Volvo 240 wagon - partial Kammback prototype

[MetroMPG]

Originally Posted by brucepick

When I originally suggested this thread (which was split off from another) my intent was to see more information on diesel_john's car. However this thread got titled with my own car's description.

If diesel_john replies to your request to write more about his aero mods, I'll make it into a new thread as well. The more the merrier!

...

Neat drawings. Don't forget: the initial angle may be in the range of 12-15 degrees, but if you use a gentle ellipse rather than a straight line, you can continue to curve towards that theoretical point behind the car where all planes meet, and exceed the initial transition angle where you depart the roof line.

Also: don't think so much in terms of adding a pyramid shape to the back of the car. The "hard" corners between the planes (e.g. sides & top) promote vortex formation; You want to round those transitions generously if possible.

I don't know if I dare build this thing.

You don't have to go to extreme lengths. Any boat tailing will help some. My 2 foot long cardboard kammback (which was mostly a roof extension, and had poorly shaped top-to-side transitions) made a measurable (if small) difference @ 55 mph: http://ecomodder.com/forum/showthread.php?t=94

[brucepick]

Originally Posted by MetroMPG

...
Neat drawings. Don't forget: the initial angle may be in the range of 12-15 degrees, but if you use a gentle ellipse rather than a straight line, you can continue to curve towards that theoretical point behind the car where all planes meet, and exceed the initial transition angle where you depart the roof line.

Also: don't think so much in terms of adding a pyramid shape to the back of the car. The "hard" corners between the planes (e.g. sides & top) promote vortex formation; You want to round those transitions generously if possible.

You don't have to go to extreme lengths. Any boat tailing will help some. My 2 foot long cardboard kammback (which was mostly a roof extension, and had poorly shaped top-to-side transitions) made a measurable (if small) difference @ 55 mph: http://ecomodder.com/forum/showthread.php?t=94

Good points all. Thanks.
And especially thanks for the link to the post re. your Metro work.

Rounded edges is / are definitely a good idea, as is the continued curve towards a smaller cross section. I'm glad you pointed those out to me here. I also wrestled with a compound curve on the temporary "Kamm" thing I had on last summer. I used "dart" cuts in the plastic sheet.

I wish curves were as easy to build in a driveway! I'm good with wood, kinda OK with aluminum, could probably work with lexan/plexiglass. Giberflass is probably not in my bag of tricks any time soon.

Aw man, you mean I don't have an excuse to go to extremes!!?!!? Shucks!! I remember seeing a sig on a non-FE site "Overkill is consistently more fun".

[MetroMPG]

I love that phrase - that's a great bit of wisdom! Haha.

Don't knock your head against the wall if you can't do curved work. Have a look at the Mira's partial kammback - it's got hard edges and straight lines, and worked for him: Daihatsu Mira aerodynamic modifications

But curved shape & rounded transitions are the ideal.

[diesel_john]

brucepick, PM me if you wish, i would like to be able to send you files directly.
opps, you already did, i didn't see it.
no, It's not about patents. i give all ideas away in hopes someone will manufacture them.


30 degrees is the worst possible slope for a back window, it is even worst than 45, 60, or 90.

"But curved shape & rounded transitions are the ideal."
i agree, with reservations, that being flat plates can be set at 11 or 12 degrees relatively easy. You have to be willing to put a fin along that diagonal edge where the two planes meet. If your concerned about looks on these mods then you are already compromised before you start. IMHO, Curves have to go all the way to a point or a line or flatten out before the exit to prevent air from turning in too soon. Getting into my wacky ideas, it's not how long your tail is that counts it's how long the air thinks your tail is that counts. Imo, the number one problem with most cars is the air underneath and how the air exits from underneath.

see the top back edge of this truck, the top flatens out at the back edge. the air sees the truck being a lot longer than it is. imo, no matter what you build, the exit is what counts.

[MetroMPG]

Diesel_John - if you want to keep the information private, that's OK. But would you mind if brucepick posted any questions in this thread, so everybody benefits from the exchange?

[brucepick]

Thanks guys, for your encouragement and information.

Project stalled.
I paid a visit to DMV today, asking about regulations.
Apparently in CT, anything permanently attached behind the rear bumper is a no-go. I hope to check into this further to see if maybe the inspector was misinformed.

Instead, I'll likely put my attention on other projects I have planned - underbody smoothing and maybe an oversized roof extension similar to the pic I posted in the start of this thread.

[diesel_john]

got those bumper extensions ready yet, you need something stiff out there to mount to any way. is the front reg. the same deal?

pic below shows air dam of rubber belt (10" baler belting, tractor supply or farm and fleet)

Important: in the sketch the belly pan extends fwd of the air dam (even if just a few inches), imo, this holds the air on the dam to be channeled around the sides, imo, the air dam is not complete without this extension.

pic of a truck rear, but works on rectangular objects could be scaled down to station wagon size. i'm looking for the formula, i think it was (inset=.03 x width) and (extension=0.3 x width)

a well proportioned dam and intake, scale for your vehicle

good side skirt design, note portion almost horizontal.

[MetroMPG]

Originally Posted by brucepick

Apparently in CT, anything permanently attached behind the rear bumper is a no-go.

The operative word seems to be "permanently". E.G. what about bike racks?

I don't know how much of your driving is highway speeds, but I'm considering a permanent partial Kammback on my car that will serve as a mounting point for a longer, removable version for highway trips. = avoids the "permanent" issue.

[brucepick]

My driving is mostly highway. I live about 1/2 mile from I-95 in a semi rural area, I use it to get nearly anywhere.

Below is a pic of my car's front.
You'll see the grill panel.

You'll also see the air dam that extends about 4 inches below the oem air dam, and the side panels I added that move the air away from the body cavities.

What you don't see is the flat panel beneath the air dam. My added panel plus the oem belly pan together cover from the air dam to the front wheel centers. This spring I hope to add screening from there to the rear bumper.

[diesel_john]

looks good, can any air get in below the bumper

MetroMpg,
could you please look up the formula for this extension in the Hucho, Barnard. I think that is where i saw it.