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Old 09-10-2014, 07:37 AM   #11 (permalink)
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A little more progress, figured I would update. I got the rest of the camper shell trimmed to size. I put the shell on the truck and traced a cut line, following the contour of the existing slope of the roof. You can tell from the photo I need to do some sanding. I got a new side photo of the truck from a distance, and did the requisite AST-2 overlay.

After much study, I believe I have decided to make a cardboard form in the shape that I want for the roof, and then layer fiberglass over the form, to make the cap one solid fiberglass piece when I am completed. This is a little more difficult than I really wanted, but the easier/simpler ideas aren’t turning out as I had hoped.








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Old 09-14-2014, 05:16 PM   #12 (permalink)
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Simple usually does not turn out to be simple, does it?

Thanks for the updates. Thanks especially for the picture of the aero overlay on top of the truck. I have the same body shape (2012 Access Cab) and now I can stop wondering how much I'd improve fuel economy with a bed cover.
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Old 09-14-2014, 09:59 PM   #13 (permalink)
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It needs plan taper, so you might keep cutting. If you could remove those plastic windows and use an electrical heating tape to put a diagonal crease in it, and notch the fiberglass to bend it for the increased tumblehome then pull the lower corners in a bit — then it would conform to The Template.

Alternatively, cut the top of the window at 22 and put as much compound curve into the new construction as you can. Instead of fiberglassing over cardboard (unless it's an egg-crate there's lots of opportunity for grief) think about buying preformed fiberglass panels and just glassing the joins. Use thin H-channel to capture the edges, drill holes and sew it together first.

IMHO, of course.

wdb -- That doesn't look bad. Two fiberglass rods in an X and some vinyl-coated fabric for a boattail erected on the tailgate. Why not?

Last edited by freebeard; 09-14-2014 at 10:06 PM..
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Old 09-15-2014, 09:01 AM   #14 (permalink)
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Wdb,
You should probably take my aero template overlay with a grain of salt, because my truck is not 100% stock. I have 235/85R16 tires, so with an approximate 1” increase in diameter my truck is 0.5” taller than stock. I also have Bilstein 5100 shocks raising the front of the truck 1.75”. Lastly I only have the 3 spring leaf pack in the back (2 leaf + overload), so if you have the 4 spring (3 spring + overload) your back end may sit up higher empty.

Freebeard,
Thank you for your input, I hear you loud and clear. I have extensively deliberated the lack of significant plan taper in my plan, as well as the two side vertical “wings” that will be created at the back as the top tapers and the sides don’t. This is obviously far from ideal. Unfortunately, the windows in the cap are actually glass, not plastic, so I don’t have a way to cut them. I want to try to maintain visibility, so I want some kind of windows. I am really trying to avoid taking the cap all the way down to a base and starting over, but to correctly apply the template, I realize that is what I would have to do.

What I don’t have a good feel for though is how much worse will my less than ideal implementation be than a true cab extension half template. For example, rule of thumb “internet” data is a regular cab height camper top is good for 5% to 10% drag reduction, and that has no taper whatsoever. Similarly, regular bed covers are usually attributed around 5% drag reduction. These are both obviously far from ideal. An aero shell is often credited with 10% to 20% potential drag reduction.

I am assuming a mongrel shell, with no plan taper, is somewhere between the regular camper top and the aero shell, but we all know about assumptions... And then since it is only a half mongrel shell….

Good thoughts on the construction technique though, do you know of a source of fiberglass sheets? I may re-look at my old roof to see if I can remove the reinforcement without destroying the top panel.
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Old 09-15-2014, 06:06 PM   #15 (permalink)
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Those pieces of (probably tempered) glass aren't doing you any favors. Get a piece of acrylic and cut it on the diagonal.

Once you settle on the overall form, the final Cd figure will depend on the details — i.e., the radius of the edge. A hard edge is worst. The rule of engineer's thumb is radius equals 4% of overall width, or less than 4" minimum.

The sheet acrylic can be cold rolled (I'm pretty sure) so roll the hypotenuse and leave a flat triangular tab to fit in the lower front corner. Then rely on fiberglass to fill in the gaps.

I'd have to Google for a supplier, here:

https://www.google.com/search?q=fiberglass+sheet++stock

It looks like it's called fiberplate.
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Old 11-05-2014, 07:45 AM   #16 (permalink)
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Sorry for the long post, I type a lot.

Well, after gathering all the parts, over the weekend I finally put together an air dam on the Amocat. I used rubber hay baler/ conveyor belt, two pieces of aluminum flat bar, stainless steel nuts and bolts, and blue Loctite for assembly. Two 5/16 nuts and bolts hold the aluminum sub frame to the truck frame. I am considering this version 1.0, as it is not perfect and I can already see several tweaks that I need to make.

The conveyor belt is 7” tall, but only 4”-5” of it are hanging below the existing truck frame where I mounted it, and the remainder is blocking the front of the truck frame and the upper gap between the bumper and truck frame. The bottom of the air dam is basically at the same height as the low point on the truck, the bottom of the independent front suspension skid plate about 12”-18” behind where the air dam is mounted. The air dam is designed to stand up to abuse and being drug off-road.

Initial impressions are that it is definitely doing something, but I don’t know if it is appreciably lowering drag at this point. It is really hard to tell since I installed it right at a cold snap, and the next tank had some off-road driving on it. I did a short fill this morning just to try to get a feel for any changes in fuel economy, but my results are within measurement error. Two noticeable effects were significantly lower lift on the truck and significantly lower wind noise from the front tires. The truck definitely felt more planted in turns. I think it is also forcing more air through the radiator/engine compartment.

There are some issues with this design that I am aware of, as follows:
The very center and the two sides have some curvature, but the remaining section is very flat. It would be better if the whole thing had a gentle curve. This was a compromise I made for easy mounting, and to not interfere with the existing tow/tie down loop.
The two sides stick out to far. I built the aluminum subframe out of two 72” pieces, and didn’t want to cut them down until I had it built and installed, so the air dam ended up flush with the outer edge of the tire. Knowing the air is traveling at an angle at that point, the air dam should end before the edge of the tire. Right now, I believe I am throwing the air too far around the side of the truck.
The lower edge is flat/sharp. It would be best to have some amount of lip here to hold the air, and some amount of radius to smooth the transition point.

So here are changes I intend to apply for version 1.2 of the air dam:
Use washers or spacers towards the center to increase the slope on the two flat parts of the air dam. I am going to be limited by my tow hook, but I can get some more slant to it than what I have now. A full curve imposes mounting, tow hook, and off road issues, so that will have to wait for version 2.0.
Bend a sharper turn on the outer edges of the aluminum to tuck it up more towards the center and end the air dam sooner. This should keep the air from overshooting the tire.
Rivet some rubber hose on the bottom to create a lip and radius, while still maintaining durability and flexibility. I need to test some sample pieces first.
Reading Hucho’s book, and looking at posts on this site, I am considering making the air dam shorter in the center, and leaving it the existing height near the edges.

So give it to me straight, what are your thoughts and opinions?

In other news, I cut the windows out of my camper top, and I have sketched out some designs for a full aeroshell camper top. Once I get my air dam tuned up where I want it, I will probably begin working on this in earnest. I also got my ultragauge mounted, still need to mess with the gauges I want to appear and learn the truck’s baseline.



















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Old 11-05-2014, 09:39 AM   #17 (permalink)
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I think it looks pretty good. I'd probably paint the aluminum personally. Other than that, I think it would be better if it were farther forward. I understand that interferes with the tow hooks though. Lots of OEMs put their dams back farther like you did though, so there still must be some benefit.
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Old 11-05-2014, 01:00 PM   #18 (permalink)
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It's looking good so far.

I like conveyor belting as a material, but I've only ever used it in conveyors. Is the edge finished or cut? What widths were available to you?

The bump in the center may stiffen the lower edge more than a smoother curve would. When you re-curve the outer ends would be a be good time to add triangular filler pieces.

I think you're on the right track with the camper shell. Are you still going to form it from cardboard? How would that work?
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Old 11-05-2014, 01:46 PM   #19 (permalink)
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Daox,
I definitely agree, the aluminum needs painted. I am going to wait until I am happy with the performance so that I don’t mess the paint up adjusting it. Luckily since my truck is a base model flat black will help it blend right in. I had seriously considered putting the dam on the bottom of the front bumper, but in addition to the tow hooks I was afraid it would get damaged easier off-road.

Freebeard,
The edge is “semi-finished”, i.e. it isn’t ragged or anything. It cuts pretty clean with a sharp knife. I bought the belt at tractor supply, there was only one size available there, 7” wide 15’ long.

Yes, the bump is there for stiffness reasons, without a significant curve side to side the wind would blow the bottom edge up under the truck.

Good idea, I will add filler pieces to my V1.2 to do list.

I am torn on how to build the camper shell, I go back and forth on making a removable form from cardboard and tape, and later removing it leaving the fiberglass skin. The other option is to make a permanent form out of XPS foam, and fiberglass it on all sides, making a solid structure, similar to a fiberglass surf board. I have the materials to do either. I think the foam would be stronger with less fiberglass, so lighter for equal strength. I think the foam would also be harder to build, but I am not sure. The cardboard/tape/junk form buildup sounds fairly simple.

My plan on the camper shell is a two piece hinged or hatchback design, so part of the roof would be removable for carrying tall objects. I also want windows in the back and sides. All of that adds significant complexity to any design. I have some concept sketches, I will probably upload a design in a few days.
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Old 11-06-2014, 01:40 PM   #20 (permalink)
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I like the lip! Location-wise it is where the folks at GM put it on the upcoming Colorado; interestingly, some pictures show that truck with the lip and some show it without.




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