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Old 08-18-2017, 11:29 PM   #1 (permalink)
s_t
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Aero Efficiency of Motorhome/RV vs Car & Caravan

Hi, I'm looking to build either a motorhome (RV) or a pop up caravan to be towed by a small 4wd/awd and would like some input on their relative aerodynamic merits.

We go travelling/camping fairly regularly, but I would like to be able to do big trips as we did in 2014 when we spent 6 months travelling around Australia (See Around Aus | Reflecting on an awesome trip for blog of that trip) with less fuel costs. I'm also interesting in going electric, but that is a side factor and doesn't relate directly to aerodynamics (except that lower Cd is better!).

First option: a motorhome/RV. This option has a higher frontal area (extremely rough estimate is 4.8m2). Be looking to build one on something like this LDV cab chassis (chosen as ICE front wheel drive and considering adding electric to rear axle for one/other/both arrangement). V80 Cab Chassis Overview - LDV Automotive Australia Total dimensions would end up something like 2.6m high, 2m wide, 7m long. Roof would curve up and then down, and could taper the sides in at the very back - like in very rough sketches below. I've only recently started reading/learning about aerodynamics, but would look to optimise the shape and finer details. GVM (total max weight) is 3500kg. What kind of Cd can I expect - is 0.3 viable/possible for a motorhome like this?

Second option I'm considering is a pop up caravan being towed by something like a Subaru Outback or Forester - relatively small and lightweight for 4WD/AWD and I have lots of experience with them (own an old Forester). The pop top caravan would be custom built by myself (not new to me - have built a small caravan before), and might look vaguely like in the following link, but of width about 2m, height from ground 1.6m and body length of 5-6m. Eco-Tourer Both tow car and caravan could be optimised for aero, including deflector plates on the back of the car to try and smooth the transition to the caravan. Frontal area somewhere around 3.4 m2. Subaru GVM is around 2000kg, and trailer would be probably 1200kg or so. What kind of Cd could I expect to achieve with this trailer arrangement? Is 0.5 possible? Or should I be expecting a lot more? I haven't seen any figures for large towing situations.

What I'm wanting to determine is which would be the more efficient arrangment in terms of power/energy needed. Allowing for reasonable (not completely over the top) aero improvements for both options. I think one will have a lower Cd, the other a lower frontal area, but I don't really have any idea as to the net result.

There are lots of other issues and practicalities to consider when choosing between a motorhome/RV, or car and caravan, but I want to start by understanding the aero implications. I expect we'll spend a lot of time in this vehicle over the coming decades (hence why not small!), and do a lot of km's. Thanks for any advice.

Any pointers/links to aerodynamic largish motorhomes also appreciated

Motorhome side view:
http://www.aroundaus.com/wp-content/...rhome_side.jpg

Motorhome iso view:
http://www.aroundaus.com/wp-content/...orhome_iso.jpg

Motorhome plan view:
http://www.aroundaus.com/wp-content/...rhome_plan.jpg

Motorhome front view:
http://www.aroundaus.com/wp-content/...home_front.jpg

Subaru and caravan option:
http://www.aroundaus.com/wp-content/...carcaravan.jpg

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Old 08-19-2017, 12:03 AM   #2 (permalink)
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They are just rough hand sketches, but yeah, the drawbar needs to be long enough to allow the trailer to articulate (both side to side, and up and down) relative to the car. At least half the width of the car.

Deflectors on the car can't close that gap either, but I think some big trucks get part way there by using rubber deflectors mounted on back edges of the cab -
nothing gets damaged in the case of contact during severe articulation.

I understand this car to trailer gap is a major source of turbulence which will negatively affect the overall Cd. I don't know how big the effect is though?
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Old 08-19-2017, 03:06 PM   #3 (permalink)
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gap

Quote:
Originally Posted by s_t View Post
They are just rough hand sketches, but yeah, the drawbar needs to be long enough to allow the trailer to articulate (both side to side, and up and down) relative to the car. At least half the width of the car.

Deflectors on the car can't close that gap either, but I think some big trucks get part way there by using rubber deflectors mounted on back edges of the cab -
nothing gets damaged in the case of contact during severe articulation.

I understand this car to trailer gap is a major source of turbulence which will negatively affect the overall Cd. I don't know how big the effect is though?
One study demonstrated a 12% drag reduction when the cab/trailer gap was faired in with an 18-wheeler.
As to motor home vs caravan setup:
The Vixen,5-cyl BMW Diesel-powered RV motor home could get as high as 29-mpg at around 55-mph.
VW Jetta TDIs,pulling up to 19-ft Airstream trailers have been reported at up to 30-mpgs.
I hope to test a trailer next month in the wind tunnel.I can't tell you anything 'til that's over.
Member BamZipPow has done a lot of trailer testing with his Toyota pickup.If you haven't seen his 1-wheel trailer thread you should check it out.
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Old 08-19-2017, 08:26 PM   #4 (permalink)
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Quote:
Originally Posted by aerohead View Post
The Vixen,5-cyl BMW Diesel-powered RV motor home could get as high as 29-mpg at around 55-mph.
Thanks, hadn't seen that before. Size is similar to what I'm looking at (mine slightly smaller perhaps). It's Cd is less than 0.3 according to Welcome to Vixen 21 Motorcoach Website. Makes me wonder how much the teardrop shape on the roof profile, that I was planning for aerodynamics, would improve it - if at all??? Also found a link to the NASA study Inter Action which indicated good results by just by adding a boattail.

That roof profile that I sketched ended up being pretty close to the profile discussed Aerodynamic Streamlining Template: Part-C, but drops slightly more steeply near the end. This shape would be 'less usable' during motorhome use - simple square provides more interior space and would be easier to make too. Has anyone seen a motorhome following a more optimised profile along these lines?

Quote:
Originally Posted by aerohead View Post
VW Jetta TDIs,pulling up to 19-ft Airstream trailers have been reported at up to 30-mpgs.
I hope to test a trailer next month in the wind tunnel.I can't tell you anything 'til that's over.
Member BamZipPow has done a lot of trailer testing with his Toyota pickup.If you haven't seen his 1-wheel trailer thread you should check it out.
Will check that thread out. Sounds like similar efficiency is possible either way - one piece motorhome or car pulling trailer, but perhaps the trailer option requires a little more optimisation. Look forward to seeing the wind tunnel results.
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Old 08-19-2017, 08:50 PM   #5 (permalink)
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The most aerodynamic possible motor home; the first is 7x24ft and has a fineness ratio of 3.42:1, the second is 8x40ft for a ratio of 5:1:



Here's a design for a small trailer that follows the Template except that it only tapers on the sides, like the Tropfenwagen.


The closest commercial product is the Airstream BaseCamp. Here's why you want an electric screw jack retractable trailer tongue:
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Old 08-19-2017, 09:30 PM   #6 (permalink)
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Thanks, I hadn't heard of retractable trailer tongue / drawbar - makes sense.

Don't see Airstream trailers over here, but they remind me a bit of some models of Kimberley trailers (but they don't have the rounding at the rear). Best Australian Off-Road Camper Trailer | Kimberleykampers

Those 3 wheeled motorhome shapes look good, but not sure I want to build a full custom motorhome from scratch - chassis, drivetrain and all. I might be a sucker for pain, but not that much!
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Old 08-20-2017, 12:45 AM   #7 (permalink)
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A very large old thread on this topic.

What's the best RV/motorhome aerodynamically?
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Old 08-20-2017, 01:47 AM   #8 (permalink)
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I think you would do the best mpg with the pop up trailer. You should be able to get it to be pretty close to even improving the aero of the tow vehicle if matched well. Forester's don't do all that great to begin with though, and I bet you might beat the Forester/popup combo with a v6 diesel mid top Fiat, Sprinter, or whatever other mid size diesel contractor type vans they sell downunder. They can get over 20 mpg US where my Forester not towing is under 30 mpg. Hook 3000 pounds to the back and I'm sure it would drop to 20 or less. A good in between might be one of the 3.0 diesel mid size SUVs like the M class Mercedes, the Jeep Grand Cherokee, or the VW/Audi whatevers. They can get 30+ mpg US and would be less effected by the trailer, and allow for a bigger trailer to still be in the wake. What is the gas vs diesel cost at the pump in AU? Around here it's pretty close but some places they are different.

I just saw the V80 you posted, that will definitely beat a forester towing. What about a custom 5th wheel, popup camper pulled by that V80. That would tow like a dream, have lots of room, and be able to be dropped at a location while the cab is used by itself. A 5th wheel can have much closer gaps as well if designed right. I wish we had something like that here. The Fiat with the 3.0 VM Motori v6 diesel is about the closest (and I think it may not even be available anymore). A stripped down cab and chassis is about $35,000 US while a new Forester is just over $20,000.
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Old 08-20-2017, 03:55 AM   #9 (permalink)
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Here's the Basecamp:

https://gearjunkie.com/airstream-basecamp

Quote:
Airstream today announced the launch of its newest travel trailer, the Airstream Basecamp. With a base weight of 2,585 pounds, you can (just barely) pull it behind a Subaru Outback.

Loaded with gear, it has a trailer capacity of 3,500 pounds. You’ll need a slightly larger truck for that.

But most small pickups and SUVs, and many crossovers, can also easily handle a pull-behind in the 2,500 – 3,500 pound range.
The retractable tongue may be a bad idea. It might be better to have a retractable nose that moves back and forth on the tongue.
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Old 08-20-2017, 09:26 AM   #10 (permalink)
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From a practical point of view I'd stick to the trailer idea.

RVs can only be used while your on holidays where the Suv cab be used every day.
Many interesting places in OZ are difficult to get to with our skinny roads and parking a nightmare in an RV. Van can just be unhitched and left a campsite.
Ducking into town for food or beer means packing up entire campsite in an RV, van can just stay put.

Personally I've chosen the motel path for most my trips now, I've worked out for me with the purchase cost, maintainence, rego, insurance and extra fuel of a van just doesn't make sense and apps like Wotif means you can find a motel on the fly nowadays.

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