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Peon Extraordinaire
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Discussion Starter #1
So what are you running now Dave?

Sam
 

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Peon Extraordinaire
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1,880 Posts
Discussion Starter #3
Care to share your changes? I hate to be a pest - would you care to recap the top 5-10 pointers when designing a fastback type cover?

Sam
 

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Ever see a Lamborghini? At the speeds they seek, good aero is absolutely essential and the rear of the car is the most important part.

Lambos step their rear windows down and the air flow closely approximates that of a properly sloped fastback. But at the same time the "risers" are clear and you get a pretty decent view out.

That is what I'm gonna do. I'm gonna attach (using threaded rod) three flat plates riding parallel to the tonneau. The rear one will be about 5" above the tonneau. The next two will space out 3 or4 inches apart and ride further forward. On the cab rear bulkhead, I'll attach a "duckbill" that breaks down 10 or 12 degrees. The duckbill turns the airflow down, but the gentle slope prevents flow separation. I'll post pix when I get 'er done.

There is a maximum angle for fastbacks: 12 degrees. Beyond that, the air flow peels off and starts drawing vacuum on the back of your vehicle right there. For those who have done carpentry the trig works out to a 1-5 slope. 10 degress (1-in-5.5) is better. My old fairing was 17 degrees and was still better than a flat tonneau.

With this setup, I'm looking for less drag and enough vision out the back to allow me to reduce the side mirrors. Side mirrors are 6-10% of your aero drag. CCTV cameras are not yet legal in Indiana, but I can closely approximate them.

I don't want to get too deep financially because my next step is get a junkyard bed and attack the whole bed assembly. The trick with a pickup is to reduce drag but retain the cargo hauling capacity. I think there is scope for that, but I'll have to swim upstream against what everyone else is doing with pickup beds.
 

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Peon Extraordinaire
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Discussion Starter #5
Interesting. Appreciate the insight. I took measurements the other night and will be doing some sketching in order to find the angle for my truck.

Sam
 

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If your truck is like mine, the cab is 24 inches above the bed rail.

A short bed is 80 inches long. If you use a 1-in-5 slope (10 degrees) your rear edge will be 16 inches below the cab top or 8 inches above the top of the tailgate.

A long bed is 96 inches long, so the rear edge would be a 19.2 inches below the cab top or 4.8 inches above the tail gate top.

Either way the boundary layer stays laminar all the way to the end. That's what you are looking for: Reduction of wake area with laminar flow all the way. If it goes turbulent, the boundary layer separates and you have a larger virtual wake area.
 
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