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

[theycallmeebryan]

Metro, the tail is coming out very nicely! I think i will try to use similar CAD techniques to make a mock up tail for my Ninja.... soon

[pgfpro]

Nice!!!! its really coming together.

That thing is going to turn some heads

[Cd]

Coming along very nicely !
Looks like the EV-1 from certain angles.

[Bicycle Bob]

Very inspiring! Will we see little fairings behind the mud flaps, too? The thing about the concave rear is to encourage a single large vortex to form there. See the Cobra Coupe for a decent example.
Anyone else starting at CAD might want to do a scale model first with paper, to see how it all stiffens up when the shell is complete. Monocoques are inherently great structures, as long as the skin does not buckle. Modern plywood boats have happily dispensed with most of the traditional frame members, because 'glass taped corners are stronger than mid-panels anyway. While marine ply is sometimes worth the money, carefully selected 1/8" (3mm) door skins would make that curve very smoothly. Epoxy encapsulation would make it last.
For a Coroplast box, we can use two layers to get rid of most of the bulkheads needed to support a single skin. There are heavy guage panels available, but they are harder to curve smoothly. With two layers, having the whole inside of the 1st skin slit is no problem - just run beads of silicon to bond it to the inside layer. The very thick clear panels sometimes used for greenhouses could make a rather hazy but still useful rear window.

[NeilBlanchard]

Hi Darin,

How far out does the boattail extend? Looks great -- and seeing this and the several other boattails is getting me thinking.

My xA is even taller than the FireFly...

[Nerys]

talk about getting thinking. I wonder if there is a way to gain SOME boat tail benifits WITHOUT a boat tail at all. I am thinking a JET of air at the right angle along the rear to SIMULATE the presence of a boat tail. if you could somehow suck this air in up front you would even effectively reduce your frontal area. Hmmm thought juice flowing for sure :-) Can't wait to see your tail all done!

[Cd]

Originally Posted by Nerys

talk about getting thinking. I wonder if there is a way to gain SOME boat tail benifits WITHOUT a boat tail at all. I am thinking a JET of air at the right angle along the rear to SIMULATE the presence of a boat tail. if you could somehow suck this air in up front you would even effectively reduce your frontal area. Hmmm thought juice flowing for sure :-) Can't wait to see your tail all done!

We are discussing that sort of thing actually. I welcome your input

Compressed air for drag reduction ( Coanda slots )

I know what you mean though. It's a shame that you can't channel air from some other place to fill the area where the tail is trimmed off.

[Nerys]

well you CAN channel air from some other place but every way I come up with is expensive :-) ie lots of ducting and high pressure pumps ie the expensive part

[brucey]

Looking awesome! Your angles are a lot different than mine were. I hope it shows major improvement!

[MetroMPG]

The boat tail is complete, and I just took it for a 40 km drive:





Above: failed attempt to get some quick tuft test results. These pics are screen caps from my crummy 2 mp digicam movie that isn't quite up to the task of resolving the tufts - there really are a bunch of them on the back of the car in those pics! (I'm still planning to do the webcam-on-the-car trick that I know will work.)

The drive: 40 km round trip, next to no wind, about 7 C.

Observations:

• Once the car warmed up, it was pretty clear this is one of those rare mods that doesn't need A-B-A testing with a ScanGauge to really know there's an improvement.

• I can go faster than I used to at a given MPG point. I used to have a saying: "70 at 70" - a rough guideline that meant that I could drive with load on roughly level ground and sustain about 70 mpg (US) at around 70 km/h indicated (faster actual speed, since my speedo under reports by 6%). But on this drive, once the car warmed up, I was able to hold that 70 mpg at around 80 km/h (50 mph) indicated (faster actual speed).

• Based on that, a 10% MPG improvement at highway speeds doesn't seem out of the question.

• on a downhill I regularly drive I had to back WAAAY off the throttle compared to before to keep from accelerating past the speed limit

• seemed to be able to hold 5th gear longer on a couple up uphills. I love being able to climb mild grades @ 50+ mpg!

• the boat tail literally stops traffic. A guy actually came to a halt in his Chevy Avalanche in the middle of a turn in an intersection to gape - blocking through traffic from two directions.

• It's off kilter! Only noticed this once I got it out of the garage and could stand well back and look at it from behind: the tip of the boat tail isn't inline with the centerline of the car - offset to the passenger side about 3 inches. So much for eyeballing it inside a cramped garage! Should have used the tape measure again after setting up the bulkheads, before starting to put the skin on.

• It's strong enough. I can grab the tip of the boat tail and shake the entire car side to side and up & down, with very little movement in the tail itself. Yay for "hurricane tape".