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Old 12-03-2009, 12:09 PM   #1 (permalink)
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How much of a radius to keep flow attached on aero bed cover?

Hi everyone,

I'm pretty new to the site but have been reading every thread I could find regarding aero caps and tonneau covers.

I have a 2006 Chevy Silverado 1500 4x4 and since winter is coming would like to keep the snow out of the bed and off my stuff. I've been considering a regular flat tonneau cover, but would love to be able to increase the truck's aerodynamic efficiency at the same time.

So, when reading through everything, it seems that the design of the cap is important to ensure that it actually improves the airflow.

Initially I was planning on making something fairly straight (easy to make). But am concerned that the transition would be too abrupt between the roof of the truck and the cap.

So the three questions I have are:

1) How much of a radius is required to allow the air to transition smoothly down the "ramp" of the cap - would the image attached be sufficient?

2) Can the transition from the sides to the top of the cap be abrupt, or must this also have a radius?

3) At the end of the day - am I only going to be able to figure out the optimal design by building a mock up and testing it? Or can reasonable assumptions be taken from design images?

Thanks in advance! I am so impressed with all of the knowledge on this site!

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Last edited by Fubeca; 12-03-2009 at 12:40 PM..
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Old 12-03-2009, 12:16 PM   #2 (permalink)
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Everything should be a nice smooth transition. Any abrupt changes are going to create turbulence.

For the angle, I'd look at what MetroMPG did with his kamm back. He overlaid a tear drop image over his car to get the ideal shape.

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Old 12-03-2009, 12:42 PM   #3 (permalink)
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Is there an easy way to do the overlay image? I don't know if I have any software that will allow me to do that.
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Old 12-03-2009, 12:58 PM   #4 (permalink)
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See below for my photoshopped attempt for the new Audi A8 with a manufacturer claimed Cd of 0.26. Not bad for a range-topping vehicle.



It also features extensive underbody panelling. Note the absence of air dams. The tapering of the greenhouse/trunk lid is beautiful IMHO.

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Old 12-03-2009, 01:00 PM   #5 (permalink)
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Quote:
Originally Posted by Fubeca View Post
Is there an easy way to do the overlay image? I don't know if I have any software that will allow me to do that.
Photoshop is great (but expensive). GIMP is free but a little cumbersome. I think Paint.net may be another choice.
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Old 12-03-2009, 01:18 PM   #6 (permalink)
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superchow, that image is not correct. The centerline of the tear drop must be along the ground.
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Old 12-03-2009, 01:22 PM   #7 (permalink)
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In the rear view of the A8 it looks like the transition between the sides and the top of the trunklid is fairly sharp. It would be easy to build my cap with a sharp transition between the top and the sides but I'm not sure if that will cause problems for me.

I traced the overlay, and it looks like maybe I should make the cap a little steeper.
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Old 12-03-2009, 01:32 PM   #8 (permalink)
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It might help to follow the lead of others to save yourself the trouble of reinventing the wheel.





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Old 12-03-2009, 01:35 PM   #9 (permalink)
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I'm absolutely trying to follow the lead of others - that is why I'm trying to get the angle right as it seems that some others have not seen success.

I rescaled the teardrop and now it looks like my initial drawing was pretty close.

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Old 12-03-2009, 01:36 PM   #10 (permalink)
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Daox is right: to use the template properly, the bottom of the teardrop should be on the ground plane (with the vehicle's tires) and the highest point of the vehicle's roof should line up with the highest point in the tear drop shape.

That would show ideal curvature/taper, or close to it.

Having said that, Phil's first T-100 aeroshell was angular/flat planed, and it worked:


But his curent one is literally made out of a boat hull (compound curves), and may be better.



Also, you could safely mimic Bondo's F-150 aeroshell - it also works.


http://ecomodder.com/forum/showthrea...d-cap-583.html

Just eyeballing your image and comparing it to Phils & Bondo's , I would guess that it would work fine (though I would radius the shoulders a bit). The "ideal" image would have slightly less of a slope.

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