious if you think the same method could be done with a GPS? You won't have to do anything except note the start time of each of your runs, then go home & download the data to your PC to see what the coast-down times are. Good (and free, which makes it even gooder) software that I found is SportTracks, which I talk about in my thread here: http://ecomodder.com/forum/showthrea...orts-8103.html. You should be able to get fairly accurate coast-down data that way (certainly as good as the Instructables method).

[wyatt]

ASME recommends 10 bi-directional runs. You start at 110 km/hr and begin the coast down, and start taking readings at 100 km/hr... I don't remember all the details, but Aerohead knows more on the subject.

[Ernie Rogers]

I see a lot of good ideas here. Here are my additional thoughts--

I just did mine with a stopwatch that collects multiple times (actually, my sport watch). I timed at 10 mph intervals from 70 to 10 mph. You really need a road with no traffic and no cops. A smooth, level mile is long enough. I averaged the results and then graphed.

Graph speed versus time, and the slope of the curve is the acceleration. Measure the acceleration (from the graph) at a high speed and also at a very low speed. Then get your Cd and Crr from the equation--

F = Ma = Cd A 1/2 rho V^2 + Crr M g

I did the calculations in metric because I felt more secure there. Important: the first term with Cd dominates at high speed. The second term dominates at low speed. My procedure is this--

You can guess a Crr that seems fair. plug that into the equation with your numbers for the high speed, and calculate Cd. Then, plug that Cd into the equation with the numbers for the low speed and calculate the Crr. Repeat the steps, put that Crr into the high-speed case, etc. After a few cycles, the Cd and Crr stop changing. Those are your answers for both.

My view is to not trust anybody's number, like the car weight (M g), unless you verify it. I got my car weight at a commercial scale. (They shouldn't charge you.) They did the weight both with me in the car and with me out. The difference had to match my weight.

Actually, it probably doesn't matter if you don't know the frontal area accurately. It's the product, Cd A, that is useful anyway.

Be very sure the roal is truely level. Pavement quality will affect the Crr measurement.

Ernie Rogers

[wwkayaker]

Those are all really good responses and answered my question. Thank you everyone!

The roads that I am planning on using are in the country (not much traffic or cops) and not used very often. Finding a smooth one will be the challenge, but I am confident that I will be able to find one.

[NachtRitter]

I recommend taking the kayaks off before you start...

[wwkayaker]

Lol! The kayaks and roofrack are only on when I go kayaking. Unfortunately, they spend most of their time in the garage. If only, I could paddle for a living!

[NeilBlanchard]

Hi,

I did a second coast down test today, and I'm having problems with the spreadsheet from the Instructables web page: the air temperature today is a lot different than when I did the first coast down. The only value for this is by altitude, but shouldn't there be one for temperature?

Also, my velocities at the end of the test were considerably lower than last time -- either my tires are very much worse (they are probably better, since they are nearly worn out) or I have a drag in a rear brake (a bearing had to be replaced) -- you can sometimes hear it dragging.

Another part of my problem was that I could not do the test as long as the 70 seconds on 4 out of the 6 coast down tests.

This method is pretty flawed, IMO -- you have to guess at the tire rolling resistance, and the air density bugs me, too.

[Ernie Rogers]

Hello, Neil,

I'm sorry about leaving you hanging on the measurement (versus guessing) of rolling resistance--you can measure it with the same roll-down test you use for air drag, just a little different. Alternatively, if you can find a slope where the car just barely rolls out of gear at low speed, then the rolling resistance coefficient is equal to the tangent of the angle, which is also the road grade. (A grade of 1% means a rolling resistance coefficient of 0.01.)

Rolling resistance of tires increases as the rubber gets cold.

Now, about air density--

At sea level and 59 degrees F, the density is 1.225 kilograms per cubic meter. For another temperature (T), the density changes in this way:

density = 1.225 x (492 + 59) / (492 + T deg F)

For example, at 100 deg F, the density is--

density = 1.225 x (492 + 59) / (492 + 100) = 1.14 kg /cu.m.

If you are far from sea level, then you need to put in the right number for your elevation instead of 1.225.

Or, cheat and call the airport weather station. They have the correct density number for the airplanes to use.

Ernie Rogers

Originally Posted by NeilBlanchard

Hi,

I did a second coast down test today, and I'm having problems with the spreadsheet from the Instructables web page: the air temperature today is a lot different than when I did the first coast down. The only value for this is by altitude, but shouldn't there be one for temperature?

Also, my velocities at the end of the test were considerably lower than last time -- either my tires are very much worse (they are probably better, since they are nearly worn out) or I have a drag in a rear brake (a bearing had to be replaced) -- you can sometimes hear it dragging.

Another part of my problem was that I could not do the test as long as the 70 seconds on 4 out of the 6 coast down tests.

This method is pretty flawed, IMO -- you have to guess at the tire rolling resistance, and the air density bugs me, too.