Project: making a permanent Metro Kammback extension

[MetroMPG]

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]

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]

looking good!!!

[OldGuy]

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

[aerohead]

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]

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]

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]

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]

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]

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]

Excellent!

About time you made the kammback a permanent blackfly fixture!

[Jigsaw]

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]

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]

Originally Posted by aerohead

That 2.5:1 teardrop can serve you as a template for a proper roofline profile.

I found the teardrop image in your gallery:



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:



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]

Originally Posted by dcb

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 .

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.

Originally Posted by Jigsaw

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.

Originally Posted by homeworkhome53

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]

MetroMPG -

I copy-catted your 1/2 teardrop here :



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]

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

[MetroMPG]

I can't believe I forgot to do the aerocivic.



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]

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]

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