Trailer Frontal Area - Fiberglass RV


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Old 11-01-2016, 10:26 AM   #1
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Trailer Frontal Area

a recent post brought up the subject of trailer frontal area and its effect on rated towing capacity.
The example was a 3,500 pound rated tow capacity towing a 2,000 pound trailer that had a frontal area that exceeded the maximum. In this post no actual square footage was supplied just the fact that it exceeded Ford's maximum for the vehicle in question.

Now my question....Is there a chart or formula that translates extra frontal area into its effect on towing capacity ???
Does extra frontal area reduce towing capacity or just decrease MPGs ?
Does extra frontal area cause engine overheating because of additional
"drag" caused by wind resistance ?
Do all McDonald Happy Meals include French fries ?
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Old 11-01-2016, 12:18 PM   #2
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Quote:
Originally Posted by Uplander View Post
a recent post brought up the subject of trailer frontal area and its effect on rated towing capacity.
The example was a 3,500 pound rated tow capacity towing a 2,000 pound trailer that had a frontal area that exceeded the maximum. In this post no actual square footage was supplied just the fact that it exceeded Ford's maximum for the vehicle in question.

Now my question....Is there a chart or formula that translates extra frontal area into its effect on towing capacity ???
Does extra frontal area reduce towing capacity or just decrease MPGs ?
Does extra frontal area cause engine overheating because of additional
"drag" caused by wind resistance ?
Do all McDonald Happy Meals include French fries ?
Fries are a choice, you can substitute apple slices, but that begs the question as to whether the meal would still be "happy".
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Old 11-01-2016, 01:50 PM   #3
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I believe frontal areas that exceed recommeded maximums reduce towing capacity, reduce mileage, and contribute to engine overheating and AC problems in the heat of summer.

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Old 11-01-2016, 05:40 PM   #4
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Drive slowly and ignore "frontal area" BS. Do not exceed trailer's tires speed rating and you will be OK.
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Old 11-01-2016, 05:55 PM   #5
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I 2nd this. If you go fast and the TV automatic is downshifting, you have too much headwind, or you are climbing a hill, take note and slow down a bit. That's all.

If Ford wants to be believable about frontal area specification, they have to provide a lot of ancillary information. They are opening a Pandora's box.
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Old 11-01-2016, 08:49 PM   #6
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The frontal area on most fg trailers due to there rounded and streamlined shape would be hard to measure and compare to most flat,blunt stick built trailers
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Old 11-01-2016, 09:32 PM   #7
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Warning: What follows is provided for those interested. It does not answer the question in the OP:
1) Computing the pounds of drag for a flat plate is easy. If we are going 60 MPH, the drag is 9.2 lbs per foot for each square foot of the plate's area.*
2) If the plate is >not< flat, then we need to adjust the above number by using the "Coefficient of drag" (Cd). For a really "slick" car (e.g a Prius), the Cd would be about .25, so the drag for each square foot of frontal area at 60 MPH is about 9.2 x .25 = 2.3 lbs. A Jeep Wrangler has a Cd of about .6, so the drag for each square foot of frontal area would be 9.2 x .6 = 5.5 lbs.
3) If a trailer is 6 feet wide by 7 feet tall, it has a frontal area of 42 sq ft, so using the Jeep and Prius as "worst" and "best", the trailer could be expected to have drag between 97 lbs and 232 lbs. Now, there's a lot more to the story--the TV has a frontal area of its own, and has already "broken the wind" (tee hee!) for the trailer, which would decrease the drag load on the front of the trailer. On the other hand, the trailer has a >lot< of drag due to turbulent air swirling underneath it, theres skin friction from all the vents, hinges, windows, rivets, antennas, etc that truly does add up to a lot, and (most of all) there's "suction" at the back of the trailer due to flow separation that typically produces more "drag" than the front end of the trailer. So, as a very rough WAG, I'd >guess< that somewhere between 100 to 250 lbs of aerodynamic drag is not a bad estimate. I'm quite sure that a square-cornered "stick-built" trailer would typically have a higher Cd than the typical FG trailer.
4) Now, 100 lbs of aerodynamic drag puts a strain on the engine and drivetrain much higher than 100 lbs of extra weight would, because each pound of drag is pulling, pound for pound, against the engine.
An example: If we are pulling our trailer up a "10% grade" (quite a steep hill), that angle is 6 degrees. The "strain" caused by 100 lbs of trailer being pulled up a hill like that is just 10 lbs. If our aerodynamic drag at 60 MPH is 200 lbs (see above) then it would be the same strain on the engine (and gas mileage, etc) as pulling a 2000 lb trailer up a 10% grade. All the time. Obviously, in real life we'll be paying >both< the aerodynamic drag penalty and the "weight" penalty.

As I warned--none of that does us much practical good unless we have real-world Cd information for our trailer. But, at least at the back-of-the-envelope level, we can see that drag can easily be more important than weight (and Ford is probably right to at least try to include that in the "mix" as they offer recommendations).

* at standard conditions.
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Old 11-02-2016, 10:50 AM   #8
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[QUOTE=samclem;615702]Warning: What follows is provided for those interested. It does not answer the question in the OP:

On the other hand, the trailer has a >lot< of drag due to turbulent air swirling underneath it, theres skin friction from all the vents, hinges, windows, rivets, antennas, etc that truly does add up to a lot, and (most of all) there's "suction" at the back of the trailer due to flow separation that typically produces more "drag" than the front end of the trailer.
--------------------------------------------------------------

I for one am very interested in streamlining and started ( joined? ) a thread on her long ago but it went nowhere, mostly because fb owners already have fairly effective streamlining. But I would like to see some (like the above) who have some science or math knowledge join in.
As a truck driver long ago I have seen truck streamlining over the years progress and remember an article in Mechanix Illustrated 40 - 50 years about how streamlining would affect gas mileage as well as handling. ( With drawings and diagrams ) I drove a White Freightliner cab-over-engine that had a front like a 6x8 plywood slab ; now trucks are all long nosed and slippery. And I believe we can make subtle changes that will affect gas mileage and ease strain on the engine and transmission as well as minimize crosswind and headwind difficulties.
At the Bonneville Salt Flats I've seen trucks go 200+ mph and the record is closer to 250 mph. Cars with small engines approach 300 mph. Think of it as efficiency rather than speed. And efficiency applies directly to fiberglas trailers and their tow vehicle. I hope a few more join this discussion : for now, an air dam on the TV is fairly simple, as is a smooth underbelly on the TV and the trailer. And of course removing the air conditioning from the roof would be a big change, though perhaps not for the average fb trailer owner. I hope some more will join in with things they have tried or believe, rather than just poo-pooing what I have said ! ! LOL David in Fresno and Sonora, CA.
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Old 11-02-2016, 11:23 AM   #9
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These are both great posts, and I also would like to make my rig much more slippery. I'm actually working on a fairing of sorts right now for my pop-up truck camper, but would love to see what you folks are doing for a FG trailer too.

Samclem, have you done any calculations on how to mitigate the suction created at the back of a trailer? I wonder if this is maybe even a worse drag than plowing through the air with the front??? As DavidG suggests with semi's - I have seen some pretty involved structures on the back of their trailers to minimize drag. And those lower side panels too.
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Old 11-02-2016, 12:01 PM   #10
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My Ollie Elite ll is a wonderful example of how streamlining makes a big difference. My last trailer was a stickie toy hauler that was the same length and height and weight as my Ollie. It was about a foot wider and square.

I consistently got 9 mpg average while towing the toy hauler. When I sold it, I towed it from 5000 feet elevation down to sea level to deliver it and got 9 mpg. Then I towed my Ollie back up from sea level to 5000 feet and got 12 mpg. Since then, every trip is about the same, 12 mpg.

The only real differences are the rounded shape, streamlined front tongue and propane area, relatively smooth bottom and narrower width of the Ollie. With this I use about 1/3 less fuel.

While towing, I can hardly feel the Ollie back there, whereas the toy hauler just felt harder to pull. It was harder, and even though my truck always has plenty of power, it just felt like it was doing a bigger job, and the mileage numbers backed that up.
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Old 11-02-2016, 12:43 PM   #11
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[QUOTE=Raspy;615744]My Ollie Elite ll is a wonderful example of how streamlining makes a big difference. My last trailer was a stickie toy hauler that was the same length and height and weight as my Ollie. It was about a foot wider and square.

I consistently got 9 mpg average while towing the toy hauler. When I sold it, I towed it from 5000 feet elevation down to sea level to deliver it and got 9 mpg. Then I towed my Ollie back up from sea level to 5000 feet and got 12 mpg. Since then, every trip is about the same, 12 mpg.

The only real differences are the rounded shape, streamlined front tongue and propane area, relatively smooth bottom and narrower width of the Ollie.
----------------------------------------------------

Does anyone have ANY info on the streamlined tongues ? In Europe I see many trailers with streamlined propane/ batteries on the tongue , possibly more effective there because caravans are mostly towed with cars. Here, smoothly style pickups often tow trailers and 'cut the wind ' . Or are those covers mostly for looks, or cleanliness ? David in Fresno and Sonora
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Old 11-02-2016, 12:55 PM   #12
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BTW, did everyone notice that way below this discussion, similar threads came up about streamlining and frontal area, at least on my computer.? (Says Similar Threads ). Scroll down and you'll some comments from the past by the usual suspects.
Still , I would like to see some new ideas . David
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Old 11-02-2016, 01:31 PM   #13
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Originally Posted by TomandCallie View Post
Samclem, have you done any calculations on how to mitigate the suction created at the back of a trailer? I wonder if this is maybe even a worse drag than plowing through the air with the front??? As DavidG suggests with semi's - I have seen some pretty involved structures on the back of their trailers to minimize drag. And those lower side panels too.
No, I haven't done any calculations. With stuff like this it normally boils down to actual wind tunnel tests, very complex computer modeling, full-scale tests, or "rules of thumb." But quite a bit of money has been spent modeling tractor-trailer rigs, with the results being what we see on the roads today: Virtual elimination of slab-front cabover tractors for long-haul use, the incorporation of "skirts" to reduce drag underneath the trailer, and now we see some good foldable fairings on the back of trailers to significantly reduce drag (improving total fuel economy by about 10% at highway speed, which is tremendous when we think about how much fuel these trucks burn).
Some snippets: From this Canadian government paper:
-- Under-trailer side-skirts are good for an approx fuel burn reduction of about 3-7% (so, maybe a drag reduction of about 10%?)
-- Fairings to reduce the gap between tractor and trailer have produced results that vary. The upper practical limit appears to be a drag reduction of about 7%.
-- Rear-body fairings: These have been studied a lot, and for trucks the problem is designing one that does a good job but also can easily be gotten out of the way so that the doors can be opened. A "perfect" rear fairing would smoothly come to a point well in back of the truck, but this would obviously be impractical due to the maximum trailer length issues, etc. But, as far as "bang for the buck," just some flat panels that form a "boattail" at the top and sides of the trailer can do a pretty good job. More in this wind-tunnel study of many tractor-trailer devices done by Lawrence Livermore. Interestingly, int at study, the under-trailer skirts saved about 150% more than the aft-body boat tail fairings.
-- BTW, some folks believe that small vortex-generators on the sides/tops of trailers right at the eft portion can do some good, but from what I read they don't reduce drag very much. They do apparently help reduce turbulence when passing other trucks, which can make big trailers unsteady.

For a little FGRV? The rounded corners help a LOT. I'd say the low hanging fruit is probably:
-- Skirts to reduce under-trailer turbulence.
-- Underbody "junk": can we smooth things out under there?
-- Is the axle flipped? Does it need to be? Reducing the height of the trailer can help a lot, especially since the trailer top isn't in the "shadow" of the TV.
-- Can we reduce the junk up in the breeze? Those large MAXX Fan enclosures, rooftop AC, antennas, awning enclosures, luggage racks, etc all take a fairly big toll. A piece of regular round pipe at 90 degrees to the airflow is tremendously draggy.
-- I think >some< designs might be worth addressing the rear area. A Lil-Snoozy migth be a candidate for some boat-tail panels.

Still, it is important to keep this all in perspective. If we did everything possible to "clean up" our little trailers, it might improve fuel mileage by about 2 MPG. We can get that same improvement by slowing down about 5 MPH. And, unless we are driving every day, fuel use isn't our biggest budget line anyway.
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Old 11-02-2016, 02:09 PM   #14
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This post has produced some amazing results. I learn something new everyday!

The frontal area issue is far more significant than I ever suspected.
The MPG gain for tractor trailers utilizing the side skirts on the trailer is impressive!

I do not own a fiberglass trailer but find them interesting in many ways.
My current trailer has a low profile with a sealed bottom, comparing it to another higher profile trailer I owned years ago it tows like a dream.
Time for the RV industry to focus more on aerodynamic design as opposed to more TVs and slide outs !
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Old 11-02-2016, 03:09 PM   #15
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I have long thought that there would be a market for underbody panels for cars and trucks, not only for aerodynamics, but to keep dirt and road salt away from the underside. But AFAIK, nobody has attempted to make and sell them.

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Old 11-02-2016, 03:50 PM   #16
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I have long thought that there would be a market for underbody panels for cars and trucks, not only for aerodynamics, but to keep dirt and road salt away from the underside. But AFAIK, nobody has attempted to make and sell them.

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Old 11-02-2016, 05:58 PM   #17
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Turbulence VS Laminar Flow

The SHAPE of the trailer is the biggest factor for wind resistance or DRAG.
Flat surfaces - both front and rear - rough surfaces and square corners cause turbulence.
Rounded shapes and smooth surfaces are more likely to afford laminar flow.
Drag tends to increase by the square of the speed - if I remember my physics classes rightly. SO, as others have said, slow down. But if you have to keep up with traffic, keep your tranny out of overdrive. Transmission (and engine) overheating is mostly caused by a slipping torque converter. The oil gets hot and transfers the heat to the radiator coolant via the Transmission oil cooler, which is in the bottom tank of the radiator, unless you have a "Towing Package" which includes a separate oil cooler. But that "radiator" is likely in front of the engine radiator so the air passing through it makes for less cooling for the engine.
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Old 11-02-2016, 06:16 PM   #18
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Laminar flow around our trailers would be great if we could get it (it can decrease surface friction drag by about 80% compared to normal turbulent flow). Unfortunately, due to the "dirty" air our trailers operate in (in the turbulent wake of the tow vehicle and near the surface of the road), we won't be able to get laminar flow over an appreciable portion of our trailers. Even most aircraft don't get laminar flow over the majority of their fuselages unless they have been very well designed and (especially) if they operate at relatively low airspeeds (e.g modern sailplanes).

Still, there's plenty of room for improvement.
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Old 11-02-2016, 06:30 PM   #19
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Originally Posted by samclem View Post
Laminar flow around our trailers would be great if we could get it (it can decrease surface friction drag by about 80% compared to normal turbulent flow). Unfortunately, due to the "dirty" air our trailers operate in (in the turbulent wake of the tow vehicle and near the surface of the road), we won't be able to get laminar flow over an appreciable portion of our trailers. Even most aircraft don't get laminar flow over the majority of their fuselages unless they have been very well designed and (especially) if they operate at relatively low airspeeds (e.g modern sailplanes).

Still, there's plenty of room for improvement.

Completely irrelevant to the original post - but I found when towing a modern sailplane in its trailer, the MPG on the freeway increased. IE got better. I put this down to the trailer cleaning up the drag bubble behind my Dodge Grand Caravan. This held true for many hundreds of miles in both directions.... Sailplane trailers are not as "big" frontally as FGRVs though. YMMV
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Old 11-02-2016, 06:43 PM   #20
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Maybe we should build travel trailers shaped like sailplanes! Any photos of yours, Jim?
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