Casita vs Bigfoot MPG - Page 3 - Fiberglass RV


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Old 03-27-2013, 09:06 AM   #29
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This image also explains why Airstreams tow better after a good hail storm.
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Old 03-27-2013, 12:20 PM   #30
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I'm waiting for somebody to tow the Lil' Snoozy backwards for a 1,000 miles and compare fuel consumption with towing it forwards. Then we will have the definitive answer.
All kidding aside, wouldn't it be interesting to order a Snoozy built "backwards"? Why not set the body on the trailer in the reverse position, with a side entrance up front and the interior rearranged however necessary for the right weight distribution?

If I had all the money in the world, this is the kind of experiment I'd love to undertake just for the heck of it!

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Old 03-27-2013, 01:45 PM   #31
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Great discussion so far. Further to the discussions of back end design above, I do not believe that a square back works well at all. The best design (see the posted aerodynamic diagrams Thomas posted above) would be a very long tapering (in all directions) tail. For lower speed ground vehicles the optimum tapering angle is somewhere about 20 degrees (as was pointed out above, too steep a taper doesn't work well -that's why the T@B rear profile doesn't work well). But this optimal design gives a very long shape with little usable interior volume, so designers of low speed vehicles (cars, trucks, trailers) just chop off the tail. It's bad aerodynamically, but the aerodynamic alternative is impractical. Now with aircraft and their much higher speeds (drag is proportional to the square of velocity), the trade off is different and designers keep the long tapering tail.

Most modern small cars utilize the "Kamm back" or "Kamm tail" (see drawing posted above). Notice that the top and sides taper inwards in the rear portions of a modern small car. Designers found that if you start the taper and just chop it off before the shape gets unusable, you get a good part of the benefit of the long tapering tail. It's as if the initial taper gets the air flowing in the right direction and it keeps on going that way after the end of the vehicle is reached.

So a chopped rear end works alright if the top, sides, etc. are tapered first, but it doesn't work so well if there is no tapering. That's why the Snoozy rear is not very aerodynamic. A lot of fixes have been tried for square backs ie. rear vortex generators, vents, etc. but really only minor improvements result. So why don't trailer designers build in a tapered rear? Generally two reasons - cost and losing usable space (how can you jam a 6+' bed into a tapered rear). If make the trailer physically taller and wider to allow tapering while maintaining the space necessary for headroom and beds, dinettes, etc. in tapered areas, then you are increasing the frontal area which can be a bigger factor than the drag coefficient. And in any case, building square is cheaper.

Francesca - I think that a Snoozy built backwards would not work well at all - the back (now front) is a square brick with poor aerodynamics, and the front (now back) is sloped too sharply so the air flow would separate from the body causing drag. It would be fun to try all these shapes in real life.
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Old 03-27-2013, 11:37 PM   #32
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Interesting article on motorcycle aerodynamics including discussion of Kamm tail (squared off back). Tony Foale Designs, article on motorcycle aerodynamics.
Excellent! Something like the bottom image in this first graphic (the Kamm tail) would be a great trailer (although it could be flat in the middle of the front, because it is tucked behind the tug)

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This image also explains why Airstreams tow better after a good hail storm...
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Old 03-27-2013, 11:40 PM   #33
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I generally agree with GP's whole post.
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So a chopped rear end works alright if the top, sides, etc. are tapered first, but it doesn't work so well if there is no tapering.
... but it would still be better than the rounded rear corners that most of us have. Those are a structural and aesthetic choice, not an aerodynamic one. The rear of the current Bigfoot shape doesn't seem bad to me, by comparison with any other current production trailer, and specifically in comparison with a Casita (or my Boler).
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Old 03-29-2013, 07:53 PM   #34
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This image also explains why Airstreams tow better after a good hail storm.
How's bout dimple the TV and toad (ala Mythbusters)?
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Old 04-03-2013, 09:57 PM   #35
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3L V6 Escape + 17 Bigfoot @ 2400lb = 12 MPG. Unless there's something horribly wrong with the trailer axle my mileage sucks. I would've been better off with an 40ft motorhome, I believe.

Granted, the car is small compared to the trailer, there might be a hell of a lot of aerodynamic drag involved.
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Old 04-03-2013, 10:20 PM   #36
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I think you are underestimating the weight of the Bigfoot. I get 15.3 mpg towing a 17B Escape with a RAV4 V6. Your engine is a bit larger and your trailer has more frontal area, and is heavier ( I believe ). I think you're doing OK.
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Old 04-03-2013, 11:17 PM   #37
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I think you are underestimating the weight of the Bigfoot.
I, uh, took it to a scale before heading back home. (I'm that paranoid. ) It showed 1240 kilograms, that's roughly 2400 lbs I believe... Of course, totally disconnected from the vehicle and hitch resting on the scale too. By then it had about 1/3 fresh water in the tank, which I emptied before the trip.
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Old 04-03-2013, 11:50 PM   #38
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I, uh, took it to a scale before heading back home. (I'm that paranoid. ) It showed 1240 kilograms, that's roughly 2400 lbs I believe... Of course, totally disconnected from the vehicle and hitch resting on the scale too. By then it had about 1/3 fresh water in the tank, which I emptied before the trip.
Actually, 1240 kilograms equates to 2733 lbs! The Bigfoot's are heavy, but very well built.
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Old 04-04-2013, 12:30 AM   #39
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3L V6 Escape + 17 Bigfoot @ 2400lb = 12 MPG. Unless there's something horribly wrong with the trailer axle my mileage sucks. I would've been better off with an 40ft motorhome, I believe.

Granted, the car is small compared to the trailer, there might be a hell of a lot of aerodynamic drag involved.
Drag is definitely your enemy, and you're probably near your GCWR with that Bigfoot etc....

But I don't think the approximately 8 mpg drop you're seeing from the 3L V6's
non-towing average of 20 mpg
is excessive.

Francesca
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Old 04-04-2013, 10:35 PM   #40
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3L V6 Escape + 17 Bigfoot @ 2400lb = 12 MPG. Unless there's something horribly wrong with the trailer axle my mileage sucks. I would've been better off with an 40ft motorhome, I believe.
I would be delighted if the mieage of my 38 foot motorhome improved to 12miles/USgal

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Granted, the car is small compared to the trailer, there might be a hell of a lot of aerodynamic drag involved.
Yes, a car and travel trailer are terrible aerodynamically.
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Old 04-05-2013, 05:50 PM   #41
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12 mpg is about the same mileage I get with my 3.3L Toyota Sienna towing at 60 mph. I'm willing to buy the extra gas as a trade for Bigfoot quality and interior size.
Front Dinette Photo by tomNjo | Photobucket
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Old 04-06-2013, 10:16 AM   #42
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3L V6 Escape + 17 Bigfoot @ 2400lb = 12 MPG. Unless there's something horribly wrong with the trailer axle my mileage sucks. I would've been better off with an 40ft motorhome, I believe.
How fast are you going while under tow? Have you tried to improve the MPGs by slowing down? I know 5mph makes a huge difference in MPGs with my combination.
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