I don't have mountain-sized hills around here, but I did find one on the other side of town that's much closer than my usual test route. The length of time it takes for me to get out to my flat route to do a set of controlled tests has me doing less of them these days...
Anyway.
I can stop at the top of this hill with the engine off and tranny in neutral, reset the SG, release the brakes and the car will accelerate down the hill and coast to a stop after about 1 km total. The hill itself is not particularly long or high, but it's fairly steep. I did 4 runs...
1) In its default configuration:
- driver's mirror deployed
- rear skirts on
- windows up
33 mph max
1100 meters coasting to full stop
2) both windows down:
33 mph max
1096 meters
3) windows up, hatchback wide open
(see also Peakster's
hatch-propped open vs. hatch-closed experiment)
23 mph max
1000 meters
4) windows up, hatch closed, rear wheel skirts removed
33 mph max
1090 meters
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Observations:
- clearly not rigorous or scientific (baseline not well established, no return to baseline)
- surprised that wheel skirts may have a bigger effect than windows, even though the window openings cover a significantly larger area
- even after seeing Peakster's test, the hatchback effect surprised me in its ginormity
What I like about this hill/method:
(a) it removes more variables from testing - the driver & cruise control - which should improve the quality/repeatability of results
(b) it's much closer to town, so I'm more likely to return here to run tests.
(c) even though the max & average speeds are low, it appears I may be able to observe effects of relatively small aero changes (if you call a pair of wheel skirts a small change).
Edit: what I don't like about the hill is that it's not a clean hill + flat run out. It's a steep hill, followed by a couple of distinct (gentle) downhill grades, ending on a gentle uphill grade.