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Happy Tripping
Senior Member
Joined: 27 May 2014
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Posts: 473
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Topic: The Mystery of The R-Pod Total Frontal Area Posted: 12 May 2019 at 7:00pm |
Ultralight trailers like the R-Pod are touted as being able
to be towed by smaller vehicles such as vans and small SUVs. That is their big selling
point, a big tow vehicle is not required. Enter - Wind Resistance.
Wind resistance is very important for travel trailers. The
more that there is, the greater is the stress on the powertrain and wheels of
the tow vehicle and the lower the gas mileage is. If the wind is from the
front, the speed of the trailer adds to the wind resistance, if from behind, it
decreases it. Additionally, wind resistance at slow speeds like 45 mph is
relatively minor, however as speeds are increased, it increases much faster
than the actual increase in speed. Going from 55 mph to 75 mph is a 35%
increase but wind resistance, and the strain on the powertrain, increases about
250%. (People with ‘L’ or ‘M’ rated tires take note) . Obviously, with all
these variables, ‘wind resistance’ is not a static number.
The wind resistance of a travel trailer is roughly
proportional to the ‘Total Frontal Area’ but protruding things like fenders,
rooftop a/c units or awning mounts, and the underside of the trailer, also have
effects that increase the effective total frontal area. To me, the published
Total Frontal Areas of tow vehicles for travel trailers are a subject of great
mystery and seem to be nearly worthless. However, this doesn’t bother the tow
vehicle product liability lawyers. Using a complicated formula that is based on
variables that are not apparently standard throughout the industry and that also
are apparently not publicized, they have established required Total Frontal
Areas for many if not all tow vehicles. These seem to exclude smaller vehicles.
Cynic that I am, I think that the lawyers have jumped in to
give their companies the greatest protection possible in their powertrain
warranties. The poor travel trailer manufacturers in response have carefully
neglected to announce their trailer’s frontal areas in order not to hurt their sales.
The R-Pod’s total frontal area is reported to be maybe around 50 sq ft. So,
just for fun, please look in your operator’s manual and let us know if your tow
vehicle passes or fails its manufacturers’s total frontal area requirements. I know
that my entirely satisfactory Jeep Cherokee (towing capacity 4500 lbs) fails,
and I am very interested to see who passes.
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jalong
Newbie
Joined: 25 Jan 2015
Location: Fredonia WI
Online Status: Offline
Posts: 31
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Posted: 12 May 2019 at 7:27pm |
2014 Jeep Grand Cherokee maximum trailer frontal area 55 sq ft for all drive trains
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John & Sue
2016 179 - built in April 2015
2014 Jeep Grand Cherokee 3.6L 13-16mpg with 179 - 21-28mpg without
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offgrid
Senior Member
Joined: 23 Jul 2018
Online Status: Offline
Posts: 5290
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Posted: 12 May 2019 at 8:11pm |
I agree that the front area A by itself is essentially useless. The important parameter is really CdA or drag coefficient x frontal area. The Cd of rpods is pretty good. And yes, drag force increases with the square of the relative wind speed so the power requirement to overcome drag increases with the cube of the wind speed (because power = force x distance traveled per unit time).
Total power requirement is the sum of the rolling resistance and the air drag. Since rolling resistance is relative small at freeway speeds and doesn't vary much, its fairly easy to roughly back into the air drag component based on your fuel consumption if you have that at a couple of different speeds. I did that and got a CdA of about 16 sq ft for my rig, which accounts for about 2/3 of the total horsepower requirement at 60 mph. I estimated A at 45 sq ft which gives a Cd of around 0.35, which is not bad. A boxier trailer would be worse of course.
For the record there is no published frontal area towing limit for my 2012 Highlander. If there was one I'd ignore it anyway, for the reasons we've stated. If your vehicle doesn't consume fuel excessively, overheat, or run at a high rpm towing at freeway speeds then air drag is not your limiting factor, weight and weight balance will be much more important to keep track of.
Here's a calculator if you want to play around with it.
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1994 Chinook Concourse
1995 RV6A Experimental Aircraft
2015 Rpod 179 - sold
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podwerkz
Senior Member
Joined: 11 Mar 2019
Location: Texas
Online Status: Offline
Posts: 966
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Posted: 12 May 2019 at 9:47pm |
Of course the towing vehicle creates a bit of a vacuum and/or vortices behind it, depending on the shape and size of it. This probably helps reduce trailer drag in calm winds, or slight headwinds, but broadside winds get to have an effect on the entire rig, more or less.
You can't win for losing.
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r・pod 171 gone but not forgotten!
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offgrid
Senior Member
Joined: 23 Jul 2018
Online Status: Offline
Posts: 5290
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Posted: 13 May 2019 at 6:04am |
Good point. The drag of the rig is what matters, not the drag of the trailer separately. Depending on the aerodynamics of the tow vehicle the addition of the trailer might have a more or less dramatic increase on the combined drag. And if you want you can add aero devices like the Purpleline product to the TV, to try to reduce it.
The numbers I worked back to in my post were for my combined rig. YMMV (literally).
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1994 Chinook Concourse
1995 RV6A Experimental Aircraft
2015 Rpod 179 - sold
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tcj
Senior Member
Joined: 05 Jul 2018
Location: Central WA
Online Status: Offline
Posts: 141
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Posted: 13 May 2019 at 8:30am |
Originally posted by Happy Tripping
So,
just for fun, please look in your operator’s manual and let us know if your tow
vehicle passes or fails its manufacturers’s total frontal area requirements. I know
that my entirely satisfactory Jeep Cherokee (towing capacity 4500 lbs) fails,
and I am very interested to see who passes.
| 2014 Toyota Tacoma Access cab V6 4x4 with towing package. Trailer frontal area is not even mentioned in the owners manual.
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2018 R-pod 180 Hood River Edition
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john in idaho
Senior Member
Joined: 04 Nov 2014
Location: Eagle Idaho
Online Status: Offline
Posts: 611
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Posted: 13 May 2019 at 8:59am |
Seems like I saw a post a few years ago on this forum. Someone added a canoe to the top of the pickup topper and increased gas mileage a few points. but my memory may be faulty.
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lostagain
Senior Member
Joined: 06 Sep 2016
Location: Quaker Hill, CT
Online Status: Offline
Posts: 2595
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Posted: 13 May 2019 at 9:00am |
I think the design of the Pod is so sleek and appealing that it magically resists air molecules as though it glides on clouds in a near weightless environment. The only improvement, which I've previously proposed, would be some fin like wings coming off the back of the trailer at about the level of the spoiler. Once installed, it would be like pulling the trailer in the vacuum of space.
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Never leave footprints behind.
Fred & Maria Kearney
Sonoma 167RB
Our Pod 172
2019 Ford F-150 4x4 2.7 EcoBoost
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offgrid
Senior Member
Joined: 23 Jul 2018
Online Status: Offline
Posts: 5290
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Posted: 13 May 2019 at 9:16am |
Originally posted by john in idaho
Seems like I saw a post a few years ago on this forum. Someone added a canoe to the top of the pickup topper and increased gas mileage a few points. but my memory may be faulty. |
i have a kayak I was thinking of taking camping, maybe I'll try it. Another way to determine air drag is through coast down testing. All you need is a couple of miles of dead flat smooth road, no wind, no traffic, and a smart phone to video your speedometer during the tests.
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1994 Chinook Concourse
1995 RV6A Experimental Aircraft
2015 Rpod 179 - sold
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lostagain
Senior Member
Joined: 06 Sep 2016
Location: Quaker Hill, CT
Online Status: Offline
Posts: 2595
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Posted: 13 May 2019 at 9:22am |
.... and if you put the wings on the back edge of the Pod, it'll easily double the coasting distance.
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Never leave footprints behind.
Fred & Maria Kearney
Sonoma 167RB
Our Pod 172
2019 Ford F-150 4x4 2.7 EcoBoost
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