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Old 09-18-2018, 04:25 PM   #11 (permalink)
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Freebeard,
Those designs look nice, but it looks like a wind tunnel or powerful simulation would be needed to pull those off effectively. Might be a stretch for my calibrated thumb.

All,
After much thought I have been contemplating a new idea, a cross between the full body chop and the rear kamm extension. First I would do a very small roof chop at the very back of the suburban, dropping a few inches in a few feet to start getting some taper, but stopping above the rear door gaskets and not affecting the rear side windows, pillars, ect for simplicity. Second I would replace the rear barn doors (which are now too tall) with a custom rear single door. Third I would build a fixed kamm onto the back of the rear door, coming out to the bumper. Fourth I would build a secondary kamm hinged off the door that would fold out extending the length past the bumper several feet, but with an electric actuator to close it back within the first kamm for parking lots and other tight quarters.

This would allow for about five and a half foot worth of taper with no appreciable loss of cargo area nor additional permanent length to the vehicle. I believe hinging the second kamm would be too complicated without switching to a single door.

Thoughts and opinions?

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Old 09-18-2018, 04:41 PM   #12 (permalink)
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tail

Quote:
Originally Posted by aardvarcus View Post
Freebeard,
Those designs look nice, but it looks like a wind tunnel or powerful simulation would be needed to pull those off effectively. Might be a stretch for my calibrated thumb.

All,
After much thought I have been contemplating a new idea, a cross between the full body chop and the rear kamm extension. First I would do a very small roof chop at the very back of the suburban, dropping a few inches in a few feet to start getting some taper, but stopping above the rear door gaskets and not affecting the rear side windows, pillars, ect for simplicity. Second I would replace the rear barn doors (which are now too tall) with a custom rear single door. Third I would build a fixed kamm onto the back of the rear door, coming out to the bumper. Fourth I would build a secondary kamm hinged off the door that would fold out extending the length past the bumper several feet, but with an electric actuator to close it back within the first kamm for parking lots and other tight quarters.

This would allow for about five and a half foot worth of taper with no appreciable loss of cargo area nor additional permanent length to the vehicle. I believe hinging the second kamm would be too complicated without switching to a single door.

Thoughts and opinions?
Online,there was an image of a GM SUV model with a rather significant boat tail.They claimed 29% drag reduction with it.For simplicity,you'd be looking at close to 15% better highway mpg at the old double-nickel speed limit.A bit more at higher speed.
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Old 09-18-2018, 10:22 PM   #13 (permalink)
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The bottom half would still need work.
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Old 09-19-2018, 09:23 AM   #14 (permalink)
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Aerohead,

Thanks, 15% would take me from 24 MPGish to 28 MPGish so with my anticipated re-gearing gains that should put me over my initial goal of 30MPG.

Freebeard,

Yes, the bottom half definitely needs some improvement. I was a bit concerned that if I did the underbelly or some of these other projects before I did the upper part that I wouldn’t be able to discern a major improvement.

I have plans for a full belly pan, except not under the exhaust, from nose to toes also doubling as skid plates for my light offroading. Also I plan on replacing the front and rear bumpers, and adding a flat bottomed side step/rock slider combination down the sides. Also in the aero plans are fender deletes, wiper relocate/lowering, rearview mirror replacement, grill ducting/modification, fore/aft tire spats, smooth wheel covers.

Attached is a picture of the rough plans, ignore the sharp edges and the very conceptual nature of this drawing. I already have been collecting parts for many of these modifications. I have some ¼” aluminum for the belly pans and bumpers, I am looking for some other aluminum pieces, such as tube, to be able to fabricate all this. With a flat/flush bottom, I am not sure if I would even need the air dam anymore.
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Old 09-19-2018, 05:21 PM   #15 (permalink)
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You could continue your kamm down below the level of the taillights, just when you get close to the lights, make the kamm from clear material.

+1 on doing more aero underneath. You know it's a right mess under there.
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Old 09-20-2018, 08:16 AM   #16 (permalink)
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Elhigh,

Thanks, good thoughts.

Everyone,

Right now my rear windows start about 1.5” below the roof crown in the back. If I do my idea of starting taper at the back of the roof by sloping only the roof sheet metal, the metal could end up a few inches below the tops of the windows. If I don’t want to impact the windows, it seems like I can:

A: Slope the entire roof equally and have two small window “tail fins.” I don’t know how bad from a drag perspective this detail would be, and if that would be offset by enabling the additional length of the kamm back.

B: do more contouring only in the center to keep the edges above the window. This wouldn’t set up any taper for the outside of the top of the roof.

C: Not do the pre-taper on the roof, and start the idealistic taper just at the rear edges of the windows. This would shorten the effective length of the kamm.

Attached are three concept drawings of A, B, and C as well as photos of the window and rear roof. Ignore the roof rack as these are old photos it has already been removed.
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Old 12-17-2018, 01:16 PM   #17 (permalink)
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After much internal debate on the best way to add boat tail to the Suburban, I have finally made a firm decision on how to proceed. It is basically a variation on the last few ideas I posted, which is a small rear roof chop, a short section of fixed boat tail, and an extending folding section of boat tail. The stock doors will stay, and the boat tail will be affixed to them except for a small transition piece. Obviously there are some complexities with how the items are shaped so that they don’t bind upon opening the doors.

The rear tail lights will be relocated to the bumper, with a second redundant set on the edges of the tail when deployed. Also the third brake light will be relocated, and a rear camera will be added for visibility. When retracted, nothing should stick out past the rear bumper. I plan to build the add on devices out of sheet aluminum covering an aluminum tube frame. The suburban roof chop will use factory style steel. Electric actuators will control the device so that I can be deployed and retracted on the go.

While this is obviously a huge undertaking, I plan to attack it in three parts, so first will be the roof chop, second will be the fixed tail, and third will be the extending tail. I would love to hear everyone's thoughts and comments on this.
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Old 12-17-2018, 02:05 PM   #18 (permalink)
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Zeroth should be to post your construction details here. I see some excellent opportunity for four-bar linkages.

I'd be stoked if you adapt this design for the rear bumper/difusser. Just replace the canvas and fiberglass rods with your solid panels.



The leading edge of the difusser extension moves down and forward to connect to the undertray.
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Old 12-18-2018, 09:25 AM   #19 (permalink)
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Quote:
Originally Posted by aardvarcus View Post
...........small rear roof chop.....
The thing I like about making/building things is that it is an opportunity to simplify a drawn design/idea/concept.

Chopping off a small rear part of your roof is just a line on a sheet of paper but the reality is much more daunting.

Just don't do that part, problem solved.

Focus your energy on what you do, and do it the best you can.

You don't want to be the guy that mutilates or fixes that which is not broken.

You want to be the guy that builds something cool looking, right?

If you have any doubts draw up your design with and without the rear roof chop. I think you will find that you can do this (at least 90% there) without a chop.

Right now that chop is 90% of your project resources, and you can do without it in my opinion.
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Old 12-18-2018, 03:00 PM   #20 (permalink)
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Freebeard,
Learned a new word. The detailed plans are in my next post below, in text form and crude sketches/diagrams. I am unsure about extending the bottom past the bumper, was planning to extend the bumper just a bit and will tie it into the undertray, but I will ponder on that some more.

While pondering on my other construction details I also realized that given I now plan to relocate the tail lights, I can put a hinge in the outer edge where the tail lights used to be and hinge the entire tail assembly without binding. So the new adjusted plan is that the tail extension is hinged separately and not mounted to the doors nor split down the middle.

Kach22i,
Noted. I originally wanted to chop the entire roof… yeah gave up on that. The roof chop I proposed was to avoid the windows or any structural parts of the roof, just the sheet metal to keep it as simple as possible. I was trying to squeeze every last inch of taper out of the plan that I could, but I will re-evaluate whether or not it is worth the gains because as you noted I can do the project without it.

I 100% agree on focusing on what is important and doing the best I can with that. The project is intended to be phased, so that I don’t get bite off more than I can chew at one time and I minimize the time when the vehicle is “down”.

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