Plan for my cargo trailer
 

I have a 6x12 enclosed trailer with standard (not ramp) rear door. The front surface is rounded but the junction between sides and top (or front and top or rear and top) is 90 degrees. The top is pretty much flat.

It has really killed the mileage I get with my Ram 1500, Pentastar engine and 8-speed transmission. So I'm thinking of some mods.

On the sides at the top I was thinking of cutting a piece of 4" pvc pipe lengthwise and attaching each half-pipe along the edge (on the sides, so it merges smoothly with the top surface). This would give some radius but I don't know if that is enough to do any good.

On the front, at that edge, I cannot do it so easily as it is curved. So instead I was thinking of putting a lip up there to separate the flow going over the top with that going around the sides, more or less like the ones I've seen pictures of used on air dams. Maybe 6" wide?

At the rear I am thinking either VGs, or a tapered box, or VGs on a tapered box. I'm thinking acrylic for the box to avoid having to make a cut-out for the tail lights and any reflectors. Extending back maybe 2 feet just to keep it reasonable, which means if it's 12 degrees it will have come in about 5 inches all around (not sure if it's worth doing on the bottom). I should be able to mount on the door hinges so it will swing out of the way when the door is opened. It would have a sharp transition and I wonder about that.

Then I think, "why bother?" I should just drive 5mph slower and avoid all that work. :o

Can you post images? That should be able to help us understand your situation better. Thanks!

If the leading edges (sides & top) are radiused, then proceed directly to the tapering addition at the rear of the trailer.

I'd make the rear extension as long as possible. The smaller the cross-sectional area where you truncate (assuming you retain attached flow), the better your results.

Or do both, and double your savings. :)

Sorry, I should have posted photos in the first place.

Hmmm, for some reason I can't upload the picture of the front to photobucket even though I can look at it on my own computer. You'll have to take my word that the front curves around to the sides, but there is no radius anywhere on the top surface.

http://img.photobucket.com/albums/v6...ps9bb906c9.jpg

Here is what I mean about adding a radius to that top edge:

http://img.photobucket.com/albums/v6...ps7f8f29ce.png

The front top edge would benefit.

Past a certain length, the square edges on the sides don't matter, except to cross-winds. Here are your options. Your proposal is labeled 'fairing'. The 20% loss in the inter-vehicle gap may be more than 20%. Does your Ram have an open bed, tonneau, or aerocap?

http://ecomodder.com/forum/member-fr...dynamics-c.jpg

I say try some side skirts and at least some rear strakes.
Most of the big rigs that can't use foldable boat tail for some reason are now using the strakes.

trailer
 

*If you had a topper on the RAM,and something to fill the gap,you could take all the air off the front of the trailer,like the zero-clearance buses in a NASCAR-style draft
http://i1271.photobucket.com/albums/...head2/Gaps.jpg
*For the tail cone,a design has been tested with 25-degree top,10-degree sides,and 10-degree bottom diffuser,which showed lowest for all of the group tested
http://i1271.photobucket.com/albums/...ead2/Scan4.jpg
*All edges are sharp,as you already have and would be about as simple to fabricate as anything you might want to take on.
*While your upper surfaces aren't ideal,NASA was able to get to Cd 0.238 with their sharp-edged van
http://i1271.photobucket.com/albums/...Untitled36.jpg
*In lieu of a tapered boat tail you could also consider an aerodynamic box-cavity.It would be a compromise with respect to performance,but would be simple,lightweight,easy to fabricate,and you wouldn't have to mess with your lighting.I don't have an image but there's a bunch on GOOGLE Images
here's a link
http://patentimages.storage.googleap...101-D00003.png

I had seen that illustration in other threads, but was not aware of the 10 degree sides and didn't notice that the last item was the lowest Cd either (I was concentrating on the 3rd and 4th from the top). I also do not see what is different between the last two items. What does the cross-hatching represent?

I think the taper on a boat tail would be pretty easy (no need to compromise with that square box), but getting the transition smooth like the NASA van would not be. Hope that doesn't hurt too much. Also I have a feeling that adding the bottom part of the cone is not worth too much without also having a belly pan on the trailer. Or maybe a dam at the front of the trailer and skirts on the side, to exclude air? A belly pan would be pretty easy on this trailer, just hang some plastic roofing under there. Skirts and dam would not be that tough either, provided I find some material that can take a beating (I was thinking of plastic baseboard molding).

Right now in the Ram I have a home-made half-tonneau. I realize the gap is a big problem and probably is a lot of work to fix, which is why I'm thinking of just winging it with a lip on top to separate the flow for now. A vortex trap or two might not be too difficult, for what it's worth; but I'm guessing they don't help unless the Ram has a high cap.

Base on the jump from 0.262 to 0.214 Cd, I'd bet that the gray shading represents plan taper and the others are flat-sided.

The vortex traps are designed to prevent air cross-migrating under cross-wind conditions, so—yup wouldn't help.

If you curve the boattail panels instead having them flat, it would be lighter and/or stronger.

You mean taper as viewed from above? I think you are right, because the top pic has no taper anywhere, and the next one (and following) investigates the change of a single factor, the top angle. I think so, anyway. Maybe aerohead can confirm.

Good point about curving the panels, and it also reduces the transition problem. I have an idea how to do that, so I will try to put at least a small curve into them.