Help Analyse my Ford E350 Teardrop RV Concept
 

As a new (and amateur) student of aerodynamics I would like some feedback on a concept I have banging around in my head. I want to travel and camp around America for 6 months to a year. I have a family so I can't exactly drive a metro, but I still want to maximize my MPG. Driving an RV is out. Those boats get 5-8 MPG. I have build and welding experience to the months of fabricating would not be difficult. I am debating buying a 2003 or older Ford cargo van with the 7.3 diesel, cutting down the cargo box and welding up a pivoting roof that would fold down to form a teardrop-ish shape. The construction details are already pretty well formulated, but what I am wondering is whether it is worth the trouble. Would the semi-teardrop give enough benefit to warrant the extra effort and cost of the folding roof? The pivoting roof would have to be framed in aluminum and the entire construction would be much more complex and more expensive than a solid build. I figure that a 7.3 diesel in a typical Ford van gets 18ish MPG to start with when driven by a typical lead-foot truck driver. Clearly I can improve on this with some good driving habits, but more important to me is estimating how much the drop roof will improve the fuel economy. It has to be better than a 5 MPG improvement to pay for the extra cost of construction. As you can tell, the inspiration is a gypsy caravan. I haven't bothered yet to draw the interior, except a few scratchings on restaurant napkins. Yes, I realize that the shape is not a true teardrop, but aesthetics and livability do have to be factored in as well. Give me your opinions and ideas. What would my MPG potential be? What other ideas can you suggest to improve efficiency?

http://www.poodwaddle.com/images/van1.jpg
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http://www.poodwaddle.com/images/van3.jpg
http://www.poodwaddle.com/images/van4.jpg
http://www.poodwaddle.com/images/van5.jpg
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Looks cool. I don't think it improves on a stock cargo van at all, it's wider and taller. Better then a class C RV but similar to an airport shuttle van. I think just adding a pop top to a stock cargo van would be more aerodynamic but with a little less floor space. I also disagree the average employee driving the bosses 7.3 van gets anywhere near 18 mpg. No doubt you could, but an average nut behind the wheel Wilmington be lucky to see 12-13, probably less then 10 on an airport shuttle van.

I am not sure on the 18MPG but the gas vans get 12-13. gas powered box vans get 8-10. The 7.3 diesel is a real monster and is used in quite large shuttle buses with plenty of power to even tow a trailer behind. Even some drivers of the smaller shuttle buses claim to get 17MPG on the highway, 11+ in town. Unmodified, the diesel van gets 17-22. Even people who have added a power chip say they get 18, so I think 18 is a reasonable claim...and of course as an RV most driving will be highway.

I think that you are right about the greater frontal area. Squaring off the corners and raising it up a bit will, no doubt, reduce aerodynamics. A pop-top on a standard van is just not reasonable. My family needs more space, and I will not be taking my family around the country in a tacky pop-top van. Sprinter vans are a bit less tacky and a bit better on fuel. They might tempt me, but most are unibody so they limit the modifications possible.

I know what some people claim on mpg, but fleet managers who count every bean say the diesels seldom do much better in the real world, and never good enough to make them cost effective. They deal with drivers who go pedal to the metal when moving, and then let it idle for 2 hours at stops. They only buy them because the workers ***** if they don't get a diesel to trim trees or whatever. I bought a 2005 Duramax because of such MPG hype but never saw anything worthy of writing home about. Power was great but mileage just a few ticks better then my 454 gas, no better then my 5.7 hemi, and I actually was/am trying to get good mpg. I would go with a 7.3 in the Ford world, stay away from used Sprinters with the modern DPF systems, fleet guys will tell you the big maintenance problems they get at 150-200k probably when you will see them used. I personally love the new Promaster because of its FWD and low floor in back. The 3.0 ecodiesel does get good economy compared to gas. Some Ram drivers get over 30 and I do actually believe them.

How big of a family? How much driving would you really be doing? Do you plan on keeping it after the trip?

In my opinion, your design wouldn't be big enough for more than 1 or 2 people for such a long trip unless most of the family will be sleeping in tents. Fuel costs, while high, might not justify the cost of doing such a mod (that's up to you, though). Along with that, there's the resale value if you plan on selling after the trip. Your modified vehicle could end up being worth more to the right buyer after modding it, but there's no guarantee. You could buy a 10-15 year old Class C RV, drive it for a year, and likely be able to get out about what you put into it. You could even add some aeromods to improve the mileage (which isn't quite as low as you think). http://ecomodder.com/forum/showthrea...d-e-18151.html

Shuttle bus mileage is a horrible suggestion for comparison to what a motorhome owner might see. Shuttles idle for hours on end to keep their interiors warm or cool depending on climate/season and generally get driven hastily to airports or conferences to make up for clients running late. I've driven shuttles locally and have a friend who drives a ~150 mile loop in Texas to and from the Houston airport. He's had people board the van 30 minutes before take-off and 90 miles away, who offer him a couple hundred bucks to get them to the airport on-time...basically asking him to do in a 10,000 lb shuttle what couldn't be done in a Viper. Believe me, if a shuttle got 18 mpg it'd be a genuine miracle and if it did, a conscientious owner could expect 25 mpg...neither is gonna happen.

Sometimes I think people make a MPG claim based on a best case scenario. "I get 18mpg on the highway" really means they got 18 mpg one time, when they drove slow and reset the computer once up to speed. Their actual whole tank, hand caculated mpg is probably 13mpg. Our van on a 360 mile trip got 29 mpg hand caculated, round trip, last week, which means we had ling streatches of 35 mpg. We only average 19 total over the last 2 years, albeit 80% city. So should I tell people a 2011 Town and Country gets 35 mpg highway? Even saying 29 is a bit misleading as it's outside the norm. I wouldn't technically be lying either way, just 26 is more realistic.

The shuttle bus mileage estimate comes from people who have purchased them for use with handicapped family, not from fleet bus usage. It is, no doubt highway mileage of a mostly empty bus of the shorter type.

My mileage estimates were based more on my own driving expectations for this trip. It makes little sense for me to calculate people's commute mileage when I will be highway driving. My driving (me, wife, and young son) would be mostly smaller highways since I enjoy seeing the sights. I have little interest in cities and on interstates I drive no more than 60 (65 if I feel pushed by traffic). I have been toying with similar designs for a Mitsubishi Fuso as well, which would have the added benefit of extra space. Yes, this van design might get cramped at times. We will probably stay in a hotel once every couple weeks just to stretch our legs. After the year I will probably give the vehicle to my parents who love traveling.

Mileage quotes are like winners in Vegas. Gamblers will always tell you what the won, "I won $1000 last weekend!" They conveniently forget that they let it ride and lost it all 5 minutes later.

"I got 70 MPG coming down Donner Pass the other day... but I got 5MPG climbing up the other side."

That all said, there are subtle differences in vehicles. These full sized vans with the 7.3 diesel are not all the same. The rear end might be geared low for work and acceleration, or high for highway speeds and MPG. Some might have big fat work truck tires with 40 PSI. Others might have street tires that pump up to 80PSI... and are completely worthless on snow. Some are driven by morons who think the accelerator is an on/off switch. So I suppose one could honestly say that highway mileage of a Ford E-250-350 with 7.3 diesel is anywhere between 10 and 20. Some claim higher and are probably lying. Some claim lower and are probably idiots driving a poorly maintained POS... or I could be wrong... it has happened once or twice before.

So back on topic then... Is there a reason for the cab over portion? Is that going to be big enough for a bed there? Losing that harsh aera above the doors would be nice. Putting a radius down the side of the roof also is important in cross winds. Basically like the back part but make the whole top that way. Just cut the bottom lower and run a flat portion down from the top after the radius ends. I wish my Hi-Lo camper had better radiuses, an Airstream top on a Hi-Lo bottom would be perfect. Just so you know the seal between the 1/2s of my Hi-Lo is far from perfect. It does OK in the rain, but take it on a dirt road far and it's a mess inside. I wish instead of an overlap seal, the top just rested down on a plate and made a seal, more like a tent pop-up camper. Our new camper has tip out tent beds. If you could work one of those in there you get a big jump in floor space as the bed no longer takes up any room. One queen tip out, and one couch bed would be perfect for 3.

Why reinvent the wheel?

A well chosen family car pulling an Airstream (or one of the older competitors as I do) can use less fuel with better space efficiency. And it can be ready to travel shortly after purchase. One of these is not "old" up to about 25-years. And at that point just needs refurbishment.

I use a one ton Dodge. Pulling my 35' trailer is 15-mpg at 58/mph in the South Cental US. Solo, not below 24-mpg same conditions, empty or loaded, rain or traffic.

The TD SUVs from Europe can do close enough to 30 while towing. The small cars with a small AS over that.

The aero qualities of the trailer preclude the need for a truck (with very few exceptions).

Airstream Trailer & Motorhome Owners Community is a wealth of info.

The link in my signature shows an earlier trailer sold.

Thanks for the suggestions. One of my primary goals is to design the back more like a building would look, less like a vehicle. The roof line (including the part over the cab) is a uniform arc. If I altered the part over the cab, it would sacrifice the uniformity and, when set up, would look rather odd (I think). There may be a way to do it and you have sparked the idea. I also like your idea of having the top seal like a pop-up camper. I have considered the sealing issue and it is a major problem. I remember a hard sided pop-up that my parents had when I was young. It might be an option and would solve several problems (including the weight of the massive hinged roof).

If you look at the back you see the windows in the doors and windows on either side. The primary bed (and a dining/sofa area) would be at the back. The bed would actually block the door in back (that is why I have the second door). I want the bed there because I want the views. It isn't ideal for several reasons, but then this camper is just 11 feet long inside so there are no ideal solutions. The slide out/tip out option might give me a better solution - if not for the primary bed then at least a better choice for bed #2.

The rounded roof edge would be much better aerodynamically but much worse aesthetically...I think. It is a very difficult choice. Is it a vehicle that becomes a house, or a house that moves? Where to a draw the lines of priority? For me it is first a home. I live in it for so many months that it has to be comfortable.

I have modified the design to drop a bit lower. This reduces the frontal area and the drag.
This is probably the most it can be reduced in any dimension. Other than this it will just be mods in things like the way it folds down on the sides.

Thanks for the advice. It has helped.

http://www.poodwaddle.com/images/van8.jpg

ecomodder is all about reinventing the wheel. We aren't satisfied with simply purchasing the newest, most fuel efficient vehicle. We mod.

I built a teardrop trailer and it worked so well I want to step it up to the next level. Yep, I want to reinvent the wheel.

As far as an airstream goes they are beautiful beasts and have their appeal. But they are designed for the masses. Their layouts do not appeal. Perhaps a remodel would be better, but it would never be mine.

Yeah, you've got a lot of people telling you that this will not work out like you expect it to. It seems like it would be a lot of work for what might be minimal gain, if the others' advice is to be taken.

That being said, it looks very nice, and I'm sure if you pulled it off, people would think it's pretty damn cool, regardless of what they are saying now.

The back porch is awesome. If I were going to do anything like this I would also go with an all outside kitchen. Slide out like those Australian Bush campers, under an awning. I hardly want to cook inside my big camper, I can't imagine doing it in something this small. I'm a fire and flesh type of guy. My buddy says I cook better on a campfire then he does in his gourmet kitchen at home.

As a Ford van owner and guy who travels and camps a lot, I think you're denying your family one of the largest benefits of a vehicle of this type and that's the ability to move about whilst underway. With your roof lowered you'll be reducing what should be a home on wheels to a passenger vehicle and be forfeiting the majority of your available space to see a minimal benefit in mileage. Think smiles per gallon. That being said, all is not lost with regard to fuel efficiency; see the threads about camper and motorhome aerodynamics. Just build a simpler, more reliable, (fewer leak points, etc) lighter, fixed body that's pretty aerodynamic and give it a tapered tail, wheel spats, fender skirts, a flat belly, etc. The whole movable roof thing is neat but it overrides the recreation portion of recreational vehicle. If you've never traveled in a motorhome you probably don't fully appreciate your passengers being able to use the restroom or change seats or get a drink from the fridge while you keep moving down the road. Believe me, traveling with a family will be MUCH better if they can use your entire rig all the time vs just when parked and the roof is raised. I've traveled pretty extensively with my family both young and old. First it was with 5th wheel travel trailers towed by extended cab pickups. Then crew cab pickups. Then bumper-pull campers towed by large SUVs. Then a camper towed by a fullsize van. Those were all reasonably comfortable modes of travel but the more interior space, the better. The van was winning until we tried a motorhome. Whole different ballgame with regard to comfort and willingness to travel.

What everyone has said so far +1.

And. Have you considered towing a small car to do your exploring with? Use the motorhome as a base camp. Not having to drive the motorhome into town just to get milk, or for a day trip into the next town, will save gas. You do have to buy a small car though.

How about a skoolie? Could take the back half and give it a good boattail. Not sure what mileage you could achieve. Pretty big, but can be cheap to get into.

And speaking of boattailed skoolies and outdoor kitchens:
http://cdn.blog.rvshare.com/wp-conte...559.jpg?de6dcd
Only the fridge is inside. Pantry, sink, stove, and BBQ are all outside. And yes, needs more boattailing.

Big Woodie bus

Both big school busses, and short busses, along with lots of cargo vans and things like ambulances can be found for good prices at publicsurplus.com
I like this better then the GSA auction page, that stuff seems to go for full retail.
We bought a nice 3 year old highway patrol car off publicsurplus. I saw a running, driving 5.9 cummins bus go for under $1000 in my town. The motor and it's Allison transmission was worth 3x that and the bus would scrap for $500.
Here is an example, this will go cheap because it needs a rebuild but the dual rear wheels are better against rollovers then single, a wheelchair lift can be sold for what the bid is at.
http://www.publicsurplus.com/sms/auc...ew?auc=1660563

Thanks for that idea. I will see if it works in this design. The kitchen, like you say, is micro inside and would be a pain to live with for everyday cooking. I think that just might be a great way of expanding the space.

Nobody has really answered my initial question though. I am curious how much the drop top will improve mileage over a solid upright build. I am guessing 3-5MPG but I would love more experienced opinions.

You might have a good point. If I choose the solid structure design then I will probably chop off the back porch (save a MPG there) and have the back slope down with a small window in back. The loss would be sad aesthetically, but the build would be much simpler and quite an eye catcher going down the road.

The back porch is sized to fit a couple bikes, perhaps a scooter if I decide to build it solid/ without the drop top.

Space efficiency. A Moho of any sort is inefficient. Rather like a pickup versus a car pulling an open trailer.

And the layout of the aero all aluminum travel trailers is proven. Weight over the axles dictates interior, for the most part. Can't screw with physics.

The engineering exercise is commendable. But will prove difficult. Easier (better) to spend time camping and learning more about what constitutes overall economy. Fuel burn is less than half.

Not to mention the reason for camping in the first place. Backing into details too far is counterproductive.

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Look at a motor home called A Vixen, it's not boat tailed and gets 30mpg

I would love one myself but their hard to find and I haven't found much in reliabily and maintenance needs.

I would love to drop a 6.2 c-code diesel in one and upgrade it for towing

There was one here on Craigslist for about $20k. Guy had a 10 year mileage log with lots of high 20 mpg fill ups. There is this really nice one near Bozeman Montana as well, I bet they are looking for around $20k as well.
Vixen-td-Vixen21-21-BMW-diesel-524TD-turbo

I have looked at those. I like most everything about them. They are clearly designed by a hypermiler and could use a few modern tweaks but are amazing vehicles. I am not, however, shopping for something that looks like a vehicle. I want a platform to build my own design and that limits my options to framed trucks with beds over 10 feet long. Mostly this means vans built by the big 3 who put in overpowered engines like the 7.3

As a hypermiler I much prefer the small BMW Diesel in the Vixen over the monster 7.3 that is the only thing Ford used that was worth getting. I am starting to seriously lean toward a Mitsubishi Fuso and studying what mileage it can be tweaked to get. It is very difficult to find info on them because drivers have so many different boxes and loads. Their aerodynamics are pathetic and the frame is built high and heavy, but the engine is excellent. The choices all kinda suck.

Might I recommend a 6.2 diesel?

It (like the 6.9 diesel) was designed for fuel economy not power.

The vixen was just a GM c20 extended cargo van with a full fiberglass shell and extended control arms.

Couple appropriate rear end gear ratio, with a c-code motor and a 5speed stick and the 6.2 achieves 30mpg in an oversized boxlike vehicle.

Some bread trucks came in a stick shift form with a 6.2,

6.2s may only drive 200k under heavy use but the blocks can be had for as little as $50 and rebuilds top to bottom start at about a grand.

You will find GM platforms are easier to locate and cheaper to maintain despite motor issues.

Having owned an oversized 7.3 casino bus, never again comes to mind, everything but the motor was garbage and rednecked.

Just my 2 cents

Thanks for that. I did not realize the Vixen's heritage. The GM diesel seems quite hard to find and reports on it's fuel efficiency suggest that it might be slightly better than the 7.3

I have also considered something with the Cummins 4BT like a Dodge 1500 or a cube van. Those things are simple to work on, offer lots of tweaks, and look like they have the potential for great mileage (the engines, not the cube vans)

You asked for a critique of the design and the conversation has drifted to diesel engines. :confused:

I like the lift-up top. The hinge line might even overhang the windshield, like a sun visor.

I'm looking at the covers for the front edge that changes angle as it lifts. Aerodynamically you want a radius in the front edge. Actually you could pull the curve back toward the door and then taper the sides behind the rear axle. Give it a continuous curve from front to back. The joinery gets more complicated.

What are your fabrication skills? What materials will you work in? What level of surface finish?

The overhang presents an interesting question on aerodynamics. It will be easier to draw than to explain so here is a photo of a Ford bus.

http://www.poodwaddle.com/images/luton-bump.jpg

How does the air flow over a Luton bump? Does it (A) create a parachute effect, or (B) create an air pocket that pushes the air over efficiently? Obviously the sides release air and this might cause more of A than B, but if we ignore the sides (pretend that they are enclosed as well) then what does the air do? Parachutes (the old type) have a hole in the middle. Without it the parachute would rock back and forth as the air spilled over the sides - essentially an air pocket is being held in the chute. Would it be the same on a bus?

I suspect that the air is captured by the bump to some degree and so is a big aerodynamic no-no...but then I would have thought that tailgates down was also more efficient. Visors/cab shades might be the same... in theory.

I would prefer to build out of steel square tube for several reasons. For one thing I already have the equipment and experience. Steel is just so easy to work with. But if I choose a hinged top then I would certainly want aluminum. I also have experience in wood and fiberglass and have even debated building the top like a cedar strip kayak (fiberglass-wood core-fiberglass sandwich). This can actually be quite light and very very beautiful. I built a kayak many years ago and found the technique both strong and light. But if I chose to keep the surface unpainted then it would be high maintenance (resins yellow over time and need to be recoated by varnish each year).

The one thing I have little experience with is sheet metals. The corners would require folding or even rolling depending on the design. The simplest solutions would not be the most aerodynamic.

Neither. Any surface exposed in the direction of travel experiences a positive pressure gradient. The air sitting undisturbed at A or B has only two choices, sideways or up. Start with 20 streamlines that are all put into competition. Air under the bump is going to move sideways and wrap around the A-pillar. Within a fraction of an inch the air is gripping the body. That's skin friction. At the distance you've drawn the streamlines air is interacting with air is interacting with air, in a [diminishing] fractal recursion. Wind tunnels want 8-10% blockage to get an accurate rendition close to the body.

The only time air moves forward in relation to the body is in the wake, within recirculating separation.
_________

How would you get from square tubing to a finished surface? Sheet metal seems logical. When you built the Kayak was there a lot of tapering the strips necessary? I guess you could do them in bulk. I've known a person who breathed a lot of resin fumes and then croaked. :eek:

All four edges of the roof need some radius. Forward facing edges a minimum of 4% of the gross width, typically about 4". 1/4-round down the sides. At the back it makes no difference, but you might as well do that too so it all matches.

Here are some experiments I did with a material called Polymetal. It is an aluminum/plastic/aluminum sandwich. The same joins could be done in sheet metal. Rolling sheet materials is actually easy, the problem is finding rollers long enough for big pieces. Three 10' steel pipes and six skateboard wheels jammed in the ends would suffice.

http://ecomodder.com/forum/member-fr...1-100-0866.jpg

Note the bottom one has a radiused edge with no framing. It could be sandwiched around a piece of angle iron if that's needed.

I ran the strips through a router. Every single tiny one of them had a tongue and groove. The strips are laid over a form and stapled in place. They take the shape quite easily and when done (months of work) the surface is shaped with sanding and planing. After that the outer fiberglass is applied. When that cures the kayak is removed from the form and flipped over (and separated in two pieces). Anyway, that is probably more detail than you wanted. Corners and complex curves are quite simple with cedar strip method. The same method can be done with some other materials. People have tried various foam cores, but cedar has the advantage of crush strength. The point of the sandwich is that the core acts as a light weight separation to strengthen the two sides, like cardboard really. If I chose to round the roof to side corners it would be a complex curve, probably difficult for sheet metals.

My son inherited his grandfather's redwood canoe, which I watched him build, so I'm familiar. It seemed the pinched, vertical ends of a canoe would require less tapering than a kayak (before the t&g treatment). Still, as you say, a lot of work.

I found some material at the recyclers. I don't know what it's called and I can't find an example in Google Images, but it's a sheet metal strip with 1 1/2" radius and an 1/8" step along the sides. I got two 8' lengths I'm going to use for cabinets; but a similar strip with 4" radius joining two curved edges would produce something like what you see in U-Haul trailers, only curved.

https://www.uhaul.com/reservations/i...d5x10Large.png
https://www.uhaul.com/Trailers/5x10-Cargo-Trailer-Rental/MV/

Another set of thoughts. For space and utility maximization:

There's a beautifully restored 1974 TRAVCO for sale in FOrt Collins, CO. V8-440 Dodge power. Front disc brakes. Ninth Moho resto by owner.

Use it well and resell it later. Probably be close to break even on purchase and sales price. This is the approach the ex and I took with a truck/trailer combo. Sold the trailer, and came out ahead versus apartment rent, hotels, etc. And this was after spending on repairs and upgrades.

Leave the aero alone. It's already good. A Travco is highly admired, and valuable. A Cummins conversion wouldn't pay for itself very soon (and would need computer control six speed auto or manual six speed to be worth it). EFI on the 440 and the special cam from the FMC specialist, Morgan Hill RV, would be as far as I went).

Other than that, a third body color [stripe] and then go to ZIP DEE for a complete awning package all around that uses those colors. Same for interior window treatments. Use AIRSTREAMS suppliers.

The other upgrade to make it "better" is a gasoline generator. Bound to be a way to fit in an ONAN behind a custom door. (And I'd convert the roof AC to a Coleman low profile 15k but Mach 8). A solar package won't help, but AGM house and engine batteries would.

See Hendersons Lineup for suspension and steering upgrades.

Trip planning is how to get a vehicle down the road. When one is living in it 24-hrs, a few hours on the road every day or every other week isn't as important. MPG is relative. But not to cars. This remains a fundamental mistake in thinking with every RV thread on this site.

A Moho is a truly excellent way for a family to travel. And the short length of this one is a tremendous starting point. No second vehicle being towed. Etc. This IS an economical Moho.

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initial question
 

I'm seeing your thread for the 1st time today (wish it had been on the Aero Forum).
I'll have to spend some time thinking on it.You've combined elements which have never been tested so it's gonna have to be a 'guess.'

To baseline you will have to figure out how far above below you are against the vixen Rv, bigger =lower

Is that a Cavalry bugle I hear? (you need to get out more :))