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Old 01-25-2013, 06:37 PM   #1 (permalink)
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Need coast down help

How would leaving a vehicle in drive with an automatic transmission show up as Cd, Cs, and Crr numbers in a spreadsheet. I know it won't actually change the numbers in real life, but for the purposes of coast down testing, it will. I would think it should actually show up as a Cs (skin drag) since it should be dependant on velocity, but I seem to be having trouble making things work out.

My friend and I have done lots of coast down testing on his van. We always use a GPS enabled device to gather very accurate data, we always do bi-directional runs, we always use the same stretch of very flat, very low traffic road, etc. The problem I am running into is that the numbers always want to optimize out differently between runs. In theory, the Crr number should always be the same, but setting it as constant and floating Cd is what has gotten me thinking I may need to add a Cs variable. The problem is less pronounced on runs that are about the same weight, but on ones that significantly alter the mass, we wind up with the Crr variable altering significantly (if mass goes up, Crr comes down, if mass goes down, Crr goes up). Mass is taken into account in my spreadsheet, so I am wondering if the problem lies in having left the transmission in drive, thus adding mechanical resistance that will show it's self the same as skin drag.

Does anybody have a good idea as to how to characterize this? I am thinking the real answer is going to involve a bathroom scale, some foam and another set of coast down tests (this time in neutral).

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Old 01-25-2013, 07:07 PM   #2 (permalink)
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Yeah, I would think you need to have the transmission in neutral. Seems like you'd be getting "engine braking" otherwise - unless you KNOW your torque converter "un-locks" when coasting. ???

In my mind, it's just too many variables to really get an accurate estimate of Cd from coast-down testing (although I've got to admit, I haven't tried using the formula - spreadsheet - so I don't really know what I'm talking about - LOL)

I have had some luck with coastdown testing in older cars I couldn't hook a ScanGauge to. In that case, I also used a GPS for speed, and went to a deserted road where I could start the test at over 100 mph (usually 105), and then coast it all the way down to 55 or 60. I'd do multiple tests, both ways of course, and get an average. Then repeat with whatever aerodynamic change I was testing. Kind of tedious, and quite a bit of variation from test to test, but hopefully, the averages "averaged-out" enough to get useful data. Usually, there was a clear "trend" in the final numbers that I was happy with (confidence I had "useful results")

But I never attempted to get any actual Cd numbers from it - just to see if "this is better than that". If it was, I'd use it. And it was in connection with a race car we were running at Bonneville Salt Flats for top speed. I had another car of almost the same body (2 years different) that I'd take out and test the aero mods on by coast-down testing. I eventually got up to over 182 MPH on the salt before I went broke! LOL

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Old 01-25-2013, 11:14 PM   #3 (permalink)
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yeah, you need to be in neutral
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Old 01-26-2013, 04:07 PM   #4 (permalink)
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transmission

Quote:
Originally Posted by wyatt View Post
How would leaving a vehicle in drive with an automatic transmission show up as Cd, Cs, and Crr numbers in a spreadsheet. I know it won't actually change the numbers in real life, but for the purposes of coast down testing, it will. I would think it should actually show up as a Cs (skin drag) since it should be dependant on velocity, but I seem to be having trouble making things work out.

My friend and I have done lots of coast down testing on his van. We always use a GPS enabled device to gather very accurate data, we always do bi-directional runs, we always use the same stretch of very flat, very low traffic road, etc. The problem I am running into is that the numbers always want to optimize out differently between runs. In theory, the Crr number should always be the same, but setting it as constant and floating Cd is what has gotten me thinking I may need to add a Cs variable. The problem is less pronounced on runs that are about the same weight, but on ones that significantly alter the mass, we wind up with the Crr variable altering significantly (if mass goes up, Crr comes down, if mass goes down, Crr goes up). Mass is taken into account in my spreadsheet, so I am wondering if the problem lies in having left the transmission in drive, thus adding mechanical resistance that will show it's self the same as skin drag.

Does anybody have a good idea as to how to characterize this? I am thinking the real answer is going to involve a bathroom scale, some foam and another set of coast down tests (this time in neutral).
The big boys always did them in neutral.They might have even added a special de-clutching device for the driveshaft of automatic transmission cars ( I'd have to check that).
Auto makers nolonger do coastdown tests.
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Old 01-28-2013, 03:12 PM   #5 (permalink)
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I am currently trying to figure a way to put the Ca (Coefficient of Automatic Transmission) into the Cs number. I think to get the numbers right, I will have to use a bathroom scale to figure out the force to overcome rolling resistance, and we will have to do some coast down testing in neutral. It would be nice to have a completely disengaugeable clutch, but I don't see that happening . The worst case is that we will only be able to show a net positive or net negative for our runs, which isn't too bad (but I would like to have the numbers...).

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