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Old 03-06-2006, 08:30 PM   #1
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Does a truck cap help wind flow?

I intend to tow a 28' Classic with a 3/4t short-bed pickup. Would a bed cap (not a tonneau) increase towing efficiency by smoothing wind flow?

Thanks.

Sam
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Old 03-06-2006, 08:33 PM   #2
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Old 03-06-2006, 08:40 PM   #3
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If you can find one, get a fiberglass cap with a turn-up at the rear, this will help deflect the wind up and over the trailer.
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Old 03-06-2006, 08:41 PM   #4
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It would also give you more weatherproof storage. I would think anything that would decrease drag would be an improvement. Unfortunately I haven't seen any wind tunnel tests
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Old 03-06-2006, 08:44 PM   #5
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Oh man, I gotta chime in on this

Got a degree in Aerospace Engineering and used to work for Boeing. Now I build bridges...but I still love my airplanes!

If you're pulling a trailer, then a cap will most definitely help you. What will happen if you don't have one is that you will get vortices forming off the back of the cab that will then slam into the front of the trailer. That flow will then need to bend around the trailer again.

If you do have a cap, then the flow is pretty much smooth across the truck. When it gets to the rear, it will try to converge but...run into the front of your trailer and then stay relatively attached as it flows down the sides. Kind of like two NASCAR stockers drafting. An Airstream is better than a box SOB at allowing the flow to converge smoothly at the end, but it still will create turbulence as the air trys to come back together.

Now when you're not towing, it's just the opposite. The cap creates more drag than not having it. What you've basically got now is a giant brick and the air rolling off the back of the truck will really create some vortices.

Granted, we're not going 600mph here and it's not the same deal at 60 as it is at 600 (or even 150). But, the ideal would be to have an inflatable cone that comes off the back so that the air doesn't "slam" back together but converges smoothly. DOT would love it wouldn't they??!!

I've actually seen a device called the "Tailcone" on the backs of some semi truck box trailers. It would be next to useless, aerodynamically. But it looks cool.

Remember, the F-4 Phantom is proof positive that with enough engine, even a brick can fly!
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Old 03-06-2006, 08:49 PM   #6
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Quote:
Originally Posted by JimGolden
Remember, the F-4 Phantom is proof positive that with enough engine, even a brick can fly!
I remember we used to call the Thunderchief "Thud" for the same reason, a 1:1 glide ratio. But I digress... Ideally, you chould remove the cap when you areen't planning to tow for a while. That brings up the question of where to store the thing when not in use. I suppose you could just leave it on the bed of the truck, if you don't have a place to store it, or slap together a pair of sawhorses in the back yard to hold it off the ground when you are not using it.
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Old 03-06-2006, 08:50 PM   #7
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Thanks, guys.

How about an air deflector on top of the cab, like the 18 wheeler's use?

Sam
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Old 03-06-2006, 08:51 PM   #8
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Old 03-06-2006, 08:55 PM   #9
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No, too much drag. \Maybe for a SOB.
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Old 03-06-2006, 08:55 PM   #10
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Another downside to the cap is the weight impact.
Mine weights around 160 lbs but there is no way I could do without it.
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Old 03-06-2006, 08:56 PM   #11
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Does a tonneau have a good, bad, or no effect when NOT towing? It would be nice to have lockable storage.

Sam
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Old 03-06-2006, 08:56 PM   #12
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The deflector will be too far away from the front of the trailer to help at all, and will only be a second source of drag.
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Old 03-06-2006, 08:59 PM   #13
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Quote:
Originally Posted by Sam Davis
Does a tonneau have a good, bad, or no effect when NOT towing? It would be nice to have lockable storage.

Sam
I had a tonneau cover on my Dakota, it seemed to make little difference in fuel consumption. I suppose if you wanted to have both, you could install a cover inside the bed rails of the truck, to use when not towing, and a cap on the top of the bed rails for towing, although it would be duplicative at best.
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Old 03-06-2006, 09:00 PM   #14
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A cap like Garry's would be the type I would recommend if you were to purchase one. Note the upturn at the rear, to help move air over the top of the trailer.
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