Casita vs Bigfoot MPG

[LeonardS]

Originally Posted by David and Nancy

You are welcome, Francesca. We love our Bigfoot, but aerodynamic it is not. Too many flat surfaces. I can feel the wind grabbing it. Egg-shaped trailers just fly better. Bigfoots are basically square with slightly rounded corners.

I've been looking at Escape 15's and Trillium 4500's. Both look a little boxy compared to Eggcampers and Casitas and Scamps. But...are they really boxy enough to effect mpg like the Bigfoot?

[GPJ]

You really don't need that much rounding to obtain a real aerodynamic advantage - some of the aerodynamic studies I have looked at indicate that rounding (front sides and top) with a radius of 6-8 inches gives much of the available aerodynamic benefit. Beyond that there is a rapidly diminishing return. As has been pointed out above, if all else is equal, frontal area is a big factor in total drag. If your trailer is taller and wider - you get lots of room inside, but pay the piper at the gas pump.

In your example, I'd say the Escape 15 and the Trillium 4500 would have mpgs very similar to the Scamps and Casitas due to aerodynamics, frontal area and also weight. The Bigfoots really are another class of trailer - bigger inside with heavier double wall construction. Remember, Nancy and David's Bigfoot is a 25' model according to their info - you can't really compare it to a 15' egg.

[Francesca Knowles]

Originally Posted by LeonardS

I've been looking at Escape 15's and Trillium 4500's. Both look a little boxy compared to Eggcampers and Casitas and Scamps. But...are they really boxy enough to effect mpg like the Bigfoot?

Front surface area is the main difference between Bigfoot and the other brands you mention, especially since without exception all fiberglass trailer backsides are equally wrong for efficient aerodynamics.

Per comparison of Trillium with Escape:

The Escape 15 and the Trillium 4500 shells have a common progenitor and one of the things they share are identical frontal profiles/areas.

Francesca

[Brian B-P]

Originally Posted by Francesca Knowles

The Bigfoot's plenty sleek- not that much "squarer" than many of our rigs. It just has more frontal surface area than most fiberglass trailers, which is likely what causes the extra drag.

I agree that the frontal area is a - or the - big issue, but there is a substantial difference in front corner rounding, and...

Originally Posted by GPJ

You really don't need that much rounding to obtain a real aerodynamic advantage - some of the aerodynamic studies I have looked at indicate that rounding (front sides and top) with a radius of 6-8 inches gives much of the available aerodynamic benefit.

[Brian B-P]

Originally Posted by Francesca Knowles

Front surface area is the main difference between Bigfoot and the other brands you mention, especially since without exception all fiberglass trailer backsides are equally wrong for efficient aerodynamics.

I agree that the typical egg is all wrong in the back, but again the current Bigfoot shape is more squared, which is actually the better design for the back end.

Originally Posted by Francesca Knowles

Per comparison of Trillium with Escape:

The Escape 15 and the Trillium 4500 shells have a common progenitor and one of the things they share are identical frontal profiles/areas.

I agree. As for the rest...

Originally Posted by LeonardS

I've been looking at Escape 15's and Trillium 4500's. Both look a little boxy compared to Eggcampers and Casitas and Scamps.

Sure. In rough order of most round (egg shaped) to most square:

1. Oxygen
2. Boler(1300/1500/1700)/Scamp/Casita
3. U-Haul/Burro
4. Trillium/Surfside/Escape
5. Compact, Bigfoot, Biggar, Boler 1650/etc

... and of course I have left many out. Within the models of a brand (and of the same approximate vintage, for those few that have actually evolved their shape with time), they are usually all similarly shaped.

The Snoozy doesn't fit in this list, because it has a rounded front (even though not a great approach to rounding for aerodynamics) and relatively sharply cut off back, as I believe they all should. It is annoying to me that essentially all of the designers - other than the Smoaks - shape the front and back similarly, almost as if there is no regard to which way the trailer is going. Just a pet peeve...

[Glenn Baglo]

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.

[LeonardS]

Originally Posted by Glenn Baglo

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.

Now THAT made me laugh out loud!

[Thomas G.]

Interesting article on motorcycle aerodynamics including discussion of Kamm tail (squared off back). Tony Foale Designs, article on motorcycle aerodynamics.


This discussion of the effect of side winds is interesting, too.

[Thomas G.]

This image also explains why Airstreams tow better after a good hail storm.

[Francesca Knowles]

Originally Posted by Glenn Baglo

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!

Francesca

[GPJ]

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.

[Brian B-P]

Originally Posted by Thomas G.

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)

Originally Posted by Thomas G.

This image also explains why Airstreams tow better after a good hail storm...

[Brian B-P]

I generally agree with GP's whole post.

Originally Posted by GPJ

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).

[Lora]

Originally Posted by Thomas G.

This image also explains why Airstreams tow better after a good hail storm.

How's bout dimple the TV and toad (ala Mythbusters)?

[andix]

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.

[Glenn Baglo]

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.

[andix]

Originally Posted by Glenn Baglo

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.

[GPJ]

Originally Posted by andix

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.

[Francesca Knowles]

Originally Posted by andix

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

[Brian B-P]

Originally Posted by andix

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

Originally Posted by andix

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.