Building a wind deflector

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MoInEd

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May 27, 2020
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Edmonton, AB
We have a 21' travel trailer (single axle) that we tow with our Audi Q7 (two adults and two kids). The Audi has excellent gas milage (on flat highway, doing 100km/hr consumption is under 7L) but when pulling the trailer the consumption is around 20L/100km. The main issue I think is the very bad shape of trailer and the drag (air). According to my friend (Ph.D. in Mechanical engineering) it's like pulling a wall behind you. I have engineering background myself and have experience in design and building things from scratch.

I have searched for commercial wind deflectors and can only find one such product but doesn't seem very good:


I have decided to build one myself. My friend is doing some simulation/analysis to come up with the best angle/sizes but I have some good ideas already and will attempt building it based on my own design soon (will tweak based on his calculations later).
My (very rough) estimation is it should reduce my consumption by 10-20% but can't be sure until I build and test it! I have already bought the parts, have my design and ready to go. It will be easy to install/remove on my roof cross bars.

Anybody has every done one like this?
 

donn

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Sounds about right. The biggest issue is likely not wind resistance although that does play a part. Sadly your biggest issue is,load vs small motor. And that you cannot do anything about with your current setup.
Wind deflectors used to be a popular option years ago, but most folks figured out it simply was not worth the effort.
 

Gary RV_Wizard

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The problem with wind deflectors is tuning them to the rig so there is measurable benefit. Simply mounting an angled board (wing) up front somewhere is rarely effective and once in awhile may even be a negative. The devices marketed as generic solutions, e.g. wings & anti-turbulence tabs, have never proved effective at all. They get "proven" on a couple of test bed rigs, but seldom translate well to other vehicles.

Engineering analysis via modeling might be enough to tune the size, shape & angle of a deflector for a specific vehicle, but validation in a wind tunnel is the only reliable method. If you have a fleet of more-or-less identical trucks & trailers, the analysis and testing may be financially worthwhile. For the rest of us, it's an interesting hobby project.
 

steveblonde

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calgary alberta
They used to make them and people with 5th wheels would mount the things on the roofs of their trucks. As a kid i always thought they looked cool lol. But fact of the matter is after studying them they found them to be of zero benefit. And the reality of the situation is gas mileage and rvs dont belong in the same sentence its an oxymoron. And if you can afford a Q7 you probably can afford the gas for it. That Q7 is a nice suv
 
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MoInEd

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Edmonton, AB
I have about $180 in parts for this and am going to test a few cases. If I can bring my consumption down 15% or more that's paid off in one summer of camping.
This idea is definitely saving on gas millage on big semi-trucks as most of them have the wind deflector on the top of the cabin and it does help effectively with gas millage. I don't need wind tunnel tests for my own setup. I make some trips regularly and if the millage on those same trips are dropped considerably on average that's a reliable test.

Will report back.
 

Matt_C

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SE - Mich
MolnEd,
You have three problems and only one of them is the frontal area of the TT.
First, your Q7 is kind of a slippery shape. You could kill the 7l just by adding a roof rack. And, I bet you are adding extensions to the mirrors to tow with it.....
Second (in order of appearance to the air, is the frontal area and block coefficient of the coach body. Just rounded corners would help, but it is there and you air deflector may help some, but it will have to be larger than the one in the picture you linked. It will have to almost contact the front of the coach.
Third issue that most people neglect is the hole you make with the front has to get closed up. Have you seen the trucks with extra doors at the rear? Those are not a joke.
I think you can play around with this a lot, but you chance of getting to better than 15l is not real good and I bet you can get close to that just with careful driving.
I would offer to run some simulations for you, but all of the code I have is leftover from my naval architecture days and it is for water. There is a difference.
Matt
 

SpencerPJ

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I have about $180 in parts for this and am going to test a few cases.
Sounds like an interesting project. Certainly will take a lot of towing to recover that $180 in gas, albeit it sounds like it's just the engineering side of you, not the value side. Certainly will be a conversation piece at the campground (y)

Do keep us posted, pics would be a plus. Good luck
 

MoInEd

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If I can get 15% reduction in consumption (which I am optimistic I can) that would be about 3.5L/100km. Given that I easily do 4-5000km each camping season that would be about 140L which (given the price of premium gas) easily pays for the parts I have purchased.
Yes, I am aware of the big gap behind the vehicle and trailer and I don't have a good solution for that.
Will post pic's when done and do some tests....
 

scottydl

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This idea is definitely saving on gas millage on big semi-trucks as most of them have the wind deflector on the top of the cabin and it does help effectively with gas millage.

Those do help, but keep in mind semi cab deflectors are curved for max effect and are essentially a part of the body lines of the truck. No significant gaps, flat spots, or airflow interrupters. I also wonder how much difference a deflector would make for shorter stints, like most of us do RV towing (occasionally) compared to daily driving (most of the time). Which is different than semis that spend near 100% of driving time on long-haul highway towing, for hundreds of thousands of miles each year. Even a 5% improvement would add up, with that scaled usage of a wind deflector on commercial trucks.

Either way, you've got the will and the way, so I'm very curious to see what you learn!
 

TheBar

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MS
I had a 3" bug deflector on my last new truck. It was almost vertical maybe 80 degrees is why I think it worked. At 40 mph it didn't do much but at 70 June bugs went up over the roof instead of splattering on the windshield. But the biggest shocker was behind a truckload of bricks. We saw a brick come rolling down the load then bounced and came directly towards the windshield at my passenger. He dove for cover but when the brick passed over the bug deflector it shot up over the roof. Luckily nobody was behind me. So air deflectors do re-direct air but they must increase drag to some degree. I would think the size and slope is critical to that balance.

Where a Class C cab roof meets the curved underside of the cabover bunk has to be a wind trap. This conversation has me thinking a bug deflector at the front of the hood might direct the airflow higher to the sloped front of the bunk and eliminate some drag. A 3" deflector has to cause less drag than that 12" air trap.
 

KeinNWO

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Apr 9, 2021
Posts
3
Location
Kenora, ON
We have a 21' travel trailer (single axle) that we tow with our Audi Q7 (two adults and two kids). The Audi has excellent gas milage (on flat highway, doing 100km/hr consumption is under 7L) but when pulling the trailer the consumption is around 20L/100km. The main issue I think is the very bad shape of trailer and the drag (air). According to my friend (Ph.D. in Mechanical engineering) it's like pulling a wall behind you. I have engineering background myself and have experience in design and building things from scratch.

I have searched for commercial wind deflectors and can only find one such product but doesn't seem very good:


I have decided to build one myself. My friend is doing some simulation/analysis to come up with the best angle/sizes but I have some good ideas already and will attempt building it based on my own design soon (will tweak based on his calculations later).
My (very rough) estimation is it should reduce my consumption by 10-20% but can't be sure until I build and test it! I have already bought the parts, have my design and ready to go. It will be easy to install/remove on my roof cross bars.

Anybody has every done one like this?
I have looked at commercially available ones aeroplus and icons. Based on information available on net, there is probably a 1mpg savings if optimally designed. An aerodynamic shaped vehicle will dump the air in front of the trailer creating turbulence.

The back of the trailer creates a vacuum causing gas consumption. Aeortabs are vortice breaking devices that break the vacuum behind the trailer. I am building a wind deflector for my tundra, not liking what is out there or available. The aeortabs may be another 1 mpg. Check these out. I want to extend my range to 4 hrs driving time between fills.
 

SmokerBill

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Sep 25, 2010
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213
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Washington State
I think that using a couple of hypermiling techniques would give better MPG savings results (for no cost, except time), especially if driving hilly terrain. Gradually accelerate on the downhills and feather the gas pedal to gradually slow down on the uphills.

Pulling a trailer, you'll still be competing against the wind, or using it to your advantage. I've seen incredibly high MPGs when pulling my TT with the help of a strong tailwind, and very low MPGs when driving into a headwind.
 

MoInEd

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May 27, 2020
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Edmonton, AB
Still in progress, here are some pics. Should be done in a week or so. Will take it out to the highway with the trailer sometime in May to do two test drives: 50km at 100km/hr once with the deflector and once without. My friend (Ph.D. in Mechanical Eng) helped me to do some calculations. He has a device that can be installed between the fuel line to measure (very precisely) the consumption of the engine and store the data for calculations. I wanted to calculate the downward pressure this will create at the back of the SUV (and on the roof carrier). Looks like (even with overestimating things) that downward pressure is very little (like 50lb or less).
 

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MoInEd

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Edmonton, AB
Ok, today I did a test run, all on highway doing 100km/hr for about 40km (20km each way round trip to my house) twice, once with the wind deflector and once without. The savings in fuel consumption wasn't as much as I had hoped for, about 10%: with deflector consumption was 15L/100km and without it was 16.6L.
I think the size of the deflector is small compared to the trailer and the gap between the two is large. I don't even notice anything is up there. My mechanical engineer friend says tweaking the design will most likely improve the savings.

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steveblonde

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When you factor in cost to manufacture time spent designing etc etc your probably at $1000 devide that by the cost of fuel $1.45 liter for premium and the fuel saving 1.6 liters per 100 kms the number of miles you actually pull the trailer plus the extra money spent at the car wash cleaning the bugs off the math is very upside down on savings. Its not just material cost which im sure by now has exceeded that initial $180 now you need to build a deflector for the cross wind lol
 
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MoInEd

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Edmonton, AB
Although I had hoped for 15%, a 10% reduction is still non-negligible. The cost of materials to me was around $200 (not including my time which if I include would be several folds more). I typically pull the trailer around $4-5k year and when loaded (trailer and car) I average around 21L/100km, so if I get a 10% saving in gas that is about $140 per summer. Not a lot really.
This was simply a prototype to test an idea. Generally, most trailer are very crude in aerodynamic design and except some that have more round nose very little thought is put into these, mostly because of cost savings.

I wonder how much an average person would pay (in $$) extra to have a better design of the trailer that would cut gas consumption, say by 20% or more....

Big semi-trailers all have wind deflectors to the top and side for a reason.
 

steveblonde

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" Big semis" are 14 ft tall and 53 ft long they also are a lot closer to the cab of a truck than a bumper pull. And they weight considerably more than a bumper pull travel trailer with multiple axles. Its pretty hard to compare the two.
Aero dynamics play a big part in FAST moving objects like bullet trains, sports cars etc if you look a most semis they are hardly what you would call dynamic the same with most motorhomes that are built on bus chassis think class A buses.
I pull a 5er that weights 17000+ lbs is 13'6" tall is 44'5" from king pin to rear bumper my mileage is 9 mpg 26l per 100 towing from Airdrie to Lethbridge and back when not towing i get 14.3 l per hundred 16mpg that the price you pay if you want to play, if your time is $100 hr as an engineer and you have 15hrs in plus materials that buys a lot of gas lol
 

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Frank B

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Just 'draft' a big, square semi when traveling. You don't actually have to follow that close to get the benefit. With our 1-ton diesel and 30 ft travel trailer, I can feel the difference in the gas pedal and still not be close enough to be dangerous.

I don't do that often as some semi drivers find it annoying, but when traveling across Montana into a headwind, it makes a BIG difference. Zero cost. 😊
 
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