Deep Blue- Suburban Streamlining
 

Edit: I reset the project, the new version is below:

https://ecomodder.com/forum/showthre...tml#post599156

-------------------------------------

All,

I have a build thread on the Central Subforum, but I wanted to make a thread in the Aerodynamics Subforum to focus more on the design than the build. I have a 1994 K2500 GMC Suburban, nicknamed Deep Blue, which has been heavily modified. I am getting to the point where I am going to start some more major aerodynamic modifications. Attached is a picture of the patient as she sits. MPG is sitting at around 23, goal is to be over 30.

Current Aero Mods-
*Roof Rack Delete
*Flush Bumper Swap
*Conveyor Belt Air Dam- V1.0

My next big project is I want to make kamm back attachments to add to the rear windows of the barn doors of the Suburban. I would love to chop the top and do a full boat tail, but I don’t have time for that much body work right now and I want to keep the project marching forward. I have considered a folding/nesting design but I believe that is beyond the scope of this Version 1.0 design.

Attached is a theoretical shot of what I am imagining right now, basically the kamm would extend to the rear bumper so the overall vehicle doesn’t get any longer. I was thinking on the sides of tapering at a 4:1 aspect ratio AST top view template, which I am sure I have saved around here somewhere. I will remove the roof mounted third brake light and install a light into the kamm shell. I am considering construction out of extrudedpolystyrene (XPS) foam covered in fiberglass, similar to how I made the bedcover for my Tacoma.

Thoughts?

I think that will be a great start! Why not a couple inches further and maybe bring the sides down toward the rear bumper? Maybe a flush rear bumper so no air can get inside the bumper and act as a parachute?

The halves of the Kamm-back swing with the doors? That angled cutline suggests it.

30-mpg
 

For Deep Blue to get 30-mpg on the highway,you'd need to drop the Cd to around 0.168.
You'd be looking at the GM Precept concept car,circa 1999.
That would require everything I did for the Spirit of Ecomodder,and no less than 8-feet of boat tail,unless you could get rid of your mirrors completely.
A new Colorado/Canyon Diesel will get the mpg,but your looking at around $40,000.00?,and expect to drive 62-mph.

19BoneStock88,
Thanks, the reason I stopped it higher was to not block the tail lights. I want to swap out the bumper to a custom design, preferably integrating with a diffuser from the belly pan.

FreeBeard,
Yes, the plan was to directly attach the kamm-back in two pieces to the barn doors, probably with a few fasteners and high strength tape. Because the hinge is at the outside door face, near the top it can be out near the side edges but not bind upon opening. I figure this is the simplest way to build it, as any other method I have thought of will require separate hinges/lifting to allow the doors to open.

Aerohead,
Well, that doesn’t sound promising then, at least not until I can do the full chop-top. I was hoping the MPG would be achievable at a more achievable figure, as I figure I am at 0.45 right now. I may should have mentioned I am expecting some of the increase to get to 30 to come from gearing changes (discussed in my build thread) not solely from aerodynamic mods. I need to get basically 30% better MPG than right now to break 30 MPG.

gearing
 

According to Gino Sovran (he's kinda the God of GM's research arm),you'd do all your aero and tires and whatever else,then fit the gearing to maintain the same BSFC for your engine.If not,you could lose a significant portion of your mpg potential.If you have Hucho's book he goes into this.

There's Hucho, then there's funkhoss:

https://ecomodder.com/forum/showthre...mpg-33961.html

Aerohead,
I agree with the concept of gearing changes as aero improves(and I have read most of Hucho’s book) the problem being I am turning more RPMs than necessary as is, with some of those potential gearing gains already on the table. I have found a several BSFC charts in open literature for the 6.2/6.5 diesels which lead me to believe being moderately loaded (e.g. 50-100%) at low RPMs (e.g. 1200 to 1800) is ideal for this engine.

Freebeard,
Thanks for the link, I know funkhoss had referenced his project in my other thread, but I didn’t read his build thread until now.

Everyone,
Didn’t get much time to work, but I did manage to get a good picture for an overlay plus did some measuring/figuring. The taper at the rear of the roof is meh, very shallow then quickly very steep, so it will be of little use. Also the angle at which the Suburban sits cancels out some of this taper. Attached is my overlay.

Had a neighbor regularly got 27-mpg with his 6.2 4WD Suburban. His father a GM exec, and son no dummy. Quizzed him on it. A friend got same with first year 6.2 pickup. Both automatics c

My 4T 2WD Cummins will also see 27-mpg. Stock. 58/9-Mph. On cruise control.

All of these in south central US. LSD, not ULSD.

I see mpg gains with both Power Service and Howes Meaner Cleaner. Not big, but adds to margin.

https://ecomodder.com/forum/member-f...w12-650-16.jpg

Golf GTI W12-650. The best treatment I've seen for a box shape. Chopping the top loses interior volume too, but it's more work. What you have are three external ducts that feed the wake with high-speed jets of air. Another example the Citroen Tubik:

https://ecomodder.com/forum/member-f...n-tubik-21.jpg

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?

tail
 

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.

The bottom half would still need work.

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.

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.

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.

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.

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.

https://ecomodder.com/forum/member-f...pe-ii-boat.jpg

The leading edge of the difusser extension moves down and forward to connect to the undertray.

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.

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”.

Current Detailed Plan:

Step 1: Roof Chop (Maybe)
Add a temporary third brake light to the rear doors. Cut out the Suburban roof from the back just above the doors weatherstripping and then 30” forward, stopping inside the window framing. Fabricate a patch panel using the roof of the parts truck and/or sheet metal tapered on an English wheel. Also fabricate a tapering Z patch for the sides near the windows, probably with a brake and bead roller. Weld all the items together onto the roof. Overhang the roof slightly over the doors to shed most water off past the weather-stripping. Cut off the upper portion of the doors, stopping at the weather-stripping. Put a cap/patch on the top of the doors to keep them sealed as needed.

Step 2: Fixed Tail
Relocate the rear tail lights to the bumper/other location. Beef up the tail light area to accept a heavy duty hinge. Create templates of the shape of the rear taper near the front and back of the tail. Bend aluminum tube (1.5”?) using a tubing roller/bender to match the templated shape. Cut/bend extension pieces to run between the two and weld the assembly together. Hinge the assembly off the tail light area on the passenger side, add a latch on the drivers side, and add areas to mount hinges for step 3. Using 0.040/18ga aluminum, create contoured panels to skin the frame and to transition from the frame to the Suburban body. Some mockup and field fit will be required. Weld the panels to the frame.

Step 3: Hinged Tail
Similar to step 2, create templates and build three frames one for each side and one for the top. (I am not sure about doing one for the bottom, I kind of liked leaving the bumper exposed from a “bumping” standpoint.) Hinge the three frames from the fixed frame, with the hinges for the top rearmost and the hinges for the sides closer to the vehicle. Offsetting the hinge pivots back slightly should allow clearance between the panels so they can shut without binding. Connect the frames to one or more electric linear actuators.

If you look at the picture in Permalink #18, the pivot point is inset from the deployed position. You can't avoid a small inset, but more might be better to position the panel. The downside is the stored position isn't as compact.

On the aluminum frame, consider two hinges; one would have to resist torquing. Have you dimensioned the panels? The top panel looks long to store correctly.

I couldn’t agree more about avoiding the roof chop. Waaay too much work for too little return.

And (the clincher), what degrades utility means it’s the wrong vehicle in the first place.



A folding boat-tail (big truck Trailer Tail-brand ) has been in a few of my daydreams for my CTD Dodge. Front air dam and side skirts of appropriate conveyor belting. Same to fill wheelwell gaps and cab to bed topper.

But all of it easy enough to remove pending sale (which should always help drive ownership duties).

Beginning, middle & end.

.

I always wondered how Aztek Cd compared to otherwise similar minivans.

Freebeard,
I see what you mean on the inset. Because the panels are curved at the mating surfaces, I will have to use inset and/or backset to make them clear without binding, however inset or backset reduce the effective length of the panel when constrained to nesting within the fixed space so I will try to minimize it. I will be sure all hinges are used in pairs. The folding panels are about 3’ for the top and 2’-6” for the sides. Attached are some accurately dimensioned drawings, a top view and side view, each square represents 3”.

Kach22i/Slowmover,
I examined the bodywork much more closely and based on your inputs I am abandoning the roof chop. There is some slight taper built into the top of the roof, but it gets too steep near the back. I will make the fixed tail meet the top right where the slope gets too steep. Thus step 1 is gone, other than the relocating the third brake light and patching that hole in the roof.

Everyone,
The taper shown in the drawings is based loosely on AST-II (side) and AST (top) rates of change, but are predicated on the assumed length that you are picking up from the existing slopes of the rear of the body. I need to more accurately compare the angles to see exactly where I am starting. I am also looking for more aggressive side tapers, I have a big collection of pictures and posts from Aerohead that I will do some overlays on. The feedback has been great! :)

I think the worst thing would be the vortices forming at those wing root stubs left over.

They look like little car fins or vortex generators.

side taper
 

I'd recommend figuring out where any plan-taper begins on the sides,then respect the taper contour based upon the template,or you're guaranteed separation if you get too radical.
The flow attachment on the aft-body can only tolerate a pressure recovery of that of a 4:1 streamline body of revolution's.
You can be more 'gentle', but you can't be more aggressive.

Attached are my AST-II side view and AST top view overlays. I also did a few other top view overlays, but AST was the fastest “proven” design, other than Meir which basically was matching but got more aggressive past 40% (which I am not going that far back). I believe I have these drawings lined up accurately based on the existing taper of the vehicle. I also did a sketch of the bodywork viewed from the back, the boat tail will start at the base of the existing tail lights.

:thumbup: That's what it needs.

I assisted my step father in restoration of several 67-72 Chevrolet and GMC trucks in the mid 2000s. On one of those, a "Camper Special" we cut the roof off of an old Uhaul Dually which had clearance lights and corresponding raised buckets to replace the rotted roof panels on the 3/4 ton.

Despite measuring 5 times and using oem pieces, this was a painstaking undertaking.

Since you are removing your factory tail lamps, you will have to build filler panels. This makes me wonder if you would be better off pie cutting the length of your chop through this section of the body. This would leave your roof and windows in tact. You would end up with misaligned door hinges and a gasket that would need attention, but that seems to be much less work that what you have been describing.

--------------Project Reset, ignore the above ----------------------
I am resetting this project, and now would like to get some fresh perspective on my revised plans. (Thanks to all who previously provided me advice.)

I have another vehicle to drive now, so I want to take the time to do this project justice. For those not following my other thread I have completely rebuilt this vehicle basically from scratch. 1994 GMC Suburban K2500, 25MPG highway at an assumed CD 0.45. The project runs and functions fine, but I still need to redo the interior and complete any aero mods I want to do to it.

I want to cut the vehicle in half. :rolleyes:

This is not a joke or a pipe dream. :eek: In terms of time investment I already have an inordinate of hours in this build so this is large undertaking is not unheard of. It is the only way this project will be anywhere near what I envisioned when I started. I have been slowly collecting metalworking tools and doing some metalwork to build my skills. My current plans (up for your comments) would have me cutting out the bodywork surrounding the cargo area (roof and sides) and grafting in new pieces along with repositioning existing pieces to hit the templates. Front and mid doors remain untouched. Frame and floors remain untouched (except narrowing the rear cargo floor if needed).

This will be a great learning project for me (trial by fire). There is no residual intrinsic value in this vehicle’s body/frame at this point (worth far more in powertrain parts) so I am not destroying any sort of value. I come to the people of this forum for advice to help guide this project to success.

Edit: Timeframe wise I will be starting in the fall/winter if all goes well so I have a few months to plan and purchase parts.

Above I posted the what, my current thinking of how to accomplish this is:

Convert barn doors to factory stepside tailgate, move over the factory bodywork from windows down starting just over the wheel wells back to the lights accomplishing a taper on the sides of about 9-10” per side over 6’, mirroring AST.

Chop top around the back of the middle row doors. Find a reasonably tapered large hatchback (suggestions?) and surrounding frame and transplant it on the very back of the vehicle to hit AST-II. Build/modify filler panels from the point of chop to where the hatch starts (I don’t know of any 6’+ hatches right off). This will have a taper down of about 15-16 inches.

Figure out about how much space exists between the new sides and new roof. Find and transplant window glass and make filler panels as appropriate.
Then I will do way more bondo work than I should have to…

All of this is up for debate and suggestions, so let them rip. Thanks in advance.

Is the plan to keep this a 4-door, or to convert it to a 2-door?

The reason I ask is that it seems like you can get more body taper if you start more forward, but you lose much utility.

https://jalopnik.com/for-10-000-is-t...d-s-1789661546

https://www.carscoops.com/2019/01/ol...ans-gladiator/

https://www.reddit.com/r/****ty_Car_...erokee_pickup/

I view this project as much in the spirit as the SUV to Pickup truck conversions links above.

And in that sense, making this a pickup truck with an removable or hinged aero-cap may allow better access to the aft spaces.

Just food for thought, I still think this is a solution to the question never asked but cheer you on in your resourcefulness.

Kach22i,

The plan is to keep it a four door, I need the middle doors. I can cheat forward a bit on the roof but don't want to mess with tapering the sides near the middle row doors. I have looked at many SUV to Pickup conversions, this is definitely similar. I want the vehicle to look like an Avalanche that someone put a sloped topper on. (There are no GMT-400 factory Avalanches.) Hence the stepside bed. Maybe even a few Avalanche badges...

The big difference would be I am not planning on adding a rear bulkhead at the end of the "passenger" compartment, which are typically added on the SUV to pickup conversions. I would be maintaining the open cargo area, which adds the complication that the "aeroshell" needs to be 100% water and air tight. The reason I suggested a hatchback graft was to try to maintain this usability you referenced but rely on a factory setup for the difficult to manufacture hatch hinge/seals. I would love to custom fab a 100% hinged cover, but am afraid it may not turn out as needed. But when has that stopped me...

https://ecomodder.com/forum/member-f...w12-650-16.jpg

I've always liked the solution VW used on their GTI-w12-650-16.

If you graft a hatchback onto it could you add to the overall length or is there some constraint?

Freebeard,

I could add to the length but probably wouldn't want to go more than 6-8" for a fixed piece past the tailgate/doors, coming out about to the bumper. Vehicle is already fairly long. I would definitely want some overhang past the tailgate. Hinged extension is a possibility, but probably not as part of an initial build.

https://ecomodder.com/forum/member-f...n-tubik-21.jpg

I'm thinking of something that would look like a rear-end collision, with internal ducting like the newest race cars.

The company I work for sells a tri-folding aluminum hard-cover that is water tight. If you sent me the dimensions you're looking to have the back frame to be I could maybe give you an idea of what models we have that would be pretty close to that, and you might just have to adjust your sizing slightly.

Full disclosure: they're a thousand bucks a crack, and I don't know how they would feel about warrantying one on your project, but our field failures are very rare and are usually due to improper install.

Anyways, here's a link to the sales page if you want to check them out:https://www.agricover.com/hard-tonneau-covers/

If you do buy one, don't buy it direct from us though, just google Lomax tonneau cover and find the cheapest seller. They prefer to just sell to our dealers and let them make individual sales, so the price on our website is always higher than you can find them elsewhere.

Hmmm, grafting in a hatchback from another car..... What about something from a station wagon? 90's B-body wagon? Dodge Magnum RT? Taurus/Sable? Maybe too far from the template to be worth it?

Magnum RT on a Camaro as food for thought below.

I can’t believe I’m suggesting it, but Pontiac Aztek. Guessing the angle of the top and overall size aren’t perfect, but might be in the ballpark. Should be cheap, wouldn’t be surprised if they’re getting lined up to be crushed