Quote:
Originally Posted by NeilBlanchard
Holy moly Batman! That is some serious data crunching.
How did you determine the Crr? In my coastdown tests using the Instructables spreadsheet, this is the hardest part to get -- it is basically an educated guess?
@drmiller100 -- I suspect you are correct. The aerodynamic drag swamps everything else at a constant speed. And accelerating (from a stop and/or uphill) is huge but if you can limit the time it lasts in proportion to the whole drive time, then it becomes less critical.
|
Hi Neil,
Well, I did spend a ridiculous amount of time "trying" to filter data that was very noisy. I like working with math, and this study was a serious study in noise filtering, and definitely shows the need for more and more stable data.
The Crr was calculated using the same technique as posted in the Instructables site. And please note that ambient air temperature make a "huge" difference is exactly what Crr number is generated.
Colder temperatures in the winter, compared to summer, changed the Crr value by about 20% or so. I don't know how accurate this calculation is because of the current GPS accuracy. But the the calculated number was generated by a "best fit" calculation was iterated many times until the error reached a minimum from the actual data compared to the "model" data.
The assumption that air drag swamps everything else at a constant speed, is only correct if the speed happens to be 150 mph or above. If however the car is a Honda Insight, and the speed is 45 mph, then the Cd and Crr are equal to each other. Obviously at slower speeds, the Crr predominates.
Jim.