Still trying to learn stuff.........box cavity?
 

Hello again,

I'm still trying to learn about aero concepts, if even in general terms for now.

Can anyone give me a quick tutorial on the idea of a box cavity on the rear of a vehicle and how and why it works? Nothing in depth mind you. Just a simple explanation.

And as usual, thank you all.

box cavity
 

As viewed from the rear,the perimeter of the box would be inset from that of the body 'slice' onto which it's attached.
When viewed from the side,the trailing edge of the box would fall where an imaginary 'template' line would intersect that point in space.
Rather than a gentle curving surface,you'd just a rearward-facing step.
The flow would separate from the body as the downslope hit 22-23 degrees from horizontal,but re-attach right at the end of the step,capturing a vortex of circulating air.
Flow just outside the cavity would skim across this pool of captured,swirling air,and undergo decelerative pressure gain,such that when it did separate from the trailing edge of the cavity,it would be at a higher pressure than before,and behind a smaller wake,effectively increasing the base pressure,while reducing pressure drag.Not as good as a boat tail,but way out ahead of nothing!

Review this: EcoModder Forum > EcoModding > Aerodynamics > 'Fluid Tail'

http://i1271.photobucket.com/albums/...itled12_20.jpg
http://i1271.photobucket.com/albums/...titled7_22.jpg

The box can be beveled as above, or stepped, as below.

https://ecomodder.com/forum/member-f...31-1-26-08.png

My favorite would be the concave truncation.

http://i.kinja-img.com/gawker-media/...zaltgbbtlo.jpg
http://jalopnik.com/the-renovo-coupe...-ft-1621969757

Here's a boxed back window over a difusser:

https://ecomodder.com/forum/member-f...ni-desktop.jpg

I'm assuming the trailing edge would be sloped about 12° (to be in-line with the template). But viewed from the top, would the rearward taper be the same 12° or something closer to 7°? I think I remember when I was attempting my aerocap for my old Ranger, something had been said about the angle from front to back being more like 7° instead of 12°. Or does the template keep curving more aggressively as you move rearward. Seems like I had planned my old aerocap at 3° down at the cab out for a foot. Then another 4° (7° total) for another 2 feet, and another 5° (12° total) for the remaining 3 feet. Or does the template keep curving more aggressively beyond about 12° as it moves rearward?

I guess I'm still not seeing how 12° down angle from the template would would ever intersect with 22° to give a point for the trailing edge of the box.

Give me a bit. My mind is remembering pictures I have seen here that might help me get a better grip on this one.

Edit: Can't seem to find the pics I'm thinking of, but now my mind is seeing some other options for my truck. I may have some more truck specific questions later If I can get my head wrapped around how it might work.

Very interesting reading freebeard.

All the pictures seem to represent vehicles where the box starts at the roof line. My mind is thinking something more for the rear of pick-up either in the bed (no aerocap) or behind the cab. Behind the cab would be where the box would meet the roof line (partial aerocap?) but would the horizontal edge of a bed cover be like a second "roof line" and have similar angles or (as I am thinking) more closely follow the template?

https://ecomodder.com/forum/member-f...5201-28i9d.jpg
The whole discussion of defining [a sector of circle] by the angle confounds me. It describes the axial angle not the central angle. The axial angle goes from 0° at horizontal to 90° at the vertical. You can't just go to 12° at max camber (the widest/tallest point) and hold it, that gives a straight line. [shrug]

That said, the curves converge to a point if the height is half the width. Taller and you get the Trophenwagen, shorter and you get aerohead's Baby Template.

https://ecomodder.com/forum/member-f...ture4984-a.jpg

https://ecomodder.com/forum/showthre...6-a-32673.html

These were some of the pictures I was thinking of.

I was wondering if I could use a flip over bed extender to make box cavity of sorts.

Maybe the top of the red area covered in coroplast also to extend the surface of the bed cover to meet up closer to the template line?

It's just a quick MS Paint representation. I think the factory bed extender is tapered like in the third picture. The green coroplast sides would probably have to follow this taper but still extend enought to make a 12" deep cavity.

I wish I knew how to place the template over a picture and move it. If you placed the template at the rear with the high point being the top of the tailgate in the up position, would the resulting shaded area be the shape of what I'm aiming for?

Or something like this........

Everything I know about aerodynamics I learned on Ecomodder.

To me a box cavity has always been a fast and dirty boat tail. Not perfect, probably not good looking, but effective. I've seen some graphics of pretty ones that just needed some skin to be real boat tails, but even one panel hanging out back coming close to the template is good.

I can't find the post, but this was on an early version of The Anal Probe:

https://encrypted-tbn0.gstatic.com/i...0hSi1vlUi_GOsQ

The setups on the backs of 18 wheelers these days are a good example. Crappy boattails, but way better than the unmodded truck:

https://compote.slate.com/images/b47...a61918f2ea.jpg

It's not pretty, it's not optimal. It's a box.

Seems about right to me.

My understanding is that the primary benefit is less about having a cavity than conforming to the general shape of that curve for the rear most trailing edges. So if the bed extender falls in that area, just closing off the top and sides of it while extended would likely provide measurable benefit without having to extend it further, though every little bit often helps.

Let's not forget the Anal Probe, Sven7's last post was 05-01-2016 07:07 PM.

The proven solution for a pickup truck is the half-tonneau. See this thread https://ecomodder.com/forum/showthre...ate-36859.html, started on 09-20-2018, 09:24 AM by hat_man. Hmm.

Curiously, a half-tonneau is basically a box cavity rotated 90°.

That idea is still on the board also. I wish I could find a way to use the flip over bed extender and still seal off the tailgate opening. Then I could use the two in tandem for longer road trips.

FWIW, here's a pickup with a half-tonneau, a folding single-wheel trailer, and side pipe that are capped with Coanda nozzle slots to pull the air toward the centerline:

https://ecomodder.com/forum/member-f...ckup-truck.png

And another with an oversized aerocap for more usable interior space. The wake of the cab reattaches to the nose of the canopy.

https://ecomodder.com/forum/member-f...4-10-08-26.png

For me the half tonneau and wing would be what I would really want, but I just couldn't figure out how to make the wing and keep the truck looking somewhat normal. The bed extender idea could be flipped out for driving to work and longer trips and put away for in town jaunts. The truck would still look normal for the most part. The wing would have to be a permanently attached structure and even though would be a perfect conversation starter, would still look a little strange. Makes me wonder if a partial aerocap (pseudo box cavity) with slope and taper of about 6°-7° and maybe 12" deep behind the cab and a half tonneau would be a start and adding the flip out bed extender idea later would be feasible? The partial cap/box cavity behind the cab would be permanent but look a little more "normal" than just a wing attached to the top of the cab. Each by itself would be a benefit. I wonder if they would play nice together?

Been thinking about the flip over bed extender idea some more. Checked on used ones but man are they still expensive and most are for an F-150 or Explorer Sport Trac. Both are too wide for my Ranger. So I'm looking at making one out of PVC fittings and pipe. It doesn't really have to hold any loads in place while extended so it doesn't have to be super strong like metal. I'm thinking 1 1/2" pipe and whatever assorted fittings I can find and a little "on the fly" engineering. I have two ideas in mind and I'm looking for advice on either. I would prefer to plan the work, then work the plan so to speak. I don't like building things twice. Both would have the same view from above, only the side view would change.

I'm open to any advice. Thanks.

https://ecomodder.com/forum/member-f...02-5-26-32.png

Had I a pickup truck, this is what I'd do, except the outer ends would be beveled. It's a half-tonneau/spoiler combination.

edit:
The manager of the pizza parlor knew it gets better gas mileage, but he didn't know how much.

I've seen that picture before. It may work but I just don't like the aesthetics of it. I want to keep the truck looking as "normal" as possible. The fold out box cavity can at least be put away when not in use (like in town) and flipped out for trips back and forth to work and on longer road trips.

Reminds me of this delivery car that used to be in the next town over. Not very aero, but definitely a head turner.

Could very well be the driver(s). When I worked for them, we always fought over who got to drive the 4.3 Astro over the anemic 2.3 liter ones. I know the economy in that thing stunk! ;)

Now you're just mocking the idea. It's more like a Camper World Cup racer

https://st.hotrod.com/uploads/sites/...ies-eldora.jpg

In both cases I think the corners could beveled or rounded without compromise. If you can hide a horizontal extension, why not a vertical one? In either case you're just trying to get back to the Template line.

Wasn't trying to mock. I've had that pic on the computer for years. Always wanted to post it here.

Not sure I could get the tonneau cover to close with a vertical extension and a pivoting mechanism. I'll have to start with cardboard and see what I can come up with. Not saying it won't work, I just can't see it in my mind. Time to stop at the appliance store see if they have any boxes.

A fully-boxed tail on a 53’ van is worth having. It’s right at four-feet deep.

But with this caveat: increasing the length of the sail ALSO increases the leverage of wind against the vehicle. This is a serious concern. The crosswind has a longer lever to use.

So much so in this case I know drivers who otherwise run them WON’T when empty or nearly so (I can load 45k in the box. Today’s load is 7k. That’s “empty”).

Moving the trailer tandem axles rearwards can offset this effect (wind forces increase drastically every foot rearwards as they can’t escape the squared roof edge), but there’s a legal limit to how far back. AND it makes the trailer more difficult to get through every turn or curve with the increase in distance from the Drive Axles (Ackerman angle).

Were I the trailer owner I would have both skirt & tails (plus the nose cone arrangements we’ve seen to collapse the tractor/trailer distance from one another). The MPG benefit is obvious, and given a regular loading (25k plus) it makes handling the truck EASIER throughout the day. Wind problems are reduced, overall. But it’s also a new caution on a windy mountain pass. In a rainstorm squall line. Etc.

The pickup half tonneau looks nice. But the Aerolid is where to start. Even a simple one (as on a Dodge Cummins owners truck in aluminum sheet) provides an idea of benefits.

For a city box truck, or for a pickup, access to the box/bed has to be weighed against complexity in a tail. The forces at work back there are high. To replicate the industry TRAILER TAIL is no small matter given a cab-height bed topper.

Which is why you now see Coanda devices being used on the trailing edge of vans. Less problem and MOST of the benefit.

Tails are so 2014. (Ha!)

Obama-Age Obsolescence.


Now, for my pickup it’d be front airdam and side skirts. The rear cavity “ought” to be something I can dismount. A hitch-mount carrier with an extension (roof & sidewalls) that extends upwards from a transport case to meet my LEER bed topper roofline.

How to treat the exit UNDER that (under truck body) is where a big question mark exists (for me).

.

One thing I've noticed in this discussion (Ecomodding Central: Green Grand Prix modding thread) is this part from a Prius:

https://ecomodder.com/forum/member-v...314-155355.jpg
https://ecomodder.com/forum/showthre...tml#post603668

Compare to the High Momentum mudflap called out here:
https://ecomodder.com/forum/member-f...dynamics-c.jpg

The Prius part seems to be a much reduced version of the truck mudflap. Maybe something half-way between the two?

those are actually illegal anyways in most states

and if the cop is a real jerk he will cite you for a oversized load without permit as well



I really love tailgating those the FE is a bit higher compared to a normal 18 wheeler truck

Why would the trailer box-tail be illegal?

Do they cause the rig to be too long for maximum specs or something?

illegal
 

It used to be that any truck in the USA could run up to a 5-foot tail,without penalty,as a matter of US energy policy.What a shame if they've changed that.

They’re in no way illegal. That’s flat funny.

Truck regulation is basically road restrictions. Think turns and curves where rear overhang intrudes into other lanes. Rare; mainly mountain roads. These roads, but not those; etc. That “takes care of” OAL limits.

And they work well. I had a slight tailwind out of Amarillo yesterday at 79,300-lbs. Stayed above 10-mpg with skirt & tails almost all the way to OKC at 67-mph.

This morning when I finish this I have to pull the tractor out from under the 59k trailer to re-grease 5th wheel. Had to initially shove my way under. Fighting any crosswind, road camber changes, ruts, etc, gets old.

Will buy fuel and then also move trailer tandem axles forward slightly and also move the 5th wheel itself. End result will be FAR fewer steering inputs and/or degree & duration thereof.

Mechanical condition always trumps aero. One has to set the conditions for it’s performance addition
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