Need help with Aerocap for 05 Dodge Ram.

[aerohead]

I can't prove it,but my opinion is that,unless you can also taper the bed (which would require a custom bed),just keep the sides of the cap all the way to the bed rails,rolling the sides, such that as you reach the tailgate/bumper area,that upper edge will be like an ellipse coming off the rail top over onto the top of the cap.
If you decide later to do any boat-tailing,you can continue morphing that upper portion closer and closer to a circular cross-section.
I'm modifying my cover to have this feature for the upcoming trip to the wind tunnel.
Professor Alberto Morelli of Turin,Italy says it's an important feature for low drag.

'Kamm' used it on his famous K-form cars

Mercedes-Benz is using it on today's Cd 0.19 concept

Ideally,we'd have a cross-section like this,but it would require that custom bed.

[Vajra]

There is another method of getting a good smooth finish, called vacuum bagging. Fiberglass on, and resin. Using a large bag, you use vacuum to suck the bag down on the part. It causes the epoxy to smooth out, forcing air bubbles out. I did this with many RC boat hulls and other pieces. Never such a big scale, but I am sure a good 2 1/2 HP dust collector could provide enough vacuum. I know they do industrial scale vacuum bagging with very good results. I also notice, all to often, people don't use finer weaves of fiberglass cloth. It takes a great deal of sanding to get things perfect, but I am okay with that.

The bed rails do taper, the bed also narrows towards the back end. Almost hard to explain, but the back is not as wide as the front of the box is. Maybe it is the shape of the wheel wells dictating this. My stake pockets are not all the same, or in a straight line with each other.

I'm not to sure I follow what you are saying about the side shapes Freebeard. I understand what Aerohead is saying though.

[freebeard]

I'm not to sure I follow what you are saying about the side shapes Freebeard. I understand what Aerohead is saying though.

Originally Posted by aerohead

keep the sides of the cap all the way to the bed rails, rolling the sides, such that as you reach the tailgate/bumper area,that upper edge will be like an ellipse coming off the rail top over onto the top of the cap.

Same thing. If you take less the half an ellipse, the springline will not start upward and arc over, it will angle up and then arc. Think of a half-hexagon with rounded upper corners.

[Vajra]

Isn't that what Chaz shows in his rendering, a sort of half hexagon with rounded edges? Or are you saying a more of a bow in the center? Perhaps I am not understanding as much as I thought. Trying to picture this in my head, thought about all day today... it was a very bad day, so I needed to day dream about something. My mind says a hexagon, with a bow in the center and rounded corners coming down onto the sides. ... ... yup.. Bedtime, the quiz I took used the remained of my energy and brain capacity. Thanks for the replies guys. Interesting stuff, trying hard to process all of it.

[freebeard]

Here's hoping things look better today.

My concern would be that you hone in on the absolute optimal shape, and then crash into the harsh reality of trying to implement it. Here's a paraphrase of Helmuth von Moltke the Elder that is more to the point:

“The first casualty of any battle is the plan of attack.”

― Cory Doctorow, For the Win

There're any number of ways to look at it.

• Two twisted rectangles on the sides and a trapezoid on the top.
• A rectangle at the cab, a half-hex midway and a half ellipse at the end
• Calculating a new curve every n inches
• A superelliptic progression (we haven't even talked about that one)

Each solution will be more or less suited to various fabrication methods. You might settle on a method and let it dictate the solution.

Also aerohead made this point:

If you decide later to do any boat-tailing,you can continue morphing that upper portion closer and closer to a circular cross-section.

Adding even the height of the tailgate to the overall length should show an improvement.

[t vago]

Originally Posted by aerohead

If you decide later to do any boat-tailing,you can continue morphing that upper portion closer and closer to a circular cross-section.

Like this?

[aerohead]

Originally Posted by Vajra

This is some of what I was thinking. Although the (grey) window would have rounded corners. Red solid circles are tail lights, the others are backup lights. Lavender/pinkish dot, back up camera. I guess though, I could get rid of the window on the back then. Corners on the rear, more pulled in. If this would not reduce drag, guess it is pointless. I suck at editing. LOL

Morelli was recommending that we avoid any kind of 'angle' at intersections,and instead,'roll' the line into an ellipse,or circle.
It's only 10X more work (ouch!) but supposed to be aerodynamically superior.
It would look more like this KIA concept

[aerohead]

Originally Posted by t vago

Like this?

Yes! Exactly like that.

[freebeard]

t vago -- a next step for that would be rear fender skirts that extend back to continue the arc down outside the taillights.

Then the rear pan or bumper.

[Vajra]

Okay, now it all makes sense. I have to wonder though, with the back end being like T Vago's, wouldn't that be less aerodynamic than just terminating at the tailgate? While I can see air moving and transitioning more smoothly along the top/sides, I would suspect that the tail end would not have much less drag verses a tail end that ended like a aerolid such as Bondos, or Swede's. I understand the concept of CL verses CD, but I have a hard time seeing how air flow being smoothed out would have a greater effect on efficiency. Or... Are we talking about having the back at 7-8" off the tailgate, with a similar curvature for the tail end? If I was do to what T Vago is doing, I would use foam as well, bread and butter style vertical sections. Don't think ply wood would be so easy to deal with in the larger lumber sizes. If 1/8" inch, sure... not 1/2" though. I think I would want slightly rounder edges and a flatter center though. My thinking, the edges on T Vago's tail end, would have a not so smooth arcing action of air on the sharper corners. Or am I mistaken? I am NOT knocking T Vagos work in progress, that cap looks very nice. I just don't see how it would be benefit reducing the CD as much as other options. I will never go out further than the tailgate down. As my truck is a 2500 4x4 with 4:10 gears, I don't see a possibility of much over 2mpg max with the aero cap. If I do, I think it would be to the point of losing much of the useful area of the bed. If I had to open it to carry cargo, what would the effect of the rather round end be if open say 1.5 feet? I suspect because of the shape, shallow corners, I'd end up having to open further up to accommodate a similar load. Hmmm... And yes... I want to hone in on a optimal shape, but also, most useful design possible. Construction wise, I think I can do just about anything one the design part is done.... at least I think so...