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Old 01-17-2020, 09:07 AM   #61 (permalink)
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For anyone who didn’t notice, the project has been stalled out, largely because 1) I have other things to drive, 2) the interior needs replaced and added sound deadening to make it usable, and 3) indecision on rear body mods prevents productive work on interior due to potential body changes affecting the interior.

To get this where I want it, I need to drop the drag coefficient (CD 0.45) significantly (CD less than 0.30). This “project” is supposed to be a “learning experience” so I need to just bite the bullet and start modifying the body. The first cut is the hardest…

I know many of you are tired of my repeated revisions to plans, but my current plan is to:

Prework) Clean garage, remove fuel tank from vehicle.

1)remove the rear side windows and rear doors, separate the D pillars from the side and roof.

2) segment the rear quarter panels in front of and just after the wheel wells, pull the sides in in two stages, a shallower angle over the wheel wells then a steeper angle going back, add contoured filler panels at seams to smooth the transitions, move the rear door frame/tail lights over about 8” each side.

3) separate the roof sheet metal from the roof framing, angle the roof framing down just after the C pillars and again in the center, shorten the D pillars about a foot and re-attach, pie cut the roof sheet metal to transition to the new angles and add new metal where it falls short or wholesale replace the roof sheet metal.

4)Retrofit smaller crossover suv sized rear side windows into the front part of the opening beside the third row. Add a new pillar between the C and D pillars right after this window for added support. Fill in remainder of ex-window openings with contoured sheet metal.

5) cut down the existing rear doors and create one new rear door that fits the new smaller opening or swap one from another vehicle, add small kamm on the rear door to bring it out flush to the bumper above the line of the taillights.

6) A bunch of other adjustments and filler panels because items 1-5 didn’t quite work right, add additional supports as needed to reinforce any modified structures.

7) Prime, Way to much body filler to make it look presentable, Paint

All contours will attempt to follow the AST-II top and side profiles, but may be more segmented than a smooth curve.

Other adjustments to the vehicle (grill, belly pan, tire spats, etcetera) will be needed to hit desired drag coefficient, but the 5+’ of taper will eventually support a <0.30 drag coefficient.

So, let me have your thoughts and opinions on this approach…

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Old 01-17-2020, 10:06 AM   #62 (permalink)
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Might it not be simpler to scab on an afterbody whose template is closer to your desired form?
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Old 01-17-2020, 11:07 AM   #63 (permalink)
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Piotrsko,

I have contemplated that option, scabbing the entire body or select pieces, however I have not found a suitable donor, as most vehicles with near ideal tapers in the rear are not nearly as large (80" wide) as what I am starting with. The other issue is trying to maintain the third row seat, which is needed for this vehicle, but I could make that work by leaving the floor and scabbing on the sides and roof.

In short, if I could find a suitable donor I would go that approach but haven't located one yet.
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Old 01-18-2020, 03:16 PM   #64 (permalink)
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sides and roof

Quote:
Originally Posted by aardvarcus View Post
Piotrsko,

I have contemplated that option, scabbing the entire body or select pieces, however I have not found a suitable donor, as most vehicles with near ideal tapers in the rear are not nearly as large (80" wide) as what I am starting with. The other issue is trying to maintain the third row seat, which is needed for this vehicle, but I could make that work by leaving the floor and scabbing on the sides and roof.

In short, if I could find a suitable donor I would go that approach but haven't located one yet.
Something like a JEEP,bumper-mount,swing-away spare tire carrier might work as an attachment for a swing-away boat-tail that would allow access to back of the Suburban.I'm doing it on the pickup,and it will be the first time I'll actually maintain relatively easy access to the bed.
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Old 01-18-2020, 09:48 PM   #65 (permalink)
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How much headroom would that third row have?
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Old 01-19-2020, 11:08 AM   #66 (permalink)
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From the last time I ran the numbers, down 1 or 2 inches from stock, the factory headliner drops down a few inches and would get replaced with a tighter fitting setup. The taper where the seat is should only be down 3 or 4 inches at that point where your head is. So a net loss of an inch or two. Currently I plan to replace the seats on the third row with newer/different seats which may sit lower. If it ends up being more than that, i would lean the seat back back.
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Old 01-19-2020, 09:08 PM   #67 (permalink)
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Quote:
Originally Posted by aardvarcus View Post
For anyone who didn’t notice, the project has been stalled out, largely because 1) I have other things to drive, 2) the interior needs replaced and added sound deadening to make it usable, and 3) indecision on rear body mods prevents productive work on interior due to potential body changes affecting the interior.

To get this where I want it, I need to drop the drag coefficient (CD 0.45) significantly (CD less than 0.30).
it's more then 0.45.


who ever said 0.45 is wrong it's 0.60
1999 is 0.55
that is for the 2000-2006 model =0.55 to 0.39(depending on the year like said they did quite a few tweaks to it over the 6 years ) 2007= 0.38 (2008 hybrid = 0.34 same gen front end with a few tweaks and slightly different body hardware)

Last edited by Tahoe_Hybrid; 01-19-2020 at 09:16 PM..
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Old 01-21-2020, 08:32 AM   #68 (permalink)
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Based off your numbers, it appears you are quoting figures off automobile catalogue, or a similar database, which the majority of are not measured but parametrically reverse engineered (shown as “estimated”). GM claims a measured CD of 0.45 for a generic GMT400 pickup truck, which while is not my exact configuration is the only measured CD I am aware of in open literature for a GMT400 platform. I personally trust this number more, but will agree to disagree on this point.

The transition from the GMT400 platform to the GMT800 illustrates the transition from detail optimization to shape optimization, which resulted in the loss of many very good aerodynamic details (flush wheel covers, flush/tight composite headlight to grill, flush door handles, etcetera) but overall a better tapered body shape.

There were not enough tweaks on the GMT800 platform over its 8 year run to cut the drag coefficient by over 40%, a 1999 GMT800 Silverado very closely resembles a 2007 GMT800 Silverado Classic aerodynamically. Many of the changes were for looks, not lowering drag. (e.g. use of the early 2500HD upper hood profile on later half tons).

Regardless, the stock drag coefficient will mean little after I cut and taper the rear of the vehicle.
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Old 01-22-2020, 11:19 AM   #69 (permalink)
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Cd 0.60

Quote:
Originally Posted by Tahoe_Hybrid View Post
it's more then 0.45.


who ever said 0.45 is wrong it's 0.60
1999 is 0.55
that is for the 2000-2006 model =0.55 to 0.39(depending on the year like said they did quite a few tweaks to it over the 6 years ) 2007= 0.38 (2008 hybrid = 0.34 same gen front end with a few tweaks and slightly different body hardware)
The Suburbans have always had a lower Cd than the pickups they're based on.The addition of a 'topper' to the C-15,to create the Suburban,was generally good for a Cd 0.03 drag reduction around 1994.I believe that GM had pickups in the Cd 0.42 range by this time,which should place it's Suburban sibling in Cd 0.39 territory.
Even the 1970s pickups had no more than Cd 0.535,so I'm thinking,Cd 0.60 is kinda high,maybe JEEP CJ territory.
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Old 05-15-2020, 05:02 PM   #70 (permalink)
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The project is still stalled out, mainly because I have other things to do that have taken priority. Plus I am still having issues nailing down a design...

Got a random thought today, what if instead of chopping the top, I left the top height alone and narrowed the rear as it went back in the cargo area? Basically sever the side walls (including the windows) of the cargo area from the roof and floor, and remount them at a modest (e.g. 5 degree) angle. (Results in a max width going from 79" down to 70ish inches over six feet.) Do a bit of compound curving over a few inches at the transition (so it isn't a sharp edge) but nonetheless still a reasonably abrupt transition. Remove a triangular section and reweld the roof and floor back together to the sides. Would need to modify or build a door, so probably a new door with a second foot or so section of full boat tail. Someday maybe even a hinged actuated extension...

The question being would the airflow stay attached to a vehicle side going from a zero to five degree angle with only a few inches of smoothing? The advantage of this approach is that I can reuse the rear cargo area windows and all the metal, since I am only repositioning the existing pieces versus building all new pieces from scratch. Yes, I am aware this is still a ton of work. Thoughts?

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