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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:

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!)

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.)

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

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!

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