[wyatt]

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).