I had a try with my own car. (Apparently the picture I am modifying is of an electric vehicle already! (google image search daihatsu mira should come up with it on the first page)
Note the skirts, the rear tail (should be tapered on all 3 sides at the same angle, and have a perspex top to retain some visibility), the deflectors. Not visible is the undertray, grille blocking and mirror replacement with camera/screens.
I suppose there is no real reason why you couldn't go all out in extending the boat tail backwards, you'd just have to make some sort of fibreglass canopy for it along with extending some more brake/indicator lights back there. And technically, you could of course extend the tail I have designed to maybe double the original length. It would then become just like a regular car to park, instead of the current park-anywhere capability.
I suppose if a person was really motivated, they could make the sides of the boattail springloaded so that they would spring back against the car if they had the opportunity, and hook up the boattail part to some sort of servo that would extend the middle part out when cruising, retracting it for parking.
It might be possible to get a Cd of 0.25 or less with such a car. You'd have to experiment with instantaneous FE and adjust the angle until you got it right.
Edit: Damn... I think it looks cool! Just had to say that.
For the ultra obsessed, here is the logical endpoint (it's not obvious, but both sides are tapered as well, leading to a tiny rectangle of area behind which there is turbulent flow, approximately 1/4 of the original rear area. I'd estimate that the Cd on this would be below 2.0. There would have to be a note to not store heavy stuff near the end of the boattail.):
I think you'd need to start the wing at the top of the vehicle for it to work properly, with a gradual slope back. I think the beetle could also have a wing better designed for aerodynamics, but it would hinder rear visibility. I'm surprised they got this good a result to be honest.
It would certainly require lots of testing. You'd probably require a plywood mockup that you could drill holes into, test, then adjust again until it was fine tuned. Then get rid of the plywood and make the real thing.
You know, with the aero mods, a 1.6L turbo, and a gutted interior it would probably be a really zippy little race car. Hmm, maybe when I retire it from daily driving one of these days.
Originally Posted by The Toecutter
I figure a Metro's Cd can be brought down to the .25 level.
That's what it should been designed at. I can't believe it's .34 stock. Even my Aerostar is only .38 (though with 30 sq ft of frontal area).
Originally Posted by Mighty Mira
I think you'd need to start the wing at the top of the vehicle for it to work properly, with a gradual slope back... ...but it would hinder rear visibility... ...You'd probably require a plywood mockup that you could drill holes into, test, then adjust again until it was fine tuned. Then get rid of the plywood and make the real thing.
Yes, I think you're absolutely right on all counts. I think I may do a redesign on the wing and actually turn it into a mini-boattail. It may not be as effective as a full boattail, but I really don't like how the full tails look. I'll sacrifice some aero loss for the sake of aesthetics. The underbelly, skirts, and grill block will probably make enough difference that the losses will be minimal.
Yeah, and I probably will have to do the plywood trick before I find something that really makes a difference.
That Mira of yours is pretty interesting looking. Looks very much like a Ford Festiva.
Well, there's a mockup of the adjusted spoiler/boattail. It only extends back the same distance as the rear bumper (but it could go maybe 6" over) and it shouldn't (knock on wood) impair vision through the mirror.
Will you be building one then? Someone in the early days of the site posted a link to a guy who had added a foot to the back of my car and was getting 80+ mpg (he was doing other stuff, too, of course, )
I'm hoping to. I'm biting off more than I can chew with all the projects that I would like to do. I don't think I would get one of these built until next year, but maybe a cardboard model after I get a MID towards the end of August...???
I'm just fascinated with aero-mods right now since I know they work and I can't do all the engine/transmission magic that you Honda guys are doing...even though there is the 1.3L trans swap, light-pressure turbo, and maybe the Hemi-head to consider...
Tranny swap isn't really that bad when you get down to it, it's just plug and play, but turbo and such where compatibility modification/issues come into play are the trickier things. One day I'll try the LPT, jared is in charge of finding me one, need to bug him,