Detachable boattail concept - Fiat Punto MK1 - Fuel Economy, Hypermiling, EcoModding News and Forum

jabb · 07-30-2010, 03:00 PM

So basically for the last 1.5yr I've been making model of my Punto with game use in mind. It's got pretty accurate exterior so I thought I'd give it some aero modding.
This is what came as a result:

I thought I'd like a boattail but if I ever build it I want it to be easy to detach - I rarely go out of town so it would be just an extra load most of the time. I want it to consist of several parts so that it'd fit inside the car if needed. I also want it to be mounted on trunk lid edge and bumper edge. This, along with location of rear lights makes it hard to get proper smooth line of sides, but this should do the trick IMO. I'm not really sure about the blue part but there's no way to make it more narrow and still have it mounted on bumper front edges.
I imagine it'd be made of fiberglass with some tubes molded in on the edges to give it shape and help connecting parts. Yellow rails are there to prevent sides from collapsing. I'd also like to add a diffuser but I do not have underbody modeled yet.

Is this design of lower part going to help or hurt aero? Should I try and make a smooth transition between pink and blue parts? In virtual world everything is quick and easy to do so it's better to get it right there before actually starting IRL.

BTW - anyone could help me with CFD testing?

Bicycle Bob · 07-31-2010, 12:01 AM

If you don't keep it smooth going over the taillights onto the pink bit, it really does very little good, and masks the light anyway. Try clear panels for the sides. I'd try to integrate the back with fender skirts, and possibly some trimming of the bumper width.

jabb · 07-31-2010, 08:11 AM

So this one should blend a bit better but it will require relocation of rear lights (since I've no idea on how to shape acryllic sheets or stuff like that). Rear lights are easily removed (held on 2 bolts) and I should have no problem lengthening (?) wiring. At the moment parts are floating in the air but that's not a big deal...
I'm not sure about mounting method for fender skirts and the pink part might be a challenge in this setup. I can't make these parts any bigger since my trunk is quite small and it's supposed to hold some luggage too.

Bicycle Bob · 07-31-2010, 11:17 AM

That's a much better shape. You can probably modify the pink bits to be just flat-wrapped, unformed sheet, but if you want to do vacuum-forming, it is easier to learn than fiberglass, and the equipment can be cobbled together cheaply. You could probably just leave fiberglass untinted and pass enough light from your signals - might even improve visibility. See corrugated fiberglass roof panels.
Another option would be to use separate lights, and plug them into a trailer terminal.

aerohead · 07-31-2010, 12:53 PM

If you construct according to your second design rendering I predict Cd 0.257 for the car.
If you'll streamline your wheels/tires,you could get it lower.

ShadeTreeMech · 07-31-2010, 12:57 PM

Quote:

Originally Posted by aerohead

If you construct according to your second design rendering I predict Cd 0.257 for the car.
If you'll streamline your wheels/tires,you could get it lower.

I was going to try to put in my .02 worth, but aerohead done solved the problem.

Piwoslaw · 07-31-2010, 01:54 PM

I like what you're planning to do, Jabb

Will the rear of your boattail be open or closed? I believe that someone here tested the latter to be more effective.

I wonder if the lights you plan to put on the extension must be from your model, or if any kind of light fixture (as long as it is certified) will do. This would make it easier to install. And don't forget about the license plate.

I can't remember if I've linked this before:
Kammbacks & tail aero info

jabb · 07-31-2010, 02:32 PM

Bicycle Bob: I've read somewhere about vacuum-forming and it's not a bad idea. No matter which material will be used I first need to make plugs for molding purposes.
And I don't have trailer terminal.

aerohead: 0.257 figure sounds impressive and worthwhile. About wheels - I'm actually running on regular steel rims, these hubcaps are original FIAT parts but since one is slightly chipped they stay in my garage. I don't think there is a flat hubcap supplier here in Poland and shipment cost from UK or wherever they can be bought would probably bump up the price significantly.

Piwoslaw: I'm glad you like the idea. I think that a closing part would not fit in the trunk if it was in one piece and making it from multiple parts would probably make it flex a lot. Every additional kilogram of weight counts when it has to be supported with homemade mounts

I haven't thought about using rear lights from different vehicle but with narrow space like it is inside the tail I could run into legal problems - minimum spacing between lights is 60cm, also lights can't be mounted further than 40cm from the sides. I think that I could use small, trailer lights - shouldn't be hard to fit.
I don't think that license plate needs to be relocated with open tail.

jabb · 08-01-2010, 10:36 AM

So this is how I think it should look like.
Top consists of 3 parts of polycarbonate\acryllic and should be fairly easy to mount (middle section held on trunk lid edge, side sections held by rear lights, sides would be bolted together with the center piece).
Red section is a bit more tricky - it would overlap the transparent parts so some bolts\clips could hold it there, but I don't know how to hold it attached to car body.
Fender skirt must fit one way or another, not really worried about that now.
Black parts might actually be held by clips in holes I'd have to drill in the bumper.

If only it could be formed like this it would actually look good! I'm really happy with this design. Too bad it will require some strengthening structure in the tail part...

Piwoslaw · 08-01-2010, 11:08 AM

Me likes!

Maybe the red and black side parts could be made as one piece? How far beyond the rear bumper does that extend?

Reconsider getting a trailer hitch: I saw the set-up for less than 200PLN, plus installation. Then you could hang quite a bit on it, like a cargo carrier inside the boattail:

I believe there are carriers which fold up when not in use.

I've been thinking along the same lines in the last few days: The bottom half of the 'tail would be fixed to the rack while the upper half would lift with the rear hatch, as the kammback does at the moment.

I do wonder about the legality of such things around here. I'm only two steps away from sending a few e-mails to ask the authorities.