MEGA metro airdam

[brucepick]

Re. scraping the ground -
I found that for any given height from ground, if the air dam is further back (closer to wheels) it will scrape less than if further forward. Has to do with the angle when the road bumps up ahead of the car or when the nose dips down.

(My temporary one is maybe 6-8" forward of where the earlier one was, is also an inch lower, and is scraping a lot more often so I really have to be careful with it.)

[jwxr7]

My camera is analoge .

That makes sense bruce, my overhang is quite small already, but i could mount it a couple inches farther back if i go for the front frame piece.

[jwxr7]

I have more test results . I did more test yesterday, this time testing with and without the kammback, and another with and without a 4" airdam (instead of a 7" one). I'm still checking out the data and will post results later, this time with more physics.

Preview----- the kammback is good .

[jwxr7]

For fun I calculated the average road load for the test I posted in #7 above. The ave drag force without the airdam between 55-50 mph was 56.9 lbs and 57.8 lbs with the 7" airdam. These #s represent rolling and air resistance.

The book I reference calculated that 70% was aero and 30% rolling resistance in that speed range for a ford escape they tested.

[jwxr7]

Here are some results from the 4" airdam test. It was A-B-A bidirectional from 55-45 mph style for this test. This test was performed without the kammback.

Without airdam averages = 14.615 seconds for 55- 45 mph, and 7.225 from 55 - 50 mph.

With 4" airdam averages = 14.675 sec and 7.33 sec respectively.

So it doesn't appear to hurt aero but probably too close to call.

I may install it for looks if nothing else .

[jwxr7]

A closer look (more like something I overlooked) at the A-B-A data for the 4" test showed a larger variation between the "A"s than the "B" had from either "A", if that makes sense . So I would say the airdam made no significant difference. The kammback definitely had a significant difference though . http://www.gassavers.org/showthread.php?t=4575

[Bill in Houston]

Makes sense. Good news on the kammback, though.

[sonyhome]

Hmmmm... I went WTF is a kammback, then:

http://en.wikipedia.org/wiki/Kammback

"A popular definition is that the cut-off should occur where the cross sectional area is approximately 50% of the car's maximum cross-section. Thus a minivan is not a Kammback."

"Prior to Kamm's thesis, a teardrop shape that tapered smoothly to a point was considered optimal. Kamm showed that an abbreviated teardrop actually worked better; the air still flowed as if the entire teardrop were still there, but without the surface drag of the long point."

Cool!

[Mighty Mira]

Originally Posted by sonyhome

Hmmmm... I went WTF is a kammback, then:

http://en.wikipedia.org/wiki/Kammback
"Prior to Kamm's thesis, a teardrop shape that tapered smoothly to a point was considered optimal. Kamm showed that an abbreviated teardrop actually worked better; the air still flowed as if the entire teardrop were still there, but without the surface drag of the long point."

I'd be skeptical about using wikipedia's arguments as to why the Kammback works, and also the reference article. It's pretty dumbed down. For example, no low drag coefficient airplane has a Kammback, because a teardrop shape tapering down to a point DOES work better. It's just that once you taper beyond a certain number of degrees (as in the VW beetle), there is no advantage to be gained superior to cutting the tail off sharply at that point. It's a question of real teardrop (e.g. aeroplane) versus faux teardrop (e.g. VW bug).

One of the sources listed in the second article is in fact correct, it's just that wikipedia's main source, the first I listed, incorrectly interprets it. As a result, I'm sure that there are a lot of people who think Kammbacks are some sort of miracle rather than the engineering compromise they actually are.

If you look at the design of Kamm's actual car, you will see this. Mild taper, then chop.

My understanding of the Kammback is that the teardrop will keep offering benefits the longer that it is continued, provided that it doesn't exceed 15 degrees or so. However, those benefits will give diminishing returns. If you have a normal sized vehicle, the teardrop constrains you to having a long piece of otherwise unusable appendage that might get in the way when you want to access luggage, and makes it hard to park.

Another way to look at it, the area at the back of the car where you chop the teardrop is equivalent to the vacuum that is pulling the car back. If you are only dealing with a small area, you'd have better results implementing grille blocking, full undertray, and even front wheel skirts than going those few extra feet. So you may as well cut the car off abruptly at the point of diminishing returns.

[sonyhome]

Hmmm.. i'd say a drop is optimal/minimal energy lost in friction for the volume of matter that flows through the air.

However:

If you cut the tail of the teardrop you're loosing volume, AKA to move the same amount you'd need a bigger drop and therefore a larger surface, though the same drag coefficient (totals to more energy wasted).

In a car the point would be wasted space anyways as it has no useful cargo benefit and would indeed be unpractical.

With a proper laminar flow I would expect a small gain over continuing the shape with a useless/unused tip: Since there's nothing there, there would be no extra surface friction.

Well I'm no rocket scientist... Just trying to apply common sense.

Come to thing of it: rockets are not teardrop shaped, they have a pointy tip. Maybe dynamics change past the speed of sound...