Coastdown testing question: does starting speed matter for consistency?

[skyl4rk]

How do you record the speeds and times? Do you record seconds at each 10 km/h, or do you record km/h at each 10 seconds?

[MetroMPG]

From: Instructables coast down testing procedure

4. Record data as follows. The driver should indicate when the speed drops to exactly 70 km/h. At this time (t=0) the passenger should start the clock. The passenger should indicate every 10 seconds after that and the driver should call out the current speed to the nearest whole km. The passenger should record this value next to each time.

[skyl4rk]

Is that the SAE standard method?

I have been using a stopwatch and a digital voice recorder with no assistant. It seems to be easier to mark the time at a certain mph (for example, 60, 50, 40, 30 mph) and hit the stopwatch lap button, speak out the time, then hit the lap button again to prepare for the next reading.

Maybe the 10 second method will work too. The curve should be the same either way, but the 10 second method will get more data points.

[NeilBlanchard]

Hi,

I think the best solution (as suggested in the Instructable) is to video the process, and then watch the video and stop it every 10 seconds, and record the speed. This is another area where the SGII will be quite helpful -- if you video tape it with speed and TPS (or RPM) on 2 of the displays, you can easily see what is going on.

I was wondering why the Instructable has the test start at only 70kph -- this would seem to barely be in the critical aero range? Also, are you supposed to coast to a stop, or just down below a certain speed?

[wyatt]

The latest version of MPGuino code also has a function where you can output the data to a laptop or cell phone. I don't know how to do that (I also haven't tried), but I do know how to video tape, and I don't even have to keep track of time, since the display updates every half second. My MPGuino isn't 100% accurate and likes to bob around, but major outliers would be easy to eliminate and get the average from there.
I think we should come up with a good "how to" for coast down using the SAE standard method, and try to get our model to incorporate the skin drag also.

[aerohead]

Originally Posted by skyl4rk

Is that the SAE standard method?

I have been using a stopwatch and a digital voice recorder with no assistant. It seems to be easier to mark the time at a certain mph (for example, 60, 50, 40, 30 mph) and hit the stopwatch lap button, speak out the time, then hit the lap button again to prepare for the next reading.

Maybe the 10 second method will work too. The curve should be the same either way, but the 10 second method will get more data points.

When I did 'em,I used an low-tech cassette-recorder,yelling the velocity points as I coasted.I did all the runs,then reduced the data at home using the stopwatch there.Sure makes a windtunnel look attractive!

[MetroMPG]

I believe this PDF outlines the SAE coast down methodology:

SAE Standard J2263: "Road Load Measurement Using Onboard Anemometry and Coastdown Techniques"

http://avt.inel.gov/pdf/hev/htp001r2.pdf