Volvo 240 wagon - partial Kammback prototype
 

I'm interested in diesel_john's rear aero treatment.
http://i188.photobucket.com/albums/z...mm_smaller.jpg

The more I look at the full size pic the better I like this back.
http://ecomodder.com/forum/attachmen...6&d=1202433430
I know how hard it can be to do a back when the liftgate / body cut is on the rear panel and not on the side.

Can you please start a new thread with some details on this one? Or if the thread exists and I missed it - post the link?

Here's a pic of what I had for a couple months this past summer. I tore it off when I wanted to clean up the car so it wouldn't look like it was Ma and Pa Kettle's (google it, I guess, if that's not familiar to you).

http://i188.photobucket.com/albums/z...k/100_0027.jpg.

You can see the taped joint at upper right were top piece meets side. Had to be untaped and retaped every time I opened the tailgate. Plus duct tape residue all over the place. As with other mods, no confirmation that it actually helped. But if using a good clean design I think it has to help.

I was thinking today that you could buy another replacement tailgate from a junk yard and build a more permanent boat-tail on that. Then you could just re-bolt the original hatch on when you want to sell it or look normal hehe.

Brucepick - hope you don't mind I split this off into a new thread. It's interesting.

And I agree - I'd like to see a new thread about the Rabbit treatment.

Question about yours: was there much downward taper from the roof line? It may be a trick of the photo, but it seems to be more of a roof line extension, ie straight out.

I like the idea of the junkyard tailgate. I hate to haggle up perfectly good parts. I am neither professional nor proficient enough to indulge in that.

mattW -

Most excellent idea. That's been one of my eee-vile plans. In the meantime, why not try to prepare mock-ups that attach to the tailgate and still form a flush connection to the side and top of the car?

CarloSW2

i'll take a pic when the windchill gets back above 0. my hatch is fully functional. the side plexiglass panels slide into a slot at the bottom which makes them stable on the highway. all mounting screws on the hatch are outside the sealing gasket, so no water can get in the cab.

hatch is functnl,
side skirts are hinged so i can get my lift arms under there,
full belly pan, except under oil pan.

was inspired by Ernie Roger's bug TDI.

My general rule of thumb is no more than 15 degrees change.

can remove wheels without removing tin work now.

can only see a thin line of the rear treament out the back window.

each vehicle is different, tuft testing is the shortest path to success.
start your tail where the separation is pronounced.
i think, bottom is the most difficult to exit clean and the most important.
other pics and posts thur out this site.

Thanks for splitting it off for a new thread. I'm going to take a good look at the other pics of the Rabbit back. diesel_john, please write up as much detail as you have time and patience for. And thanks!

Mine did taper downwards. You can see it in the pic at the right corner where the top and side pieces meet. At center maybe not going down quite as much but it did go down. I was trying to be conservative and not have too much of a downward angle. Roof of course is essentially flat/level as it's a long wagon with flat roof.

diesel_john,

Can you post a gaslog? Or is there one somewhere that we can see? What kind of FE improvement have you gotten with your setup?

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. Both cars have a large rear face with the large wake that generally produces.

Anyway, based on the guideline of maximum 15 deg. angle to maintain attached flow, I did two drawings. I didn't measure the drawn angles but I think it's close enough for where I/we are now. You can see that to reduce the rear face appreciably on this car you need a reaaaaaaallly long boat tail.

I don't know if I dare build this thing. It would have to be pretty large, which probably means heavy, and also will have a large side profile to catch side winds, with all the construction issues that come from those issues.

http://i188.photobucket.com/albums/z...ideView400.jpg

Rear view:
http://i188.photobucket.com/albums/z...earView400.jpg

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.

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".

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.

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.

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?

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.

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.

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.

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.

http://i188.photobucket.com/albums/z...lip45deg-1.jpg

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.

Bureaucratic dodge: Remove your bumper. Make up a little piece of aluminum sheet bent kinda like a bumper. Attach it at the back of you tail. declare the aluminum to be your bumper.

Nope, pretty well sealed. Factory air dam is lip-mounted to bumper's lower edge. My air dam extension is screwed to lower edge of that, nearly a perfect seal. Factory one has holes that feed to the radiator. Well, have to admit that the ducting/baffling is not perfect there. That's something I could address, I could build some additional panels into that system. Thanks for making me think of that.

Anyway, the bottom panel is a lot like the one in diesel_john's thumbnail diagram, its joined to the air dam's lower edge via a bunch of small angle brackets, all the way along.

brucepick -

Stuff like this is good to know. Maybe we should check all our state regs and add it to an Ecomodder "state laws" page.

CarloSW2

The tailcone of this airplane has the optimal proportions for a boattail and is what I used as a model when designing mine. The Questair Venture is a 350mph kitplane nicknamed "The Flying Egg" for its short fuselage. The angle on the bottom side of this boattail is a bit sharper than optimum for a ground vehicle, but I didn't change it on mine since I needed the ground clearance for entering my steep driveway. Since your Volvo has a lot of overhang behind the rear wheel, it'll be more likely to scrub the pavement with the underside of your Kammback if the bottom angle isn't sharp enough. As far as the rear bumper problem, remove your stock rear bumper and design a new rear bumper into the back of your Kammback. Its not like they require 5mph bumpers any more, a bumper is just a flat peice of metal, foam, or plastic on the rear of the vehicle.

http://img.photobucket.com/albums/v4...nture_s_th.jpg

basjoos and Big Dave-

Good thoughts.
I'm going to have to look into the state regs myself if I can manage it.
Adding something I can call a bumper could be the solution.
I'll save the airplane drawing as a reference.

I've seen 12 degrees and 15 degrees mentioned. Anybody have a hard spec on that? How about a suggested radius? That would probably be as a function of length I think?

The good news re. scraping the boattail is that my bumper (bottom) is about 16-17 inches off the ground. So any boattail starts there and goes up as it goes further back. I'd have to pay attention to that though.

I'd probably leave the oem bumper in place. It's provided terrific protection in some rear-enders that I've been subjected to. You shoulda' seen the other cars! GMC Savannah van that hit me got hauled away. No lie.

Hey Bruce, long time no speak. I'm happy to see you are keeping up the 240. You seem pretty dedicated to improving the fuel economy of the 240 wagon. I would seriously consider the M47 swap I did. The difference in consumption between the no lock up AW70 and M47 is massive. We saw as much as 32mpg on the highway on a couple of occasions but 27-28mpg most tanks. I know it seems like a big project but with a little planning its totally doable.
After the accident (the 240 was totaled) Liz is driving a 96 NA 855 5 speed averaging about 24mpg with lots of city driving. I'm working on it :thumbup:
Cheers
Justin

Hi Justin -
Good to see ya again!

Yes I'm pretty dedicated. I always have some compulsive thing that I put my attention on. I've been on FE for the "brick" for a while now. The e-fan conversion project you inspired me for was summer '05.
http://i188.photobucket.com/albums/z...k/EFan_288.jpg

I've been tempted to do the 5-speed conversion as you just mentioned but you're right - it's a big project. Would cost some major money too.

I've considered a Volvo 850 5-spd for a future daily driver - but while they're more fuel efficient than the 240's, they're not a whole lot better. Also from '96 forward the 5-spd 850's became pretty uncommon. Not easy to find one of those! Pre-'95 or '96 won't take a scangauge, which is a requirement for my next car.

Sorry for the OT. Good to see an old friend again!

inneractive airfoil site
 

if your trying to develop a curve you might be interested in this site:

http://www.pagendarm.de/trapp/progra...les/NACA4.html

Hey Bruce
Quick question. What kind of plastic did you use to make your airdam and more importantly where did you find it. Did you put it together with rivits, nuts and bolts. Im dreaming up an undertray for the 850. I was planning to use aluminum but that stuff is getting expensive and hard to find.
thanks for the help
Justin

if you don't mind sorting the junk, your local alumimum recycler, will usually resell. one time i bought a bunch of road signs the county scraped. big pieces.

I like wood. Cheap to buy and easy to work. I used approx. 1/4" plywood for the airdam and 5.4 mm. lauan plywood for the undertray. More recently I used 2.7 mm. lauan for the 'grill' panel - I like the 2.7 mm. a lot. Get lauan at Home Depot etc. 2.7 mm. is $7.xx per sheet!

Folks can joke if they want about the plywood. Even without primer protection I had no rot problems due to lots of drying airflow. However, no primer = finish paint gets wrecked fast. I used oil base house primer on the grill panel and the primer survived winter's blowing road sand and everything else. Any new wood work will get that primer. I had them mix it as dark a gray as they would go. Too much pigment = not durable. I wouldn't use latex primer. Oil base is a lot tougher. Just doing small sheets, so I use cheap foam brushes and cleanup is not an issue.

The grill's dark blue finish coat took a beating though. Black pickup bed liner paint from Duplicolor has done very well on top of the primer when I put it on the air dam and in other applications. It's tough stuff, and cheap. I'm still looking for some tough clear protective top skin. I'm sure the bed liner (comes in black only) would hold up fine on the grill but I think it looks good in dark blue.

The 1/4 inch plywood strip for the lower air dam was a bear to bend. This spring I plan to build a new one of 2.7 mm. lauan.

I got a strip of aluminum angle stock and cut it to make angle brackets. That's what attaches the undertray to the lower air dam. I used stainless screws + nuts. Rust is a PITA, stainless ain't that expensive unless you want metric.

Next time I'm considering pop rivets instead of screws + nuts. Because you have to get at the back side to hold the nut, the undertray is roughly 1/2 inch up from the air dam's lower edge (for access to the nut). Using rivets I should be able to drill thru panel + bracket and then put the rivet in. So undertray can go flush at bottom edge of lower air dam.

Good info, thanks Bruce. I think I still prefer using metal or plastic but at least I know if all else fails I can rely on wood.

imho, on a blocky car, it is important that the air leaves all four sides at the rear edge of the vehicle at same time. to further define this picture, ideally the air on all four sides would have the same amount of energy, so pressures, speeds, and mass if taken all together would be equal for each surface at the back edge. simply put i wouldn't want air from say high prressure on top of the vehicle to come off the rear and plunge to a low pressure area coming from under the vehicle. so the air needs to leave the rear edge parallel to the streamlines of the vehicle. carrying this notion further fwd along the sides, the air needs to stay on the same side of the vehicle as it travels back along the car, on a square cornered car, i would put fins on the corners (they could be a on the 45) to keep the air from spilling over the sides. if the sides are angling in slightly as they come back, (11 to 15) degrees, the last couple inch should straighten out. Fins on these tapered sides have an added bonus of pressurizing the air between them slightly which causes it to speedup as it leaves. almost like squirting out the back. what do think of those wacky ideas?

brucepick -

This is not as ambitious as what you want to do, but this is what I want to do :

http://img297.imageshack.us/img297/4...w06eql0.th.jpg

CarloSW2

aero back shaped by the wind
 

my caravan was parked yesterday facing a snow storm. i did not touch the snow. this is what the wind did. the wind was coming from the front left corner, so shows what a yaw angle would do. the two back sides are different

pics 3,4,5. are my variable grillmockup, on kitchen drawer rails.

cfg83 - I had one sorta like that for a few months last summer, coroplast and duct tape. Couldn't document that mine helped but I'm sure that if its built well, it will help. Reading on the Kamm topic in wikipedia makes me think you could/should slope it down a bit steeper. The idea is to have a smaller cross section at the point where the airflow finally leaves the body. Taper it downwards a bit. Curve it down if you can, but making a curve there will probably get you a compound curve that's tough to build. Taper it in sideways too.

diesel_john - Very neat - it made its own Kamm back!

Natural Selection in action :D

I don't want to hijack but I am curious about the screening. Is a tight and light screening an effective way to do a full belly pan?

I apologize. I'd forgotten that the screening discussion was on another board, not on EcoModders. You can see it in this link; the 5th post in the thread is where the screening discussion begins.

http://www.gassavers.org/showthread.php?t=5613
I'm interested in it but need warmer weather and to make the project a priority if I'm ever going to get around to covering Sven's underbody.

Thanks for the reply and the link, most interesting.
Some thoughts.
Spray with rubberized under body coating. It is inexpensive and will cover in one application. It likely will leave a "pebbled" finish which may act as the dimples on a golf ball.
They do make stainless steel screening. Stiffer and less likely to sag.
For my Matrix an actual screen frame likely will work. It will stretch, keep the screen taught and attach with fewer attachment points. Just might be easily removable to.

I also wonder if is necessary to cover the holes.
Thanks again.