Wind deflectors

Does anyone use a wind deflector on the top of their truck to cut down on the amount of wind that hits the front of the fifth wheel?  My husband is thinking about buying one.

A wind deflector helps only if it is in the right position and at the right angle for your particular truck and trailer. Generally, this is impossible to determine without a wind tunnel test to see how the air is flowing over the entire rig at highway speeds.  It is unlikely you will get any useful results by trial and error set-up.

Thanks, Gary.  You probably saved me $400.

I should elaborate a bit. Studies have shown that tractor-trailers with "full aerodynamic treatment" can reduce power demand by around 20-22% at 65 mph. At intercity speeds, the effect if negligible and it doesn't even become noticeable until you exceed 50 mph. A 20% reduction in the horsepower demand doe snot translate to a 20% reduction in fuel consumption, but Cummins has found it could reduce fuel consumption by around 13%.

For truck+trailer, "full aero" means stopping the flow from the sides in between truck and trailer as well as smoothing the flow over the top and against the front of the truck. The roof top wings you buy for pick-up trucks for $400 attempt to address only one piece of this, the flow over the top of the truck and into the front of the trailer.  You have probably seen the huge hoods that some tractors have to smooth the flow up to the front of the trailer. You may also have seen the "wings" on the side of the  cab that stop the flow of air around the sides and onto the front of the trailer. This is actually the larger effect on a tractor towing a big box trailer.

I have not found any similar studies on pick-up trucks and recreational trailers, so there is no real evidence. However, RV trailers vary much more in shape than the box trailers that commercial truckers tow, and they have many more sources of drag, e.g. roof skylights, fridge vents, awnings, satellite domes, etc. These are not affected much by the addition of a generic wing on the top of the truck cab. And nothing reduces the inflow from the sides around the back of the truck.  Aerodynamic flow is a very complex thing and it is my opinion that the addition of some generic wing device to the top of the cab has only a slight chance of altering the wind flow in a positive way and in sufficient amount to effect fuel economy.  It is much easier and all but guaranteed that you can reduce fuel consumption by slowing down 3-5 mph. Cummins test show that every 1 mph above 55 mph costs 0.1 mpg. And it doesn't cost a dime... 

If your husband wants to study up on this, he can read this article by Cummins called "The Physic of MPG"

http://www.everytime.cummins.com/assets/pdf/MPG_Secrets_Whitepaper.pdf

I had to reply to the posts that disregard any benefits to using a wind deflector. I mounted the purple line wind deflector on our suburban that we pull a 6400 pound camper. I  can't say enough good things about it. When I was doing my research before purchasing one , the one comment that convinced me of buying one was it at least keeps the bugs off. If it keeps the bugs off I knew that it was directing the wind away from the camper and reducing drag. Before mounting it the best mpg I could get was maybe 8 mpg and that was running at 60mph. On our latest trip driving into crosswind I was getting 9.7 mpg and that was at a consistent 65 mph. And that was running at a higher gear . Before mounting the deflector I rarely was able to run in the higher gears and that's where you get the better gas mileage. I've noticed at the end of a 400 mile trip I am less fatigued . At times it feels like I'm not pulling a camper at all.

There is a big difference between a deflector on a Suburban 2 feet from the camper and a deflector on a pickup truck roof at an unknown distance from an already aerodynamic designed FW nose.  I am very happy it is working for you and that you posted your results, but it is a different question than a deflector on a truck for a FW, for the reasons Gary mentioned.

To Gary , you said in my situation my suburban and my trailer being only 2 feet away , that seemed way to close . So I measured it , from the wind deflector to where the air is directed over the camper is a good 8 feet plus. I would be curious to know the distance from a wind deflector mounted on a truck pulling a fifth wheel . For any one interested go to aeroplus website they have done a complete wind tunnel test on their deflector

I think one would also have to look at the number of miles you will be traveling.  If you get a one mile per gallon difference you will need to drive about 15,000 miles to pay for the defector.  That's a rough estimate and for me it would a few years to break even point.  Over the road truck drivers put that mileage on in a few months.

For any one interested go to aeroplus website they have done a complete wind tunnel test on their deflector

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There is no question that the deflector gadgets deflect wind. The issue is that each installation has to be individually tuned with wind tunnel testing or it's a crap shoot. Wind tunnel testing a device on one vehicle shows what can be achieved but proves nothing about how it will perform on another vehicle. Without another wind tunnel session to show where and how the air flow needs to be adjusted on that vehicle, installing the deflector is guesswork.

I did the math a 3000 mile trip without a wind deflector in my situation I average around 8 mpg running 60 mph . With current gas prices around 3 dollars per gallon it would cost me 1125 . With the deflector on running 65 mph this same trip will cost me 900 . If I drive at 60 mph the cost savings would be greater . These numbers are what  I have experienced before and after installing the wind deflector . Let's say gas prices go up to 5 dollars a gallon , the savings is even greater . I will have paid for my wind deflector in one year .

After looking at the posts on wind deflectors , my mistake it was grashley that said a trailer 2 feet away from my suburban , not Gary . To all those that have posted on wind deflectors I encourage you visit the aeroplus web site to see their results of their tests . There are also some good independent studies using the wind deflector

I'm a newbie in the RV world, but physics are physics.  Drag (essentially power required to move an object through the air) increases exponentially as velocity is increased.  Doubling your velocity increases drag by a factor of four.  Even at small speed increases from say 60 to 65, the amount of drag increases significantly, requiring more power and lowering your fuel economy. 

I'm looking forward to getting my rig on the highway and seeing what mileage I get and experimenting with finding the right cruise speed. 

I don't wear Gary gets a 20% number. It's more like 2% savings. Assume an 18  wheeler over the road. 60 hours a week at 60 mph . 4 miles per gal. equals 15 gal. / hour or about $52.50/ hour. Times 60 =$3150 /week fuel bill. A 2 % savings is about 63 dollars a week or a $3,150 savings per year. Not bad for an owner/operator.

johnandcarol said:

I don't wear Gary gets a 20% number. It's more like 2% savings. Assume an 18  wheeler over the road. 60 hours a week at 60 mph . 4 miles per gal. equals 15 gal. / hour or about $52.50/ hour. Times 60 =$3150 /week fuel bill. A 2 % savings is about 63 dollars a week or a $3,150 savings per year. Not bad for an owner/operator.

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OK, so  the math above made my head spin. Are you saying there is a ".4" miles per gallon savings? Have  there been testing and analysis on saving during cross winds? What if mileage goes down instead of up?
This reminds me on the scene where Homer Simpson had figured out that instead of typing "yes" at the Nuclear power plant he could type "y" as a response to the prompt. He informs Marge that he has tripled his productivity

To ip076 read my first post I said less drag . Enough less drag that I can run at 65 mph and run in higher gears and get better mileage with the deflector mounted than without it.