Quote:
Originally Posted by JRMichler
Your approach is good, and would be better with one change. If you want to separate aerodynamic drag (proportional to V^2) from viscous drag (proportional to V) from constant drag (fixed amount, not proportional to speed), you need a wider range of speeds.
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Yah, that is most likely the case. Also, how would one separate out the aero drag due to lift? That would cause vehicle weight to fluctuate at speed, dependent on speed of course, and would make determining C(rr) that much harder...
Quote:
Originally Posted by JRMichler
In my coastdown testing ( https://ecomodder.com/forum/showthre...yon-20405.html), I started at 60 MPH and coasted down to zero speed. I did not try to pull a viscous drag number from that data because the speed range was not large enough. I would have needed to coast down from about 80 MPH to get good enough data for that.
The effect of a barely noticeable breeze surprised me. I would not have thought that a 5-7 MPH breeze would be so large. That breeze was barely detectable. Another thing to watch is the road slope. A slope of 0.1% (5 feet per mile) will have a noticeable effect on the drag, especially the rolling resistance. |
That just goes to show that proper coastdown testing is really hard to do. Folks like yourself and
aerohead really deserve recognition for actually doing coastdown testing.
Quote:
Originally Posted by freebeard
I can imagine aerodynamic and skin friction drag, but what is constant drag?
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That would be C(rr), which is ideally dependent on the rolling resistance of your wheels.
Hm... maybe I could rig up a few different test schedules - one at low speed to solely determine C(rr), and then another to determine C(d) and C(l)? I will have to look into that
Quote:
Originally Posted by freebeard
I'm now on board with level, no up or downgrade is going to be constant.
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You can kind of mitigate around this by doing a coastdown run between point A and point B, and doing a reciprocal course between point B and point A. However, the averaging is only accurate during those portions of the runs where vehicle speed is approximately equal. However, that would have to assume the slope was in fact constant for the length of the run.
One could, I suppose, add a slope angle parameter into the equation to be solved for, along with another parameter to specify whether the vehicle is going up the slope or down it. Again, that would have to assume the slope was in fact constant for the length of the run.
It would be much more ideal to find some path that is known to be reasonably flat. Less computations that way.