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

[Cd]

And you ( of coarse ) accounted for the weight difference right ?

What is the weight of the 'tail anyways ?

[Nerys]

once your done accelerating the weight is a very very minor issue in the equation especially since I bet its not very heavy at all. I bet I could take a good dump that weighs more than that boat tail (ok not really just trying to be funny :-)

[thatguitarguy]

Originally Posted by Nerys

once your done accelerating the weight is a very very minor issue in the equation especially since I bet its not very heavy at all. I bet I could take a good dump that weighs more than that boat tail (ok not really just trying to be funny :-)

There might be more than a couple of people who think you are just about that full of it...

[Christ]

I'm only interested in the weight of the tail for leverage reasons... side loading on the rear tires during heavy cross winds, or the event that could flip the car windward.

EDIT
I don't think this is really too much of a concern, I'm just interested in the data. A guy can want, can't he?

[thatguitarguy]

Originally Posted by Christ

I'm only interested in the weight of the tail for leverage reasons... side loading on the rear tires during heavy cross winds, or the event that could flip the car windward.

If you're talking about Darin's experimental design, I think that in heavy crosswinds, it would tear itself free of the vehicle (creating another set of problems), or at least partially collapse, allowing him to pull over before flipping over.

The same design in a more permanent configuration, has enough of a symmetrical conical shape that I think that, except in extreme conditions, a crosswind would pass sufficiently around it. I think this design is safer in that aspect than more "fishtail" type designs.

EDIT: I wrote this before I caught your edit.

[Cd]

At what point would a 'tail like this one cause enough crosswind drag to cancel out any benefits from straight on ?

[Christ]

That's Ok, your answer is basically what I was after.

I'm not worried about tearing away, mostly because I would assume a real boat-tail design would be attached in such a way that it could not.

I think the effect I'm looking for data on is "Wind Vane" effects, where the rear tires become an axis if enough leverage is generated against them sufficient to (please read entirely) significantly lift the front tires in a cross wind situation at speed.

I am not suggesting that aerodynamic lift is or will be an issue! (Good gods, we don't need that again.)

What I'm suggesting is that because the wind is constantly changing direction, a well-aimed gust could be sufficient to force the tail down, creating a fulcrum of the rear wheels, while the gust force spun the car so that it faced into the wind.

Again, I don't see this as being an issue, I just want the data!

EDIT: Length is the primary consideration here, actually.. Weight only plays a small part in it, when considering the force necessary to be exerted on the structure as a whole in order to lift the front tires enough to allow the vehicle to spin laterally.

[Cd]

Originally Posted by Christ

I am not suggesting that aerodynamic lift is or will be an issue! (Good gods, we don't need that again.)

.

[thatguitarguy]

I think all these questions point to designing more of an effective "Kamm" style tail, than a full boattail, for a real world commuter vehicle. Though I think the AeroCivic was well designed and has effectively proven the point that it can be done, my ideas run more toward a "Kammtail" extension, a little longer than the one on the MetroMPG Firefly, giving the vehicle more usable cargo capacity, and beyond that, a collapsible, perhaps inflatable, extension for longer trips, when it would be more effective for long range mpg gains, and less of a pain in the boattail to deal with.

But alas, I'm just theorizing while awaiting videos and test results.

Quit teasing and get on with it.

[Nerys]

hmm these cars seem pretty nose heavy. my "gut check" says any gust of wind sufficient to be a danger to the boat tail and a pivot/weather vane motion is already dangerous enough to be lethal to the stock car unmodified. IE one heck of a gust IE you should not be driving in those conditions tail or not :-)