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Old 02-25-2011, 04:52 PM   #1
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Air Dam on tow vehicle for aerodynamics

I was shopping today for a new SUV and drove a Nissan Xterra with a factory tubular roof rack. The salesman then showed me on the "S" model an additional gear compartment built in the front of the roof rack. To open there is a catch and a compartment door flips up. There are 2 pistons on each side like on your hood keeping it open. It immediately reminded me of those air dams on tractor trailers. I know the subject has been discussed there are manufacturers of bolt ons. I asked the salesman if it could be left open, he asked me why and then the light went off, "Oh, yes, you want to tow and this would deflect the air" and we both smiled.
Anyone else seen this option in action?
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Old 02-26-2011, 08:42 PM   #2
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I talked to a guy who had one on a big rig. He claimed 2-3mpg improvement.
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Old 02-26-2011, 10:37 PM   #3
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Doubtful. The air dam would need to be located at the extreme rear of the tow vehicle and be as wide as the trailer to have any affect. You will also notice that the big rigs use side spoilers to deflect air out from the front of the trailer.
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Old 02-27-2011, 05:29 AM   #4
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This is from Volkswagen full-scale wind tunnel tests - you can see from the shape of the VW Passat towcar that this data is getting on for 30 years old, but then the laws of physics haven't been changed since then, as far as I know:



It would be reasonable to say that the changes in Cd would be roughly the same as changes in fuel consumption, so that's 15% reduction with a roof spoiler, providing your ratio of towcar to trailer is similar.

And the last column shows why European trailers all have sloping fronts - so that the low hitch weight percentages commonly used in Europe don't become negative at speed as a result of aerodynamic lift.
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Old 02-27-2011, 07:34 AM   #5
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Shape counts. We towed a 15.5 foot Box Trailer that weighed 500 lbs less than our 16 foot Round Casita. With the Box Trailer we got 20 mpg and with the Round Trailer we got 23 mpg, a 15% improvement with a heavier trailer.

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Old 02-27-2011, 10:32 AM   #6
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I talked to a guy who had one on a big rig. He claimed 2-3mpg improvement.
ROGER, i spent 23 years driving the big rigs aerodynamics is a huge concern there and it helps a lot, but 2-3 mpg no way not on a big truck, I know somebody else told you that but we're talking about vehicles that on a good day don't get much more than 7 mpg. Greg
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Old 02-27-2011, 10:55 AM   #7
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Rounded shape or sloped front lower drag that is the one thing all the experts talked too had agreed on.
Ultra light fiberglass trailers like Casita, Boler and Egg and others win with the bottom to top arc side to side arc and rounded top and side corners.

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Shape counts. We towed a 15.5 foot Box Trailer that weighed 500 lbs less than our 16 foot Round Casita. With the Box Trailer we got 20 mpg and with the Round Trailer we got 23 mpg, a 15% improvement with a heavier trailer.

Norm
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Old 02-27-2011, 12:43 PM   #8
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I did some research (a quick google search) and only found one picture of one of these boxes open.

2005 Nissan Xterra Roof Cargo Box View Photo 5

Now this one seems different from the one you described, as it does not have the two prop-rods. I would be skeptical, due to its size and location that it would help with aerodynamics. I am also skeptical it would stay open, with out using some sort of fixed prop rod. At even a slow 55 mph, there would be some considerable wind forces, let alone the wind generated from an on coming big rig. I would be afraid it would break.

That being said, it is an interesting idea. I have toyed with the idea of putting a spoiler on my TV to help with some aerodynamics. I guess the proof is in the pudding, take a little 10 mile jaunt with it up and then with it down (if you get the Xterra) and see. Let us know the results.

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Old 02-27-2011, 10:35 PM   #9
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Volvo used to make a spoiler to deflect air up over the caravan.

It may make a difference for my Boler, if I could even find such a thing nowadays.

Then again, it looks a bit too "ricer"
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Old 02-28-2011, 08:46 AM   #10
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That is the one Sam, but they have made some changes. It now has 2 gas prop supports and is covered under the 3 year warranty. The salesman said "Why not try it and see if it breaks, you will be covered"
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Old 03-01-2011, 07:13 AM   #11
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2 years when we were near JC Whitney Co I purchased the wind deflector and installed it on my 2000 Dakota. My findings Are absolutely no change in gas consumption I get a pretty constant 14 , pulling the Scamp 5er. It does keep the bugs off the front of the rig And there is less turbalance in the box of the truck.Stuff stays put in the box. The straps that came with it had to go as they were very noisy, It was not one of my better investments
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Old 03-01-2011, 08:35 AM   #12
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What did you do to rejuvenate your stove's finish?

Thank you

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Quote:
Originally Posted by Sam and Jess View Post
I did some research (a quick google search) and only found one picture of one of these boxes open.

2005 Nissan Xterra Roof Cargo Box View Photo 5

Now this one seems different from the one you described, as it does not have the two prop-rods. I would be skeptical, due to its size and location that it would help with aerodynamics. I am also skeptical it would stay open, with out using some sort of fixed prop rod. At even a slow 55 mph, there would be some considerable wind forces, let alone the wind generated from an on coming big rig. I would be afraid it would break.

That being said, it is an interesting idea. I have toyed with the idea of putting a spoiler on my TV to help with some aerodynamics. I guess the proof is in the pudding, take a little 10 mile jaunt with it up and then with it down (if you get the Xterra) and see. Let us know the results.

--Sam
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Old 03-01-2011, 10:25 AM   #13
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Lyndon's note regarding things getting noisy is worth consideration. :-) I purchased a car top bike rack last year that came with a spoiler on the front bar - no matter how many different positions I tried moving it to in the end I just took it off as it made things very noisy in the car even though it was made by the car manufacture so it was designed to use on my car.
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Old 03-01-2011, 10:46 AM   #14
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If the noise comes from two straps together, I found a paperclip shut them up.

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Lyndon's note regarding things getting noisy is worth consideration. :-) I purchased a car top bike rack last year that came with a spoiler on the front bar - no matter how many different positions I tried moving it to in the end I just took it off as it made things very noisy in the car even though it was made by the car manufacture so it was designed to use on my car.
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