Question for the Aero-knowledgeable. - PowerStrokeNation : Ford Powerstroke Diesel Forum
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post #1 of 17 (permalink) Old 05-06-2010, 05:59 AM Thread Starter
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Question for the Aero-knowledgeable.

my old custom cap rotted, and I have a new plan for an aero cap that can be made into a carrying container as well... but before I plan it out, I need to know something...
would I be better off:
A: having something sloping from the cab to the top of the tailgate or
B: from the cab to the end of the bed with the tailgate down?
My thought is that having the tailgate down while improving the airflow behind the cab would be more effective...
I'll keep you all updated if I get this going, of course.
thanks for any insight-
Dan

Dan
96 Powerstroke: 6637, downpipe to dump exhaust, 140v IDM and a shift kit.

84 Honda Sabre V65 1100: pipes, jet kit and new tires. Faster than I think I'll ever need.
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post #2 of 17 (permalink) Old 05-06-2010, 12:59 PM
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Re: Question for the Aero-knowledgeable.

I think that the Mythbusters have found that the tailgate down thing is humus.. But IIRC, tailgate down with a cargo net thingy instead improved mileage.

One of them truckless bastards.
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post #3 of 17 (permalink) Old 05-06-2010, 02:05 PM Thread Starter
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Re: Question for the Aero-knowledgeable.

that's also for newer trucks and more common in shortbeds (I have a 96 Longbed) where the low Pressure zone behind the cab was small.
also, everything changes when you hide that area from the wind...

Dan
96 Powerstroke: 6637, downpipe to dump exhaust, 140v IDM and a shift kit.

84 Honda Sabre V65 1100: pipes, jet kit and new tires. Faster than I think I'll ever need.
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post #4 of 17 (permalink) Old 05-07-2010, 01:39 AM Thread Starter
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Re: Question for the Aero-knowledgeable.

shameless bump...

Dan
96 Powerstroke: 6637, downpipe to dump exhaust, 140v IDM and a shift kit.

84 Honda Sabre V65 1100: pipes, jet kit and new tires. Faster than I think I'll ever need.
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post #5 of 17 (permalink) Old 05-07-2010, 02:28 AM
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Re: Question for the Aero-knowledgeable.

Seems like everything would be smoother if the tailgate was up. the insides of the bed would probably cause turbulence. but idk worth trying both I suppose

1996 F-250. 5 spd, 3.55s, 292k miles, 6637 intake, 3"dp 4" straight pipe, 1.0 A/R, gutted ebpv
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post #6 of 17 (permalink) Old 05-07-2010, 02:43 AM
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Re: Question for the Aero-knowledgeable.

Some thoughts:
Keep the tailgate up. Lowering it increases the size of the low pressure area behind the cab.

The goal is to reduce your wake area as much as possible without having the flow detach from the surface of the vehicle. To hold the flow, you must make changes in angle as gently as possible. Generally you want to keep the slope of the cap to less than 12 degrees – more or less a 2-in-12 pitch for carpentry types. For a longbed, that means you can have the back edge (96÷6=) 16 inches lower than the top of the cab. I don’t know how high the cab is above the rail on a OBS truck, but on mine it is 24 inches above the top of the bed rail. Therefore the top of the portion above the tailgate is 8 inches above the top of the tailgate.

My fairing is sub-optimal because it is too steeply sloped. Mine goes from the top of the cab to a half-inch above the tail gate – about a 17 degree angle. I know I have flow separation. Hopefully I will address that problem this summer.

Google up a guy named Phil Knox. He has been dinking with pickup truck aero for years. He has a Toyota T-100 that makes my truck look conventional but he gets insane MPG from that gas pig. He also posts regularly at Fuel Economy, Hypermiling, EcoModding News and Forum - EcoModder.com
It’s a hybrid/small car site but they tolerate him because he has the goods.

Here’s a thought: To get optimum aero improvement, you need to streamline the thing not only in the elevation (side) view, but the plan (from above) view as well. By doing that and sticking to less than 12 degrees angle, I could in theory reduce my wake area by 41% and retain smooth flow. Now we are talking places where only a skilled body man can go.

2000 F-350 SC 4x2 6 speed
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Slammed 4"
Rugged Air Dam
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post #7 of 17 (permalink) Old 05-07-2010, 04:42 AM Thread Starter
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Re: Question for the Aero-knowledgeable.

so would it then be better to slope at 12 degrees from the top of the tailgate to wherever that meets, or to the top of the cab?
also, thinking about streamlined vehicles- they don't make all of their change at once. could a more aggressive downward pitch be more beneficial if I eased into it? say, starting at 10 degrees, and working towards 30 by the end of the bed....
this is full custom, so it's only a matter of whether or not I can build it...
however, I'm making it so that I can also convert it into a 4 ft tall sidewall in a few moments notice... going to be hauling lots of junk this summer helping my parents clean up a fixer property.
also, thinking about streamlining in multiple dimensions... couldn't the bed sides act as fins in a way where they would not greatly harm the aero? (this, assuming I could slope it in a way to have the tailgate down)
thanks for the info,
Dan

Dan
96 Powerstroke: 6637, downpipe to dump exhaust, 140v IDM and a shift kit.

84 Honda Sabre V65 1100: pipes, jet kit and new tires. Faster than I think I'll ever need.
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post #8 of 17 (permalink) Old 05-07-2010, 11:48 PM
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Re: Question for the Aero-knowledgeable.

Yes, you want to transition from the top of the cab to the 11 degrees fairly gently - maybe a two-foot transition zone.

2000 F-350 SC 4x2 6 speed
3.08 gears Gear Vendor Overdrive
Slammed 4"
Rugged Air Dam
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post #9 of 17 (permalink) Old 05-10-2010, 05:11 PM Thread Starter
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Re: Question for the Aero-knowledgeable.

ok, and if I took another two feet to transition to 22 degrees? is this increasing the likelyhood of flow separation?

Dan
96 Powerstroke: 6637, downpipe to dump exhaust, 140v IDM and a shift kit.

84 Honda Sabre V65 1100: pipes, jet kit and new tires. Faster than I think I'll ever need.
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post #10 of 17 (permalink) Old 05-11-2010, 12:29 AM
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Re: Question for the Aero-knowledgeable.

If you let the slope get steeper than 12 degrees, separation is gonna happen.

2000 F-350 SC 4x2 6 speed
3.08 gears Gear Vendor Overdrive
Slammed 4"
Rugged Air Dam
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