Stainless Kammback for Previa.
 

Just finished this today. At this point It's still a roof and sides extension but I'm adding bigger panels soon. I can't wait to try it out. I have to find some rubber moulding for the leading edge on the sides where it touches the paint.

The pieces are all totally detachable and no holes were drilled into the body. I used existing bolt holes and bent stainless brackets to attach everything to the back door. I tried to get around 10 degrees slope and will gently curve the panels to get optimum taper. I plan to make the panel over the rear window plexiglas so I will be able to see and do tuft testing easily.

http://ecomodder.com/forum/member-or...9-p1000805.jpg

http://ecomodder.com/forum/member-or...0-p1000814.jpg

http://ecomodder.com/forum/member-or...1-p1000810.jpg

Cutting out the side panels:http://www.youtube.com/watch?v=d0pwL__s3_w

orange4boy -

Wow, I love 100% reversible mods. You're my hero!

CarloSW2

Thats a very nice bit of craftsmanship. Well done. Does it save much fuel? Improve stability?

ollie

Here are some more of the brackets.

http://ecomodder.com/forum/member-or...k-mounts-1.jpg

http://ecomodder.com/forum/member-or...k-mounts-2.jpg

It was a PITA to get the stainless to bend in just the right shape but nothing a good hammer couldn't sort out.

I'm hoping I won't get any flutter at speed. There is a small gap which will go away when I find a moulding. In the meantime, some foam draft seal should do the trick.

Ollie,

I haven't had a chance to test it yet. Will do soon but probably no a-b-a.:( At least not yet.

orange4boy -

I was gonna mention the gap later on, but I was betting you're already on it, and you are.

CarloSW2

Awesome job!


Cant wait for the results

Took the van to town today. Too soon for data but coast down on a regular run was about 5-10kph higher than before.

I did register a lot more LOOKS than before. Does that count for anything? I preached the Ecomodder gospel to a few interested people too.

Fantastic work :thumbup:

I'm so ashamed with my uglyback... :o

Please, keep us informed with your feelings and your improvements :turtle:

Denis.

On the road today with the new extensions. Coast down is definitely better. I got up to 120kph in a spot where I usually only hit about 105Kph. Mileage stayed the same but I was using A/C today.:o The egg is a greenhouse crossed with a magnifying glass. Where can I get those mesh seat backs in Vnacouver?

I can't wait to add the kamm panels. Wait, what am I doing here typing when I could be modding?

dont forget to get full side shots

hope those stay put in an accident...other wise thats a stainless head chopping blade.

Binger -

Yeah, I'm paranoid, so I'd do the panels in plastic (which are still dangerous at speed), but I'm not worried about orange4boy's design. All told, I see 6 bolts holding that down.

CarloSW2

They are rather scimitar like now that you mention it.

Actually, one of the reasons I am using steel and aluminium as a base for my mods, other than looks, is strength. Everything is tapped, threaded and lock-washered. My Dad is a little more cavalier with these sorts of things and it always made me nervous so I'm super careful about things like proper tie down straps etc.

I always cringe when I see someone driving with ten foot wood mouldings sticking out a window, or someone on the highway with a mattress tied onto the roof with strings going through the windows, one hand on the steering wheel, one hand attempting to hold down the mattress as it tries to achieve flight.:eek: Funny, now that I think about it, it always seems to be metro drivers.:D

Previa side shots coming soon...

Man, what an great looking mod!

They could hold a boat tail too.........:)

I'm contemplating a 75% detachable boattail, but I can't devote the time to it right now and I'm going to start with the most practical step first: a kammback that does not require mounting any taillights.

I may not be able to resist the boattail for too long with all this encouragement/egging on.

On the other hand, if I use it as a marketing tool with my logo on it I may be able to convince my lady to let me have at it. The question is would it be good or bad PR? The media likes to take the piss out of anyone doing their own thing. Canadians can be daisy cutters.

Tuft Testing Kammback
 

Finally I had a chance to try out the angle of the kammback. It's at about 15 degrees in the picture. There was no flow separation at that angle. tufts were flat and pretty motionless. I tried it at more like 22 degrees and the tufts didn't seem to lift, but they wiggled side to side a bit. Hard to tell for sure without seeing from the side.

http://ecomodder.com/forum/member-or...ack-tuft-3.jpg

I think I will leave it at 15 degrees which looks better, is guaranteed attached flow and leaves me some rear view. I may redo the videos tomorrow.

Here is the video at 22 degrees for what it's worth:

http://www.youtube.com/watch?v=HhnyCPnArNA

orange4boy -

I applaud your work. YOU WILL BE COPIED!

CarloSW2

even without the kamm, that side shot is money!

looks real clean!

Nice!! That's another good side of clear Kammbacks: you don't need to attach the camera on the outside:)

The Previa (my wife hates that name, it always makes her think of placenta previa, which is bad if you're pregnant) was always kind of a pig at the pump in spite of the shape. You've dragged it into not-eligible-for-clunker-cash territory.

Dang.

Mods in stainless and no new holes, you are an ecomodding monster.

More mock ups. I am trying to decide if I want a plumb line at the back or an angled line to correspond to the angled back of the Previa. The angled back looks better, I think but it makes the whole thing a bit longer than I was shooting for. The bottom picture shows the two possibilities I am contemplating. It may come down to the size of the lexan pieces I have.

http://ecomodder.com/forum/member-or...kammback-2.jpg

http://ecomodder.com/forum/member-or...kammback-3.jpg

http://ecomodder.com/forum/member-or...kammback-4.jpg

I found the anodized aluminium extrusions at Ikea in the as-is section for a buck each. Ecomodding score!

OOOoooooooooh. :)

Wow.
That is looking great so far.

Plumb line... To me, anyway.

Plumb it is.

I had a piece of 1/8" plexi around so the end is filled with the table saw shaped piece. The sides are black coroplast for now but I think I will pop in the twin wall lexan when I get some more. The bottom is not filled yet but I have some "clear" coroplast which will probably fill that function

Construction details... Drilled and tapped for 8-32 pan head stainless machine screws.

http://ecomodder.com/forum/member-or...kammback-2.jpg

I'm not totally satisfied with the look of this. I think it needs a radius at the end at the top corners and along the lexan seam. I can fix this later. I also want to make a side detail that reflects curves of the stainless scimitar panels.

http://ecomodder.com/forum/member-or...kammback-3.jpg

There she is 90% done and driveable. As a fun design accident, the trailing edge has a trip angle to it.

http://ecomodder.com/forum/member-or...kammback-6.jpg

Wake area reduction is approximately 7 sq feet. from about 28 to 21 so almost 1/3 reduction, if my math is right. It sure will be interesting to see how that feels at highway speed. Who knows what the un-modded bottom half does to the air. We shall see.

unknown quantity
 

Without the lower half she's an unknown quantity.I suspect some trailing vortices as with two points of separation,at different velocities and pressures,it's kind of a setup for that.Does seem like the wake will be positively modified though.Best to you with it and let us know when you get some numbers.

once again, stellar stuff right there


id be interested in knowing:


1. how much length does the tail add?

2. how is visibility?

3. mpg gain (naturally)

4. what police have to say about it?

(i know some get curious and ask, is why)

Thanks for all the :thumbup: everyone...

Alohaspirit,

Tail overhangs the bumper a little over a foot. The tailgate does not stay open without a prop now:( I probably need new ones anyhow.)

My rear window is cut by about 2/3. I can still see cars when they are about 3 lengths away. The lexan breaks up the light a bit but I can still tell how close they are through it.

MPG gain is unknown until I can do some highway runs. I doubt that it will affect local island driving because it's mostly hills and I rarely get above 35mph.

I have not encountered the Mounties in the van yet. I may be a little out of the regs with it because it blocks the tail lights at high angles. I will add some extra lights soon. I need the plugs and some LEDs. I'm hoping to find a strip of red for the middle stoplight.

I like this angle best: That's what the rigs will see when they haul ass up behind me.:eek:

http://ecomodder.com/forum/member-or...kammback-4.jpg

orange4boy -

From the reflection it looks like you made a clear cover for the vertical rear component. Could you have gotten away with not having it?

CarloSW2

right on man

that thing looks great

cant wait for the numbers

I asked aerohead about this and this was his reply:

So I closed it. I think it looks better too.

orange4boy -

Ok, wisdom from the master.

CarloSW2

I thought I would post these which I made last year. I can't remember the values I used but I have not been able to reproduce them since.

I notice now that the angle of the kamm on the second video is just a touch too steep as there is some separation there. Who knows how accurate these are. I have been scouring aerohead's images for the best angle for the afterbody. It seems to range from 10 to 22 degrees. I measured mine again and got 17 degrees on the actual kamm. The video is 22 degrees to the mid point and goes steeper after. I will have to do more tuft testing to be sure on the actual Kamm and I may change the angle depending on how that pans out. My tufting seemed good at 17 but I only got up to about 45 mph.

I updated the annotations to reflect my current understanding of the theory (corrections and clarifications welcome)

http://www.youtube.com/watch?v=7BJc9I1W2JE

http://www.youtube.com/watch?v=KJGkxHDrNr4

Results of first highway drive.

I have some relatives visiting from California who were more than happy to watch my tufts while on a highway run to another relative's home. Luckily, my brother in law is a serious tech nerd with a good understanding of aero.

The top section tufts were straight back, so that angle seems fine. The tufts on the joint between the top and side panels were perfectly parallel to the seam and straight and flat. This is good news because it means the top and sides are not spilling any air and are not creating turbulence.

There are some separation problems with the lower half of the side panels which I was half expecting. The tufts on the top half were good and straight but the bottom half were flapping like crazy on the coroplast section. This part of the kamm has greater angles than the top section. The side panel angles change from top to bottom due to some bad eyeballing during construction and a compromise because of the length of the aluminium stock. I bet they are making huge trailing vortices behind the Egg. This will
be easy to fix once I get home.

http://www.onera.fr/photos-en/simula...images/246.jpg

The separation on the sides seems to be such a penalty as to cancel out the benefit of the reduced drag on the top part. additionally, the open bottom of the kamm may be a big parachute. Seat of the pants coast down was about the same but possibly worse than with just the stainless extensions. The stainless extensions were a big improvement by the way so I think this is a tuning issue not a failure as such.

I think my trailing vortices are about this big.

Earth As Art - Karman Vortices
http://earthasart.gsfc.nasa.gov/imag...ices_hires.jpg

Tomorrow, I will try to pick up a taillight set so I can finish the lower half of the kamm.

I am half bummed but I think it's fixable and will make a huge difference when it's tuned.

Wait... you think your trailing vortices are over 100 Miles long?!

LOL - I wonder if you noticed the scale indicator in the bottom right corner of that second pic?

;)

Stick a couple of VG's in there and she'll be right!

ollie

I love the idea of VG's too, but I can still change the angles easily at this point to fine tune it.

I had disappointing results with the mpg and seat of pants from the new back. I thought the angles were wrong. Top was 15-17 and sides were 15-20. I changed to 12 top and 12 sides but the van still seems to be sluggish on coastdown. It was much better with just the short extensions. It's possible the cooler weather is affecting the results. Just how much difference is there in air density between 85F to 70F? If it was working well one would think that the benefit would out-weigh the change in temps. One might be wrong, though.

Or could the half shell create some massive vortex, increasing drag, as postulated in previous post?

Of course it could be a dragging brake or some other increased friction like a toast bearing. I guess I should take the sucker off and try it that way. OR...Cardboard and ductape it for now.

I picked up my new taillights. I wanted red strip LED's but i couldn't find any DOT approved:(.

Well, It turns out the problem was not the kamm back but sticking brakes. Not so much to get the wheels hot but enough to rob me of the gains from the kamm. This time it was the left rear drum.

New best of 36 mpg on the highway. MPG has been trending up.

I\'m not sure when I\'ll have time to finish the bottom half. I did get the tail lights set up. Pics ready soon.