Pickup bed aerodynamics idea. Any thoughts?

[hat_man]

I had posted earlier that I was designing an aerocap for my truck and got so much good information that I started building the "skeleton". Life threw me a curve and I had to put it on hold. I want to thank all of you who gave me advice on the shape. I am following it as best as I can. I haven't given up just a minor set-back.

I understand the concept (but not the mechanics) of "tailgate up vs. tailgate down" and how the air coming over the cab "curls back" and "stacks up" in the bed (after hitting the raised tailgate) allowing the subsequent air to flow over the bed more efficiently

The other day on my way home from work I had a crazy idea cross my mind on air flow under the truck after the aerocap is built. I had already thought of adding a partial belly pan from approx. the rear axle to the bumper using the spare tire carriage for support.

For the sake of argument, let's assume that the first picture is the shape I am shooting for.

In the second picture I have extended the aerocap out to the same distance as the open tailgate. Would this help with efficiency or mpg? How would this effect the air flow under the truck?

In the third picture (now for the crazy idea) what would happen if someone added a "reverse tailgate" of sorts to the outside of the regular tailgate? When closed it would rest just under the aerocap extension in picture two. But when opened would it work similar to the "tailgate up" concept and allow for better airflow under the truck?

I am no aero guy for sure, I just get a mental picture and wonder how it might work. The whole idea came from seeing the fourth picture and kind of smashing it together with my aerocap idea.

[BamZipPow]

Check out my threads on my aero cap, dropped air dam, and belly pan...

My attempt at an aero cap... - Toyota Nation Forum : Toyota Car and Truck Forums
Under belly pan... - Toyota Nation Forum : Toyota Car and Truck Forums
Modified air dam... - Toyota Nation Forum : Toyota Car and Truck Forums

[kach22i]

The trapped air question in the third image is an interesting one.

I would guess that the air under the typical pickup truck is messy as all heck, and you shouldn't worry about managing it "after the fact".

[ChazInMT]

Gahhhh---Don't stick something down like that! Keep in mind the thing we're shooting for is making the shape as gentle as possible in the back. Just because a tailgate up is a modicum better for fuel efficiency, it does not make it an efficient enhancement....it's just the lesser of two evils over having the tailgate down. By building an aerocap, you are way ahead of the game. Hanging something in the slipstream like that would not be too different from putting a drag chute on the back of your truck.

If you really wanted to go all out, take the tailgate off, build a longer cap and hang out the back with a lower "Diffuseresque" thing dropping to the bottom of your bumper and coming up from there.

I can't speak for what angle would optimize the efficiency here on bottom, I know for sure you want to stick with the top of the template curve.

If you want, and you keep you current design, you could raise your cap a few inches in back without hurting the efficiency of it, my analysis of your shape shows you could shift it back a bit which would move it up. It may not seem like much, but 2 inches will make for more room and easier access if you build in a little swing up door onto your cap.

Here's what I mean.

[aerohead]

*i think the upper extension could 'frustrate' the turbulence a bit,kinda forcing where it could or could not go.It would be hard to attach a numerical value to it though.
*as to the lower panel,I think that it would just be embedded within the turbulence that would already be present back there.
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*Without a complete belly pan and diffuser, the underside of the lower panel would have no 'sink flow' approaching it with which to modify,just the turbulence which would begin under the nose of the truck,reaching all the way back for hundreds of feet.

[aerohead]

Originally Posted by ChazInMT

Gahhhh---Don't stick something down like that! Keep in mind the thing we're shooting for is making the shape as gentle as possible in the back. Just because a tailgate up is a modicum better for fuel efficiency, it does not make it an efficient enhancement....it's just the lesser of two evils over having the tailgate down. By building an aerocap, you are way ahead of the game. Hanging something in the slipstream like that would not be too different from putting a drag chute on the back of your truck.

If you really wanted to go all out, take the tailgate off, build a longer cap and hang out the back with a lower "Diffuseresque" thing dropping to the bottom of your bumper and coming up from there.

I can't speak for what angle would optimize the efficiency here on bottom, I know for sure you want to stick with the top of the template curve.

If you want, and you keep you current design, you could raise your cap a few inches in back without hurting the efficiency of it, my analysis of your shape shows you could shift it back a bit which would move it up. It may not seem like much, but 2 inches will make for more room and easier access if you build in a little swing up door onto your cap.

Here's what I mean.

I like your solution very much and I've had some success with a version of it as has bondo with his F-150.I think Brett gave us some YouTube video of tuft tests he did.It's sweet!
Anything which fills in the wake like this seems ,so far ,to be beneficial.

[bondo]

Like this.................

[aerohead]

Brett,it would be oxymoronic to say that you outdo yourself,'cause everything you do is beyond O.E.M.quality.
And the fold-away feature gets he jump on utilityphiliacs.Anyone who might criticize it would just be addicted to criticism.
Thanks for the look! She's big-time sweet!

[brucepick]

Like an idea I built once, I think the reverse tailgate is d**n clever but not of much benefit, and there's a good chance that thing sticking downwards will hurt the overall aero. The dropped tailgate apparently only works on SOME pickups. But I think that where it does work, it's partially due to the pretty bad aero properties of pickups in general. So even a small improvement is welcome.

I'd say, work to build as much of a true aerocap or Kamm treatment as you can. Just go for the teardrop outline. Every half foot or foot of length (with appropriate taper) will give you additional gains. If you can only build it back 2-3 feet from the cab, fine, it will be better than no cap. That's my guess.

I finally ditched the clever rear spoiler thing I worked up, and instead built a simple Kamm type extended roof line for my Civic. Much simpler and apparently much improved aero.

[KamperBob]

Brett, love that collapsible fanny pack!