Aerodynamics help

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Aerodynamics help

Postby SkyNerd » Tue Sep 10, 2019 7:06 pm

I have a 12’x 6.5’ trailer. I’m gonna build a long teardrop with forward bunk beds and extra storage in the galley and foot of the bed.

My tow vehicle is a 2015 F-150. My trailer is already heavy and I plan to drive cross country - we drove with one of those hitch storage boxes and the bed loaded to the tanau cover from Texas to Michigan to Niagara falls and back. We averaged 21-22 mpg with one leg of the trip as high as 23.5mpg. Oddly with the truck empty and no storage box 19-20 is about average. I assume the storage box reduced the drag window and improved my mpg.

I’m looking to expect the same from the teardrop even though there is more weight I don’t want to do anything to compromise the aerodynamics to preserve as much of those MPG’s as possible

So here’s the hard part....

The truck bed is 6’ wide from the top of the bed to the top of the cab the truck width reduces to about 5’ wide.

If I max my build to 6’ wide it puts about 1/2 the trailer height in the profile of the truck while the top part will have some area exposed (approx 6”x18”x 2) about 1.5 square feet. That doesn’t sound like a lot but I’ve felt the force of the wind on my hand out the window at 70 mph.

I have a few options.
1.) Build to 5’ wide and keep the trailer profile within the profile of the truck. = smaller trailer, kids may out grow bunks faster.

2.) Build some where between the 6’ and 5’ marks and reduce the drag window.

3.) suck it up and build it to 6’ because you are an over analyzing fool and you’ll never get started on this thing if you don’t just take the plunge.

4.) The trailer is actually 6.5 feet wide. I’ve considered building all the way to the edge. But again this increases my area for drag. We borrowed a 14’ pop out once and it took my truck down to 9 mph. I can’t drive cross country at that rate, I don’t know how people do it.


I could build rounded or angled corners that come in to the same profile as the truck thus causing less of a head on drag and allowing the air to “spill” down the sides, while this wouldn’t eliminate drag it could certainly decrease it.

FYI my trailer is over kill. The price was more than right for a custom built trailer with electric brakes, jacks, #3500 axle, bull dog hitch, all out of 3” channel, but it probably already weighs 1000lbs .

Image

- Jes



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Re: Aerodynamics help

Postby GTS225 » Tue Sep 10, 2019 7:49 pm

You're right......the trailer is overkill for a teardrop. We have some folks coming in considerably under that 1000 lbs. with the teardrop fully built. (Admittedly, they are building very weight conscious, and out of foam.)
I think one of your ideas has merit. Rounded corners will help the aero, and if you build to the upper cab profile, that will also help. (Meaning 6' at mid-height, down to the 5' at the top.) The narrow upper profile will not be missed very much on the inside of the camper, but it does make it more difficult to build.
Some have experimented with a flexible "tunnel" fastened between the rear of the tow vehicle, and the front of the trailer. This would surely help smooth out the turbulence that will occur there, but it may be diminishing returns for the effort.

Something you may wish to look at and think about is the recent use on OTR trucks, of the angled "baffles" mounted to the back of trailers, and under the sides of the trailers. Those are there for a reason, and they wouldn't be there if they didn't work. They effectively break up the low pressure turbulence, thus increasing mileage, and in commercial freight transportation, that means money and a higher profit margin.

Your initial increase in economy with the rear cargo carrier shows that your thought processes are partially correct, but before I conclude anything, I should ask if you typically drive with the tonneau cover in place. "Killing" the turbulence is key here, as any vertical surfaces on the trailing side of an object will build a low pressure area, thus attempting to hold back the forward momentum of a body in an airstream. Your example of the hand held out the window will show you quite a bit, if you experiment with different angles and shapes.

Just a bit of food for thought.....Roger.
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Re: Aerodynamics help

Postby troubleScottie » Tue Sep 10, 2019 8:27 pm

One fairly lengthy discussion.... http://tnttt.com/viewtopic.php?f=42&t=70179

Just to point out, the classic teardrop is one of the better designs in spite of the 90 degree edges where the sides meet the roof. One could round the edges if one was so inclined.
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Re: Aerodynamics help

Postby SkyNerd » Tue Sep 10, 2019 9:01 pm

GTS225 wrote:You're right......the trailer is overkill for a teardrop. We have some folks coming in considerably under that 1000 lbs. with the teardrop fully built. (Admittedly, they are building very weight conscious, and out of foam.)
I think one of your ideas has merit. Rounded corners will help the aero, and if you build to the upper cab profile, that will also help. (Meaning 6' at mid-height, down to the 5' at the top.) The narrow upper profile will not be missed very much on the inside of the camper, but it does make it more difficult to build.
Some have experimented with a flexible "tunnel" fastened between the rear of the tow vehicle, and the front of the trailer. This would surely help smooth out the turbulence that will occur there, but it may be diminishing returns for the effort.

Something you may wish to look at and think about is the recent use on OTR trucks, of the angled "baffles" mounted to the back of trailers, and under the sides of the trailers. Those are there for a reason, and they wouldn't be there if they didn't work. They effectively break up the low pressure turbulence, thus increasing mileage, and in commercial freight transportation, that means money and a higher profit margin.

Your initial increase in economy with the rear cargo carrier shows that your thought processes are partially correct, but before I conclude anything, I should ask if you typically drive with the tonneau cover in place. "Killing" the turbulence is key here, as any vertical surfaces on the trailing side of an object will build a low pressure area, thus attempting to hold back the forward momentum of a body in an airstream. Your example of the hand held out the window will show you quite a bit, if you experiment with different angles and shapes.

Just a bit of food for thought.....Roger.


The builder insisted on 3” channel, I asked them to build out to the wheel wells I assumed thy would use 2” angle. I also over looked the over all width. In hind site I probably would have limited it to the 6’ and insisted on the 2” angle iron for the widened portion. Lesson learned at least I know it will support what I put on it.

I like the idea of going to 6’ because it will be much longer till my 8 year old grows out it. But if it kills my mpg I’d hate to think all my blood sweat and tears were for nothing.

The sole purpose was to eliminate losing drive time or play time setting up and taking down camp on a long road trip. After many camping trips since kids, I’ve learned the value of an efficient camp site.. unfortunately my wife knows the value of comfort. The tear drop has all the comforts for sleep while always ready to roll away. I wasted +/- 2 hours setting up a tent, air mattress, cots, a table, stove canopy cooking supplies.. etc etc...

BUT - I’m so worried that I’m going to go from 22mpg down to 13 or worse back to the 9. honestly if that’s the case I’ll bail out now and start working on plan B. I know there’s no equation to determine MPG’s but attempting to travel cross country on anything less that 18-20 mpg and I can start hearing my pocket book screaming. .

- Jes


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Re: Aerodynamics help

Postby tony.latham » Tue Sep 10, 2019 9:21 pm

There was a big difference when I bumped up from a four-wide to a five with my Tacoma.

Image

I have also heard that Camp-Inn's Raindrop gets better mileage than their normal teardrop.

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Re: Aerodynamics help

Postby SkyNerd » Tue Sep 10, 2019 9:28 pm

tony.latham wrote:There was a big difference when I bumped up from a four-wide to a five with my Tacoma.

Image

I have also heard that Camp-Inn's Raindrop gets better mileage than their normal teardrop.

Tony


When you say big difference what was your MPG with out the trailer, with the 4 wide and then with the 5 wide.

Also my F-150 is probably a bit wider do you think I’d suffer the same difference?

- Jes


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Re: Aerodynamics help

Postby tony.latham » Tue Sep 10, 2019 9:55 pm

When you say big difference what was your MPG with out the trailer, with the 4 wide and then with the 5 wide.


I built the five-wide in 2013 so I can't recall for sure but it was maybe 2-3 MPG.

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Re: Aerodynamics help

Postby Aguyfromohio » Wed Sep 11, 2019 8:17 am

Here's another good thread about aerodynamics.

http://tnttt.com/viewtopic.php?f=67&t=70301

Our trailers have pretty poor aerodynamics that create lots of drag for exactly the reason Tony points out - ours are substantially taller than the tow vehicle and substantially wider than the tow vehicle.
A bigger trailer has bigger drag.
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Re: Aerodynamics help

Postby aggie79 » Wed Sep 11, 2019 8:19 am

We have a 2011 F-150 with 5.0 liter V-8 and 3.55 rear end gear ratio. Without towing, we get 16 to 16-1/2 mpg at 65-70 mph. With towing our 5' wide teardrop (fenders extend another 12" on each side for a total width of 84"), we get 15 to 15-1/2 mpg at 65-70.

In the end, frontal area of the trailer is going to affect mileage mileage more than coefficient of drag. Think about height and width, and don't worry so much about shape.
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Re: Aerodynamics help

Postby Aguyfromohio » Wed Sep 11, 2019 11:07 am

aggie79 wrote:
...In the end, frontal area of the trailer is going to affect mileage mileage more than coefficient of drag. Think about height and width, and don't worry so much about shape.


Bingo.
We set the large size of our trailers early in our build and accepted the trade off as worth the benefits, for us.
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Re: Aerodynamics help

Postby SkyNerd » Wed Sep 11, 2019 11:27 am

aggie79 wrote:We have a 2011 F-150 with 5.0 liter V-8 and 3.55 rear end gear ratio. Without towing, we get 16 to 16-1/2 mpg at 65-70 mph. With towing our 5' wide teardrop (fenders extend another 12" on each side for a total width of 84"), we get 15 to 15-1/2 mpg at 65-70.

In the end, frontal area of the trailer is going to affect mileage mileage more than coefficient of drag. Think about height and width, and don't worry so much about shape.


This is good news. My truck hardly clears the same garage I’m storing my camper in so height won’t exceed the height of the truck. My over all trailer width is 78-80” and the truck width is 72”

My instincts tell me that I’ll wear it out or want to build a second one by the time my 8 year old out grows the bunks. I’m thinking just building to 5’ wide and using the remaining trailer surface as a step side and maybe diamond plate it for looks. This keeps the profile of the trailer 100% within the profile of the truck with the exception of a few inches of trailer on either side.

If I’m gonna keep camping in a teardrop with the kiddos I’ll need to look in to a roof tent or perhaps building a pop up once they out grow the bunks.

Thanks everybody for the info. FYI only losing 1 to 1-1/2 mpgs would be awesome. Here’s to hoping i can keep it light despite my monster sized trailer!!

- Jes


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Re: Aerodynamics help

Postby Squigie » Wed Sep 11, 2019 2:16 pm

This is pretty much all it boils down to:
aggie79 wrote:(...)
In the end, frontal area of the trailer is going to affect mileage mileage more than coefficient of drag. Think about height and width, and don't worry so much about shape.

The best way to improve the trailer's impact on mileage is to put a cap/shell on the bed of the truck. Streamlining the truck often provides a better improvement for fuel mileage than going to a trailer with a smaller frontal area.

Every vehicle-trailer combination is a bit different; but I've always gotten better mileage by streamlining truck beds, even if only a little bit - like piling gear and coolers so they're above the bed sides and sloping from front to back.

For several years, one of my brothers hung on to some old, ratty bunk bed mattresses, just for the fuel mileage gain. We'd pack his truck and mine for hunting/camping trips with the 'streamlining' method, then strap the mattress down over the top. My Ranger gained 3-4 mpg (16 mpg to 19-20). My brother's '86 F-250 (460 / 4x4 / Ext. cab / Long bed / 5-speed) would go from 11-13 mpg to 14-16 mpg. We got better mileage while hauling a 'streamlined' load than when running empty.
With my truck, there's almost no change from those numbers when pulling a sub-3,500-pound trailer with a frontal area (or rear ramp) that isn't considerably bigger than the truck.
My brother's F-250 does take a mileage hit when pulling a trailer of any kind, but he doesn't really have the right gears for towing. (I've got 4.10s, he's running 3.55s and the swapped-in 5-speed that, itself, has tall gears.)
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Re: Aerodynamics help

Postby SkyNerd » Thu Sep 12, 2019 1:18 pm

Squigie wrote:The best way to improve the trailer's impact on mileage is to put a cap/shell on the bed of the truck. Streamlining the truck often provides a better improvement for fuel mileage than going to a trailer with a smaller frontal area.

Every vehicle-trailer combination is a bit different; but I've always gotten better mileage by streamlining truck beds, even if only a little bit - like piling gear and coolers so they're above the bed sides and sloping from front to back.

For several years, one of my brothers hung on to some old, ratty bunk bed mattresses, just for the fuel mileage gain. We'd pack his truck and mine for hunting/camping trips with the 'streamlining' method, then strap the mattress down over the top. My Ranger gained 3-4 mpg (16 mpg to 19-20). My brother's '86 F-250 (460 / 4x4 / Ext. cab / Long bed / 5-speed) would go from 11-13 mpg to 14-16 mpg. We got better mileage while hauling a 'streamlined' load than when running empty.
With my truck, there's almost no change from those numbers when pulling a sub-3,500-pound trailer with a frontal area (or rear ramp) that isn't considerably bigger than the truck.
My brother's F-250 does take a mileage hit when pulling a trailer of any kind, but he doesn't really have the right gears for towing. (I've got 4.10s, he's running 3.55s and the swapped-in 5-speed that, itself, has tall gears.)


I’ve considered the camper shell as well. we usually only pack to the tanau cover to keep our stuff secure, it gets like Tetris most trips. Having the extra cargo room would be a plus for sure.

I also looked up my axle ratios, that was a bit of searching looks like my truck has 3.31 gears, that explains the mileage, looks like I’ll expect to take a bit of a hit when I had some trailer weight.

One last consideration, if I added the camper shell, do you think I’d see much difference between towing a 5’ wide vs 6’ wide teardrop?

- Jes



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Re: Aerodynamics help

Postby Squigie » Thu Sep 12, 2019 6:34 pm

The truck is likely near 80" wide*. I don't think it would make enough of a difference to worry about a 60" vs 72" wide trailer. My experience says you wouldn't notice a difference unless one was notably taller than the other, too.
I think you'd be impacted much more by a foot of height added to a 5-foot-wide trailer, than by going from 5 feet to 6 feet wide at the same height.


*(Semi-educated guess, based on the average size of current trucks - but I didn't look it up)
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