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-   -   Project: making a permanent Metro Kammback extension (https://ecomodder.com/forum/showthread.php/project-making-permanent-metro-kammback-extension-3518.html)

MetroMPG 07-03-2008 03:42 PM

Project: making a permanent Metro Kammback extension
 
4 Attachment(s)
I've started on version 2 - the permanent version of my earlier, cruder, Kardboard Kammback:

http://metrompg.com/posts/photos/aer...ail-rear-z.jpg

See: Kardboard Kammback: testing a partial boat tail prototype

Following in ebacherville's footsteps, and encouraged by a conversation with Phil, I'm going to try foam core construction, based on a cardboard mockup/base.

The mockup will be tuft tested first, to make sure it works. The main difference from version 1.0 will be generously radiused "shoulders" in the transition from the roof to the side of the car - to ensure more gradual pressure recovery and discourage vortex formation.

Here's where I'm at so far:

(forming compound curves is harder in cardboard than it will be in foam!)

http://ecomodder.com/forum/attachmen...9&d=1215114031

http://ecomodder.com/forum/attachmen...0&d=1215114031

http://ecomodder.com/forum/attachmen...1&d=1215114031

http://ecomodder.com/forum/attachmen...2&d=1215114031

MetroMPG 07-03-2008 03:48 PM

The new version will also be slightly longer than the first. I've checked out the rear view mirror, and on a flat road, can still (just) see the horizon, below the lip of the extension.

The main reason I want to tuft test it is because even though I'm following the established curve, the "roof line" angle I measure through the end of the cardboard (though it's slightly curved, not flat) is 14.5 degrees. That's about the same as the rear roofline of the Audi A2 (Cd 0.25), but several degrees more than the Insight (Cd 0.25) or Prius (Cd 0.26) - about 12 degrees each. (Though without its rear spoiler, the Prius is about 14.)

http://metrompg.com/posts/photos/prius-insight-a2.jpg

I want to make sure there's still attached flow at the end before putting in the hours to do the final version.

ebacherville 07-03-2008 03:54 PM

looking good!!!

OldGuy 07-03-2008 04:46 PM

Looking at it from here in sunny Tucson Arizona, I see a great rear hatch window sun shade developing. I will be watching.

aerohead 07-03-2008 04:48 PM

Kammback
 
Darin,This is a little scattered so forgive please.I thought I had a photo of an ideal teardrop shape over at maxmpg.I tried to go over and check and my brain froze and couldn't get in.Forgot my password.I'm in town and copy center will be closed tomorrow and for long weekend and I can't get to the house and back to get my password.

Here's where I'm going...

That 2.5:1 teardrop can serve you as a template for a proper roofline profile.This is a little weird but try and visualise this.The teardrop,if it's width (at it's widest point) is twice the height of your car (any car), when a line is drawn through the center of it lengthwise,defines the ideal roofline template form, along with it's mirror image projected under the ground.

If you were to park a scaled photo of your car under the form of the teardrop,with the bottom of the wheels on the line bisecting the teardrop( the ground plane),such that the point where your roofline just begins to descend, is exactly at the point of max cross-section of the teardrop,the upper arc described by the teardrop rotating towards the rear,describes the perfect roof curvature for any car of any height as long as the teardrop's width,is twice the height of the car .

I've been working on this for weeks now.Kamm's car fits the profile.Jaray's car fits.Lay's car fits.The Gentoo Penguin fits. Mair's boat-tail fits under perfectly.If you go "outside the box" you aggravate skin friction.If you go "inside the box",the boundary layer separates and your screwed.As long as you stay on the curve you're streamlined.

If you extend it all the way,it intersects the ground at a point which is 1.78 times the width of the teardrop,as measured from where the roofline just begins it's downward curve. Chop it off anywhere you like,and you're "clean" up to that point!

I was going to wait to say anything about this until I could post it with graphics, then I just now saw your post.I think this is like a universal aerodynamic constant.If the flow is good up to the vehicles point of maximum cross-section,if you follow the outline of the teardrop,you'll have attached flow the whole way.

By the way,all the talk about "phantom" boat-tails behind cropped aft-bodies does not appear to hold up under scrutiny.I'll be in touch soon.Happy fabricating!

P.S.,with this relationship,you can reliably predict the Cd of any vehicle who's roofline stucture basically mirrors that of the body.

MetroMPG 07-03-2008 05:07 PM

Old guy: even though we don't get quite the heat up here that you do in AZ, I had exactly the same thought.

Phil: that settles it. Before I do anything else, (well, after I come back from the sailboat races ;)), I'm going to move your albums over. We recently added gallery ability to the forum, and I'm going to set one up for your maxmpg pics this evening.
I'll be watching for the ideal shape pic as I do this.

dcb 07-03-2008 05:37 PM

So what is the verdict on open back or closed back?

Also, what is your time frame? I'm wondering if I'll have time to finish the auto CDA computing on the guino first. No biggie, but that is exactly the intended usage for it.

Cd 07-03-2008 06:17 PM

I've been wanting to see a follow up on the Kamm back for a while now.

I can't wait to see how that this one will turn out. [thumbs up]

Just my peronal taste, but I think that the car looks much better and more 'grounded' with the Kamm back and the wheel covers on.
Since the roof slants upwards towards the back and the car sits high on such small wheels, the car looks animated - almost as if it is a flea ready to jump forward.
Add to that the rounded insect like shape, and i can see why you call the car the 'flea'.
Looking forward to seeing more !

Cd 07-03-2008 06:22 PM

Not to change topics or anything, but a quick question : With the price of gas going higher every day, have your neighbors taken any sudden ( non mocking ) interest in your work ?

gascort 07-03-2008 10:43 PM

Darin, this is long overdue! I always wondered what happened to the ol' Kardboard Kammback!
I'm anxious to start my Coroplast Cammback, following in your footsteps, soon - I have to determine how I'm going to attach the beast at the top. I'll be following along closely!
Matt

AndrewJ 07-04-2008 12:37 AM

Excellent!

About time you made the kammback a permanent blackfly fixture!

Jigsaw 07-04-2008 03:49 PM

Also, please keep the prototype cardboard template pieces. I can't tell you how much I wish, someone would fabricate good templates and sell them to the rest of us who wish we could just "bolt-on" these mods to our cars. I for one would be happy to pay for a kammback template or kit . I will watch your progress with great interest!

homeworkhome53 07-04-2008 04:18 PM

Nice looking rear wheel skirts. What are you using for fasteners behind the the skirt. Are those pop rivits? Or are they scews and if so what are they screwed into. Do you have a mpg benefit that you have tested.

homeworkhome

MetroMPG 07-04-2008 08:25 PM

Quote:

Originally Posted by aerohead (Post 40843)
That 2.5:1 teardrop can serve you as a template for a proper roofline profile.

I found the teardrop image in your gallery:

http://ecomodder.com/forum/member-ae...lined-flow.jpg

It may be a distortion in the photograph, but that image is actually more like 2.7 to 1. So I corrected it back to 2.5 to 1 in photoshop (not the original - just the following picture) and then followed your instructions to compare these cars:

http://ecomodder.com/imgs/ideal-teardrop-comparo.jpg

Your CRX is the closest example of following the shape.

Most surprisingly, the excellent GM EV1 is the furthest from the "ideal" profile!

Kamm's own design follows very closely up to the last 20% of the roof line where it dives down. (All these cars - except mine - are in your gallery ... "other vehicles 1 & 2".)

And for my design, it's not perfect. It's not very clear from this picture, but there's a diverging gap between the cardboard and the teardrop "outline" roughly comparable to the Prius & Insight examples. I'd still like to tuft test it before raising it up though.

MetroMPG 07-04-2008 08:39 PM

Quote:

Originally Posted by dcb (Post 40852)
So what is the verdict on open back or closed back?

No verdict. I've seen conflicting data suggesting both open and closed styles are best :p.

Quote:

Also, what is your time frame? I'm wondering if I'll have time to finish the auto CDA computing on the guino first. No biggie, but that is exactly the intended usage for it.
Oh, that sounds cool. Time frame: finish & tuft test the current profile this weekend. Then decide if it the angle needs to be eased back. Then start fabricating.

Cd - I also prefer the look of the car with the kammback extension. As for the neighbours, they're not really the mocking types. Not because of gas prices, but they're used to me doing strange things to cars by now.

Quote:

Originally Posted by Jigsaw (Post 41088)
Also, please keep the prototype cardboard template pieces.

I may end up glassing the cardboard into the structure. Haven't decided yet. But at the very least, I can make a tracing pattern of the final product that you could use as a template.

Quote:

Originally Posted by homeworkhome53 (Post 41090)
Nice looking rear wheel skirts. What are you using for fasteners behind the the skirt. Are those pop rivits? Or are they scews and if so what are they screwed into. Do you have a mpg benefit that you have tested.

All your questions about the skirts answered here:
DIY: plastic rear wheel skirts (Geo Metro)

cfg83 07-04-2008 09:11 PM

MetroMPG -

I copy-catted your 1/2 teardrop here :

http://img75.imageshack.us/img75/791...drop50prs4.jpg

Based on how I interpreted your tear-drop, do you think the "red line" I am showing in the above picture is close to the ideal slope for my car?

CarloSW2

MetroMPG 07-04-2008 11:03 PM

Yup. Assuming we're working with the right tear drop as a starting point, that looks good.

MetroMPG 07-04-2008 11:13 PM

1 Attachment(s)
I can't believe I forgot to do the aerocivic.

http://ecomodder.com/forum/attachmen...1&d=1215227435

I'm not sure if I've done it right - matching the highest point of the car's roofline to the widest (tallest) point of the teardrop puts some of the rest of the car's silhouette outside the teardrop boundary. Not sure what the implications are.

Cd 07-05-2008 12:06 AM

If possible, can you post any images/video of the tuft tests ?
I'm sure everyone here at this site would find it useful.

Thanks !

MetroMPG 07-05-2008 12:19 AM

Video is the plan: "web cam on a stick!" (Duct taped to the side of the car.)

garys_1k 07-05-2008 06:55 PM

Quote:

Originally Posted by MetroMPG (Post 41186)
Video is the plan: "web cam on a stick!" (Duct taped to the side of the car.)

I hope that stick doesn't kick up enough vortices on its own that it messes up the flow over the body. Maybe you should use a chase car and video from that.

Cd 07-06-2008 12:22 AM

Oh boy, oh boy,oh boy.

Can't wait to see this !!

One of the guys here ( millenniumtree ) suggested using audio tape insted of yarn .
My previous tuft tests showed that sewing yarn seems to cause vortexes for the tufts further downwind. Also, the yarn tends to stick to itself and those surrounding it at as you get up to speed, thereby ruining the test.
I had better luck with thick cotton thread.
I must give millenniumtree credit for the audio tape idea - it fits completely flush with the body of the car, yet flutters like mad in a turbulent area,
Just out of curiousity, I cut up some 2'1/2 " strips of tape from a video cassette and applied then to my car as i took a short drive. They seem to work just as well, yet are easier to see....but not on a black car like you have.
As I'm sure you remember, audio cassette film mainly comes in a dark brow color, but I remember tapes that had the film being white as well.

Cd 07-06-2008 12:33 AM

Quote:

Originally Posted by garys_1k (Post 41296)
I hope that stick doesn't kick up enough vortices on its own that it messes up the flow over the body. Maybe you should use a chase car and video from that.

As long as the stick is not in the airflow area that he is filming he should be fine.
Trying to get a chase car together is a real pain because it requires three people to do it right : two drivers and a camera-man.
Another benefit of using a webcam is that they are so small that they can be used to do tuft testing underneath the car. ( Or even in the wheel wells if the camera is small enough. )

garys_1k 07-06-2008 01:40 AM

Yeah, I thought about that later. If he has the camera on the left side of the car and monitors the tufts mainly on the right side roof, no problem.

cfg83 07-06-2008 03:20 AM

.Cd -

Quote:

Originally Posted by .Cd (Post 41362)
...

Just out of curiousity, I cut up some 2'1/2 " strips of tape from a video cassette and applied then to my car as i took a short drive. They seem to work just as well, yet are easier to see....but not on a black car like you have.
As I'm sure you remember, audio cassette film mainly comes in a dark brow color, but I remember tapes that had the film being white as well.

This is cool. I was thinking of X-Mas tree tinsel, but that's crinkly. Audio/Video tape is smooth, so I think it is better for staying flush to the car.

CarloSW2

88CRX 07-06-2008 06:32 AM

how about a VW Beetle? it seems like the original VW would have a low Cd.

Cd 07-06-2008 09:33 AM

Quote:

Originally Posted by cfg83 (Post 41395)
.Cd -



This is cool. I was thinking of X-Mas tree tinsel, but that's crinkly. Audio/Video tape is smooth, so I think it is better for staying flush to the car.

CarloSW2


Let me add a disclaimer : I tested only a few strips at the base and middle of my winshield and drove for all of a mile and a half. I placed some of the strips directly behind my winshield wiper and noticed severe flutter once I reached anything past ten or so MPH. This is good for indicating that there is drag in this region, but I wonder if the cotton thread would show the direction of the vortexes better.
Sorry - I was keeping my eyes on the road in front of me vs. on the test.

After running prior tuft tests with the car, I saw the direction in which the flow traveled along the windshield.
(As you know, sometimes air will travel in directions that you would never imagine it would. Sometimes it will even reverse itself.)
To test how these strips would handle this kind of situation, I intentionally placed some of the strips perpendiculal to the airflow.
( I placed them vertically where I knew from the previous testing that the airflow was horizonatal.) I found that the strips fold over themselves and accurately show the true direction.

Still, I wanted to add that I did a really quick, sloppy test. The experts ( used to ) use yarn. Apparently there is a reason they dont use the tape.

Cd 07-06-2008 09:45 AM

The AeroCivic looks even better when you compare the entire profile of the tail : http://farm4.static.flickr.com/3157/...9157fa9a_m.jpg

( A perfect match in fact ! )

MetroMPG 07-06-2008 01:03 PM

1 Attachment(s)
Quote:

Originally Posted by garys_1k (Post 41383)
Yeah, I thought about that later. If he has the camera on the left side of the car and monitors the tufts mainly on the right side roof, no problem.

That's pretty much what I did - set up from above, to the side and slightly behind the area in question:

http://ecomodder.com/forum/attachmen...1&d=1215363682

I just got in from some runs and got some cool pics and video. I'll post later.

I won't keep you in suspense though: there appears to be attached flow on the top, but not on the sides (on the sides, I suspected I'd gone too far inside that ideal "tear drop").

MetroMPG 07-06-2008 09:15 PM

I'm revising my evaluation about the "side panel": the turbulence is from the outside mirror! I forgot to fold it in when I ran the tests (I usually fold it back at highway speeds).

I did a few more runs comparing mirror deployed to mirror folded to window up/down, the difference is clear. I'm putting together a video now to post on YouTube...

There's still a little wiggle in the yarn, but none more than what I'm seeing on the side glass ahead of the Kamm back (got some shots of that too).

garys_1k 07-06-2008 09:43 PM

Nice! Can't wait to see the vids, your work is inspiring. My Focus is pretty much as far along in the front as I can (want to) take it, I'm sure the back needs cleaning up and that's likely where it could do the most good.

MetroMPG 07-06-2008 11:06 PM

Here it is:

YouTube - Tuft testing a "Kamm back" mockup

Cd 07-06-2008 11:46 PM

Very profesionally done !

Thanks for posting !

MetroMPG 07-06-2008 11:47 PM

Quote:

Originally Posted by .Cd (Post 41362)
One of the guys here ( millenniumtree ) suggested using audio tape insted of yarn .

I've used audio tape before as "telltales" on a sail. Works really well, but the downside is it seems to have a serious static cling problem in certain conditions.

Also, not so good on a sail if you're used to red & green telltales on either side. :p

cfg83 07-06-2008 11:54 PM

MetroMPG -

Great stuff. The camera mounted on the car is the best POV for tuft testing.

CarloSW2

Cd 07-06-2008 11:56 PM

So is the Kamm back going to have a hollow cavity or a plexiglass rear window ( the red portion of the image )

If you remember, the UCDavis project car had a small cavity ...but then their .Cd count wasn't exactly impressive for the amount of work that they put into that car.

http://farm4.static.flickr.com/3281/...f8708123_m.jpg

Cd 07-06-2008 11:57 PM

Hey - I love the look of the car by the way.
Man what a difference !


I have a new question : I noticed that in the still pictures ( the one that I posted in my previous post ) the area on the side of the Kamm back isn't filled in. ( The triangular area just above the area that I filled in with red )
Surely you filled in this side piece before the test ... right ?

Peakster 07-07-2008 01:54 AM

This is such a great video! Real professional and it worked extremely well. Are you thinking of pushing the envelope further and making an attachable boat-tail in the future? I'm totally staying tuned :).

ebacherville 07-07-2008 02:00 AM

thats awsome what did you use to mount your camera for the testo do the same form my setup for tuff testing.. how did you mount that camera!!!!!!!

groar 07-07-2008 03:43 AM

Quote:

Originally Posted by MetroMPG (Post 41539)
Here it is:

Great job :-)

This gives me more and more desire to ecomod my car...
but I will only begin with front grill block and may try rear wheel skirts : your DIY article seams so easy to follow...

Have you ever tuff tested your rear wheel skirts or your front grill block ?

Denis.


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