I don't like terminal velocity tests because power increases as the cube of speed. So if you make a really good improvement, worth like 3%, then you terminal velocity only changes by 1%, which might be too small to see.
yeah, agreed... terminal velocity margins of error are multiplied.
Force is only a squared factor though, and that is really the measure you are looking for.
Plus the RR, which is a large non-aero factor, weighs in.
What I am seeing in my measurements is a RR force about 30 lbs with a aero force at 60 mph of about 50 lbs... actually one should be able to get the margin of error from this:
let's say +- 1mph on speedo reading which is +2, -almost 2 lbs of aero force.
Cd change of +- 0.1 on my VX yields +-2lbs as well.
So a aero change of +- .02 is the best I could hope to measure.
Also forgetting wind variation... and I am not convinced RR is a constant.
Either way this gives an idea of what is possible.
I am playing with coast down testing a lot, using a video camera on a second clock and the speedo. When I review, I mark the timestamps for each 5 mph tick, the calculate the deltas between in 10mph increments. I average this approach of 5 or so runs. I think I can show that the margin of error is less for a couple of reasons:
the deltas are calculated and relative: ie if I get one reading high or low the next calculation delta will absorb that error.
the averages should take wind and road levelness out of the equation (although 10 runs might be better).
PS - the difference in the 4 runs today was pretty big: the shortest was about 45 feet less than the longest.
Wow! that makes me wonder, how much shorter it would be in the winter? That may be a big factor in bad winter mpg. I have noticed cars don't seem to coast nearly as good in cold weather, could be a major cause of poor mpg in the cold.
Depending on your video software, you might be able to get times from the software/video rather than the clock, which would let you break it down to ~.04 second intervals.
Also thinking a DMM hooked up the the speed sensor might give precise readings... if you could read 1/10's of a sec and get speed to 10th as well..
hmmm... wish that damn wind wasn;t there.... or the bumps in the road :-)