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Old 12-11-2008, 04:39 PM   #21 (permalink)
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Quote:
Originally Posted by MetroMPG View Post
As for shape to follow, look at Bondo's or Phil's truck and follow those lines. The fact that you're chopping yours off before the tailgate doesn't matter. If you can round the "shoulders" (the transition between the "sides" and "top" of the extension), it'll help.
Thanks, Darin. I understand that I'm going for a teardrop, no prob. My question concerns how to chop it off for best effect. Right now, it's cut off with a rather ragged shape. I would rather, for aero as well as aesthetics, have it be a straight cut all the way around. The question is... along what plane should I chop it? Most Kammbacks seem to chop at a downward-tilted angle, which makes sense at the very back of the vehicle. (Does it? I guess it does...) But off the cab in the middle of the wake, is there any reason to chop it vertically? Does it matter?

I'm asking, b/c "bigger" was exactly my intent. I only did this version because it was quick and involved scraps, and I'm impatient. And I never intended to attach sides to this -- it was just an impulse when I realized how easy it was. So I want to know for the next project if there's a reason to chop it vertically, or tilted back like yours, or forward like a "fastback fairing".

Ultimately, I'll have a topper. But in the meantime I have a few intermediary ideas I want to try that will still allow the bed to be open.

Quote:
Originally Posted by MetroMPG View Post
If you can round the "shoulders" (the transition between the "sides" and "top" of the extension), it'll help.
It's hard to tell in the previous pix, but the corners actually are pretty well rounded. See this one:



I actually consulted your thread a lot, and I remember you saying that you wish you rounded the corners more.


I'm hoping this thread may inspire some other truckers. This mod was SOOO easy. Way easier than the wheel skirts.

All it involves are about 6 sq. ft. of coroplast scraps and double-sided Duck tape.
It's perfectly reasonable to have it all done in one evening.

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Old 12-11-2008, 05:23 PM   #22 (permalink)
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where/how

Forgive me for butting in but I wanted to toss out something that might help you answer your own question. Aero isn't that mysterious. Your truck is submerged in air,and as you advance forward,you displace a volume of air equal to your frontal area,times your distance.Air has mass(so momentum and inertia are involved) and air is viscous(so there's friction when you move through it).---------------

With the Tacoma,your trick is to allow the air to recover to the position it occupied before you came along with the minimum amount of separated flow. Once the air has reached the point of maximum cross-sectional area ( about where your head is, outside around the cab),you have converted atmospheric pressure into kinetic energy. The whole point of streamlining,is to provide a ramp which is gentle enough,such that the air can rebound(like a spring),without leaving the surface of the aftbody of the truck. Any reduction in the cross-sectional area of the truck without flow separation will produce a like reduction in drag. If you can reduce the wake of the truck 10% without separation,then you reduce your drag by 10%. If you imagine the imaginary teardrop tail,extending rearwards behind your cab,divided into 10-sections,each section with 10% less area (turbulence-free),the further back you go,the less drag you have,losing 10% for each "section." And you chop it off where ever you choose.That's all there is to the Kamm-back! Kamm and others did recommend that you chop it off when the (clean) cross-sectional area is reduced to 50% of frontal area. -------------

You can cut it anywhere you want and you can calculate your drag reduction as a direct function of the area of the remaining wake. That's all there is to profile drag! But remember,the curvature of your structure cannot exceed the curvature found with the teardrop or separation will occur,and you're right back where you began.Careful examination of your roof will tell you if Toyota gave you a little curvature to begin with,you can take it from there.
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Old 12-11-2008, 06:06 PM   #23 (permalink)
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Wow, that was a really helpful explanation. Thanks! You're perfectly welcome to "butt in" any time.

Quote:
Kamm and others did recommend that you chop it off when the (clean) cross-sectional area is reduced to 50% of frontal area.
Hmm... I see the problem, since on a truck, with the bed taking up at least 50% of the area all the way to the back, that pretty much forces a topper and a partial boattail on you.

But I think you've also suggested that you can pair a cab Kamm with a rear tonneau or Kamm extension to provide that continuity. That's probably my next mod. I've been wanting to ask you... My short 5' bed has a pretty sharp angle from the cab to tailgate, on the order of 20+ degrees. How much would I be able to bridge that gap with fancy curvature on the Kamm? I'm figuring I may have to whale-tail it a bit, and I'm wondering how high I might need to build the extension. Thanks again.
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Old 12-12-2008, 02:32 AM   #24 (permalink)
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i did a quick measurement off the rear windshield/roof angles of some sub 0.29 Cd car profiles and most where somewhere between 15 and 16 degrees. most also had a little vertical spoiler or simple trunklid like structure just before the end of the car.
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Old 12-12-2008, 07:35 AM   #25 (permalink)
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if u noticed gm and ford are using short beds for the xfe models, they add a cover. they claim the bed is short enough that the main air flow dont hit it.. i know the air hits the last foot or so of my 6 foot 3 inch bed.
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Old 12-12-2008, 11:39 AM   #26 (permalink)
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if u noticed gm and ford are using short beds for the xfe models, they add a cover. they claim the bed is short enough that the main air flow dont hit it.. i know the air hits the last foot or so of my 6 foot 3 inch bed.
Taco: Can you find an article that explains the XFE aerodynamics? It'd be nice to see an airflow diagram of this.

It still seems to me that optimally I would want the reattach the airflow at the back. Maybe with the short bed, I would have a chance for a really short wake if I get the curvature right.

LH: I actually made the half-tonneau sit inside the rails instead of on top, and it sits just below the top of the tailgate, creating a small lip. I think for those cars, the angle from the cab to the top of the lip is generally in the 12 degree range. In my case, it's about 22 degrees. It sure would be amusing to try putting a 6" tall "lip" on my tailgate to see if it helped reattachment. How's that for counterintuitive?

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Old 12-12-2008, 01:04 PM   #27 (permalink)
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Might be easy to test this out. Maybe try extending the rear of the vehicle a bit, like they do on the salt flat cars. It wouldnt have to be this dramatic, but it might help by making the bed essentially longer. The lip on the tailgate also sounds like a good idea to combat this issue. Or perhaps both with be called for to be effective on a short bed.

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Old 12-12-2008, 01:38 PM   #28 (permalink)
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Yeah, that's what I was thinking of. The other idea was to continue the curved lines of the roof fairing into a rounded tailgate Kamm and/or spoiler-type tonneau top. I think a rounded arch-style tonneau top, sloping downwards, could look cool and un-spoiler-like enough not to be laughed at. I could make it look like some kind of fancy executive beer cooler.

Actually, since I couldn't let a day go by without trying something new, I did stick a tiny flat extension on the back using the only scrap of anything I had lying around. But it looks so stupid the way I did it that I'm embarrassed to post it, and besides, it made no difference in my commute. To mate the air off the cab at 12-13 degrees, it would need to be long... I think my trig told be that it'd need to be 12-24" past the tailgate to have a chance. I think my best choice is to raise it a little , and make up some of the angle with fancy curvature from the cab fairing.

One advantage to modding a truck is that, if it's designed cleverly enough, I think a tailgate Kamm could fool people into thinking it's some sort of towing apparatus.
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Old 12-12-2008, 02:03 PM   #29 (permalink)
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^
lol
*man examining truck*

well folks it looks like some sort of towing apperatus.

*akashic returns to truck with grocieries*
HEy HEY! get away from my truck!
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Old 12-12-2008, 06:29 PM   #30 (permalink)
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akashic - did you see the teardrop photoshop templates I did in my Kammback thread? If you fit your truck into that shape, with the highest point of the cab at the highest point in the teardrop, you'll be able to see the "ideal" curvature from the tail end of the roof, aft.

I think you're right in that the end point of the ideal curve is going to be higher up than the top of the tailgate.

Here's the image:


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