Found a coastdown hill! Mini comparo: windows vs skirts vs hatchback
 

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.

Note that 3 of the runs recorded the same max speed, but had different end points.

It may have been a resolution issue (I had left the SG reading MPH, but if I change to km/h on the I may be able to detect smaller differences). I was thinking I could also adjust the SG's speed offset by, say, 100% (if it goes that high), effectively doubling the speed resolution...

I may just try that.

About the weather: the best weather to test in would be cold with high pressure and low humidity = dense air & more drag.

The key question in the first set of data is:

How reliable are the results? I didn't do multiple runs without changing variables to see how close the results would be.

So I went back...

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I did 4 runs tonight without changing anything on the car, and each was longer than the previous one.

Which tells me:

1) The drive train was still warming up (tires warming & pressure increasing, bearing temps going up, tranny oil warming up, etc.). I drove a "cold" car straight to the hill, roughly 6 km.

2) The runs I did previously would have suffered from the same effect, (I drove on a "cold" car straight to the hill) which means the effects noted would likely have been even larger if the runs had been done with stable drivetrain temps.

So that's crummy. There's no time saving in having a local test track, because I would have to drive around for half an hour just to get the car warmed up for valid, repeatable runs.

Well, you know what? This "problem" could be an opportunity to do a test.

And the test could be: how far/long do I have to drive the car to get the drivetrain to a stable temp (car coming to a stop at the same point on say 3 successive runs).

Then I could go on to do an actual test on a warm drivetrain, and not consider the warmup to be a total waste.

Or I could just go to my original tried & true test route. (It's far enough away that the car is warm when I arrive).

PS - the difference in the 4 runs today was pretty big: the shortest was about 45 feet less than the longest.

I need to make a chassis dynamometer to pre-warm my drivetrain. http://www.gassavers.org/images/smilies/biggrin.gif