Quote:
Originally Posted by t vago
Turns out that coastdown testing calculation is not easy!
Right now I am looking at using this Instructable as a start, but with using a starting speed of 115 kph and an ending speed of 15 kph instead.
I remember the discussion about there actually existing a V term that corresponded to drivetrain-related viscous drag, so there's a complication there. Not really sure how I'd calculate a C(d) either, since people have pointed out that cars have a negative lift component at speed which is going to throw off the V^2 term calculation. Hm.. at least C(rr) could still be reliably calculated... perhaps.
I think the best to be hoped for is to just do a sort of stopwatch function that starts when the vehicle slows down and passes 115 KPH, and stops when the vehicle reaches 15 kph. One could do an average of several passes in that fashion, and store the resultant elapsed time for A-B-A comparisons. |
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.
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.