Project: Geo Metro boat tail prototype - 15% MPG improvement @ 90 kph / 56 mph

[MetroMPG]

Hi Jim - just read your questions and will reply tomorrow.

Until then, I've posted the "raw" video of the tuft testing and it's available if anyone who wants to look it over:

http://www.youtube.com/watch?v=Mjff5NDREkQ

[bestclimb]

As far as Reynolds numbers and aerodynamics go the Wright brothers made some faulty assumptions that nearly held them up. They did some wind tunnel testing with airfoil shapes (single sheet of fabric curved) and found a shape that worked very well at the size, wing loading, and speed of the tunnel. scaled up and it just barely worked.

Something that would work well for testing purposes would be an airspeed indicator. Adjusts for baro pressure, temperature, head/tailwind. basicly if at 29 inches air pressure you get 50 mph air speed and the pressure goes up to 32 inches to get the 50 mph air speed you would have to slow down a bit. An ASI measures ram air pressure, increase or decrease the number of air molecules in a volume of atmosphere you have to drive slower or faster to get the same drag on your car. A used gauge can be had for 50 bucks from Ebay, and some aluminum tubing will work for a pitot tube.

[MetroMPG]

Originally Posted by MetroMPG

posted the "raw" video of the tuft testing

There are 10 segments in the video, a couple of new ones that weren't included in the edited version:

1. Clip 1 of 10 (0:49)

- tuft testing top side
- 0 to 50 km/h (~30 mph)

2. Clip 2 of 10 (0:34)

- tuft testing top side
- 50 to 80 km/h (~30 to ~50 mph)

3. Clip 3 of 10 (0:39)

- tuft testing passenger side
- 0 to 50 km/h (~30 mph)

4. Clip 4 of 10 (0:34)

- tuft testing driver's side
- 0 to 50 km/h (~30 mph)
- door mirror folded back

5. Clip 5 of 10 (0:37)

- tuft testing effects of added wheel fairing
- 0 to 50 km/h (~30 mph)
- door mirror folded back

6. Clip 6 of 10 (0:33)

- tuft testing added wheel fairing
- with front fender skirt
- 0 to 50 km/h
- door mirror folded back

7. Clip 7 of 10 (0:40)

- tuft testing bottom
- main undertray not installed
- 0 to 50 km/h (30 mph)

8. Clip 8 of 10 (0:44)

- tuft testing bottom, aft view
- main undertray not installed
- 0 - 50 km/h (30 mph)

9. Clip 9 of 10 (0:44)

- tuft testing bottom
- main undertray installed
- 0 to 50 km/h (30 mph)

10. Clip 10 of 10 (0:45)

- tuft testing bottom, aft view
- main undertray installed
- 0 to 50 km/h (30 mph)

The difference between clips 6 and 4/5 after adding the front wheel skirt was clear on the tufts on the back of the car.

(OT: still, I didn't see any coast-down or fuel economy improvement when I tested mine. Frontal area increase + poor lower aft design is my guess why mine didn't work.)

[MetroMPG]

Hack a Day picked up the boat tail project yesterday (I sent them a link):

Aerodynamic tail makes Geo Metro even cooler - Hack a Day

It's a popular topic: 50 comments so far. I thought that on average they were pretty decent. Hack a day readers are probably pretty smart, after all.

But apparently one of their own thinks the comments are below average. The last post (currently) starts out...

omg… What has happened to the hackaday commenting crowd? Normally nice people interested in hacks… motors seem to bring out the freaks…

[fusion210]

That is a fantastic video of your tail and your testing!

[Frank Lee]

I thought on average they were remarkably naive. Wow. Just... wow.

My fave is Andy's Scientific Theorem Of Gravity. I would like to test that Theorem by dropping Andy on the top of his head, and on his face, to see which one had the most gravity.

[JackMcCornack]

My gosh, those comments on Hack-a-Day are an absolute hoot! Some guys get it, some guys don't, and some would simply rather not deal with it. I'm tempted to put the comment below in my sig:

> That aside, it’s the most retarded vehicle I ever saw and living proof that
> stupid hippies shouldn’t be allowed to own vehicles. If he’d put a V-8
> engine in it I might be impressed.

Ooh, ooh, this next one's my favorite. This one has me googling <bumper sticker printers>, it's time I replaced my handmade "My powerful libido compensates for my pipsqueak car" sticker with a professionally printed replica...and a few more for my hundred closest friends. If you missed it, there's a comment that ends with...

> Bolt that on the rear of your gaymobile and I guarantee you will never get
> laid again. Besides, real men don’t drive Geo Metro’s anyway!

You gotta love it!

[MetroMPG]

I personally liked the theory that the weight of the tail caused the front end to lift, allowing more air underneath which reduced rolling resistance, thus the MPG gain (at least I think that's what he was saying).

When I said I thought they were decent on average, I was thinking in terms of what I would have expected to read on other forums. EG: AutoblogGreen commenters would have been worse, I think.

[Christ]

The mythbusters have shown you can dimple the car like a golf ball for about a 12% increase in your MPG without going having to to resort to useless empty extensions of the front or rear of the vehicle. Granted, a dimpled car is not for everyone. But it does not require restriping every parking lot to accomodate 6ft aerodynamic extensions either.

Posted at 2:45 pm on Dec 25th, 2009 by BuzKill

*Facepalm...*

[alohaspirit]

Originally Posted by Christ

The mythbusters have shown you can dimple the car like a golf ball for about a 12% increase in your MPG without going having to to resort to useless empty extensions of the front or rear of the vehicle. Granted, a dimpled car is not for everyone. But it does not require restriping every parking lot to accomodate 6ft aerodynamic extensions either.

Posted at 2:45 pm on Dec 25th, 2009 by BuzKill

*Facepalm...*

he lived up to his name