echo boattail/hatchback (warning bad art)

[moonmonkey]

i am looking at chopping up my echo ,in an attempt to hit my goal of 60 mpg.
i can hit 50 now with some pulse and glide and many other aeromods, belly pans ,pizza pans,skirts,mirror delete,grill blocks,( alot of highway miles),, i know my limetations,,i cant get 60 with current equipment, i think i can make it look decent,maybe even factoryish,, i am very handy with welding/bodywork and decent at fiberglass, so it wont break the bank, i am following in basjoos foot steps here.unless my wife see,s me with the saw, luckly im better at acually building than photoshoping,, if i keep the angles at 15 degrees or less i figure it will work, i will mock up a wire frame (that will become permanate) if it works and cardboard it for a while to make sure it works,i've lincluded some unmolested images for comparison,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,, looking for input, criteque,advise,berating,bashing,what ever you got.

[Piwoslaw]

If you are really bent on cutting and welding the car, then instead of incorporating the whole, long boattail, just do a Kammback to the rear bumper. If you start be lowering the rear of the roof and bending the sides inwards, then that will really lower your Cd. Something like the Citroen C4:



At the end, make mounts for a detachable 'tail for long distance/highspeed trips, use without the tail for everyday errands. With a heavily Kammed rear, the 'tail itself can be relatively small, maybe you could stow inside the car.

[Christ]

Based on observation of needs and the necessity that some of us share for utility in our vehicles, I'm starting to think that maybe the "ideal" shape (looking from the rear toward the front, on the ground plane) for the average "grocery getter" that gets groceries 50 miles away at 65 MPH is an isosceles trapezoid.



Not necessarily of the same ratio/dimensions as the image, but the shape is applicable to most of our purposes.

For trucks, set that shape on top of a rectangle (which represents the bed) and it's still a good basic foundation.

In fact, it seems that many auto manufacturers are using trapezoids on top of rectangles for all sorts of vehicles, and some are using a trapezoid on top of an inverted trapezoid.

[Piwoslaw]

Originally Posted by Christ

Based on observation of needs and the necessity that some of us share for utility in our vehicles, I'm starting to think that maybe the "ideal" shape (looking from the rear toward the front, on the ground plane) for the average "grocery getter" that gets groceries 50 miles away at 65 MPH is an isosceles trapezoid.

I would think that making the roof narrower than the rest of the car definitely decreases drag. I'm not sure how it effects Cd, but it reduces FA. In my aerodynamics book I found a paragraph on the work of Asano and Takagi[*], where they changed the angles of about 10 different things to find the difference they make in drag. (Instead of 3^10 tests, they were able to reduce the number of tests to 81, and each test was done twice on a 1:5 scale model in an aerotunnel.) Here is a scan of the things they tested #10 was a spoiler):



So what Christ is talking about is #2 on the picture. Asano and Takagi tested two angles of the side windows, 22 and 30 deg. From what I can find (I haven't been able to find the original article, see bottom of post), the optimal angle is 22 degrees. I would think that 30 would be better, since it would further reduce FA, but apperently the optimal angle also depends on what's going on in other parts of the car. From the picture that Moonmonkey attached, it appears that the Echo's side window angle is 30-35deg. from vertical. I drew an exaggerated narrowing of the roof. I also suggest narrowing the rear with the same angle as the Kammback, 14deg.



As for the Kammback, a 12deg. slope (and slightly lowering the rear end of the roof) would intersect the bumper line just above where the trunk lid ends. A full Kammback, ie sloped roof and narrowing sides, would reduce the Echo's wake area by 40%-50%, I guess. The detachable boattail probably wouldn't help much then, and it shouldn't be longer than 2-3ft.



[*] Asano T., Takagi M., Use of a Designed Experiment for Systematically Testing the Effects of Vehicle Shape on Aerodynamic Characteristics, JSAE Review, No.2, April 1987, p.26-33.

[moonmonkey]

thanks for all your work on this piwoslaw,,,im going to study this a bit before i use the airsaw on it. your first post talked about a detachable tail for long trips ,my daily commute to work and back is slightly over 100 miles,so im looking to do permanate mods,i like the kammback idea, but dont want to cut the roof,so i might extend the rear a little past the trunk/bumper to achive the correct angle.,if i remove the trunklid , tail lights and bumper cover i will have many mounting points, that are reversable,(if ever needed) i would also bring the sides in as you recommend, i was going to do a bi/tri wing spoiler that aerohead came up with but it would be a compromise to a full kammback/boattail,i like the citoron look, but want more cd reduction echo started at .29
which is surprising,

[Cd]

Originally Posted by moonmonkey

i am looking at chopping up my echo ,in an attempt to hit my goal of 60 mpg.
i can hit 50 now with some pulse and glide and many other aeromods, belly pans ,pizza pans,skirts,mirror delete,grill blocks,( alot of highway miles),, i know my limetations,,i cant get 60 with current equipment, i think i can make it look decent,maybe even factoryish,, i am very handy with welding/bodywork and decent at fiberglass, so it wont break the bank, i am following in basjoos foot steps here.unless my wife see,s me with the saw, luckly im better at acually building than photoshoping,, if i keep the angles at 15 degrees or less i figure it will work, i will mock up a wire frame (that will become permanate) if it works and cardboard it for a while to make sure it works,i've lincluded some unmolested images for comparison,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,, looking for input, criteque,advise,berating,bashing,what ever you got.

If you can get by with replacing the rear side glass with lexan, you might be able to get some good curvature and have it taper some.

How often do you use the back seats ( do people ride in the back of your car often ? )


BTW Here is a question for everyone : What are the legalities of replacing the SIDE ( or rear for that matter ) glass on your car with lexan that is not rated as " safety glass " ?

[moonmonkey]

cd do you mean replacing the side windows and cutting the roof pillar to move it inboard for narrowing? that is an idea i did not think of, but i was tring to not cut the roof ,it being a unibody and all it might affect the structural integrity of the passenger compartment, i will have to think on it. 90% is me in the car by myself 5% me and daughter and maybe 5% with one of her friends in the back seat,as far as legality i think since lexan is shatter proof it would not be a problem, and further i dont think any cops around my area would know or care.would suc if i had to undo it though.

[Christ]

The lexan being shatterproof actually is the problem. You can't get out of the car in the event of being trapped in a situation where you would need to break the window to escape.

The only exceptions I've ever seen were for "wing" windows, the rearmost window of a pickup truck, and windows which aren't part of a passenger area. Even those are questionable, though.

[Cd]

MoonMonkey : If I were a welder, the first thing I would do would be to start cutting and tapering the back end.
As far as structural rigidity goes ( as you are of course aware ) a good weld actually makes the metal stronger in the area of the weld.

Looks like you are in for a nightmare load of work ! Is this your only car ?

Christ, so do you think this applies to rear windows too ?
That would totally suck to have to remove your windows after doing all that work and spending all that time / money on your project. ( Come inspection time )

[moonmonkey]

no inspections in fl. cd, i have another truck to drive f-250 4x4,, it kills me to drive it $20 + a day to get to work and back as compared to $6 - for the echo ( so i would do it over a long weekend. does'nt basjoos have a big sheet of plexiglass for his rear window? i think he does, plexi= acrylic (it breaks) lexan=polycarbonate (hard to break).