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

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?

gap
 

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.:)

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 http://ecomodder.com/forum/showthrea...-c-9287-1.html, 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?

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.

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:
http://ecomodder.com/forum/member-fr...-w-caption.jpg
http://ecomodder.com/forum/member-fr...07-7-35-02.png

Here's a design for a small trailer that follows the Template except that it only tapers on the sides, like the Tropfenwagen.
http://ecomodder.com/forum/member-fr...17-trdrp1a.jpg

The closest commercial product is the Airstream BaseCamp. Here's why you want an electric screw jack retractable trailer tongue:
http://ecomodder.com/forum/member-fr...untitled-4.jpg

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! :D

A very large old thread on this topic.

http://ecomodder.com/forum/showthrea...lly-13149.html

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.

Here's the Basecamp:
https://s3.amazonaws.com/images.gear...or-700x412.jpg
https://gearjunkie.com/airstream-basecamp

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.

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.

I fergot to ask...how many people does this need to support? A single wheel setup may not be able to fit yer needs even if you load a teardrop trailer on there with some compromises. ;)

https://i.pinimg.com/736x/a6/e4/6c/a...7fec167084.jpg

Thanks, that's a good read! Still to get through it all.

Diesel is generally slightly more expensive I think, but not much in it (don't have a diesel at the moment, so haven't been paying attention to pump prices). Would only go the V80 to build a motorhome on the back, or possible tow a large 5th wheeler. Would want a caravan to be pulled by a SUV type vehicle.

A retractable tongue would certainly need careful and detailed engineering to ensure it is safe. A movable nose is a good suggestion that would likely be easier to implement safely.

Motels :eek::eek:
Sure, financially you may be correct. But I'd much prefer to go to bed in this setting:

http://www.aroundaus.com/wp-content/..._1024small.jpg

or wake up to something like this:

http://www.aroundaus.com/wp-content/..._1024small.jpg

Yes, caravans do have benefits, but so do motorhomes. When noise/disturbances across the road at 11pm has you wishing you'd chosen another spot to spend the night, just walk through to the drivers seat and drive away to another spot. Did this a few times during our around Australia trip. In general, motorhomes are quicker and easier to set up for the night, and get going for the morning. So better for shorter stays. Caravans, as you suggest, have benefits for longer stays.


Ah, thanks, but too small! There are currently 4 of us, but the two boys are now teenagers so I'm designing this thing just around the wife and I. But we will do long term extended travelling in it, so has to have some creature comforts (composting toilet, queen sized bed, fridge etc) It's a long term project - will be at least 5 years till this thing is ready to go...

What I haven't detailed yet are my hair brained plans re electric and solar. I figure I can fit 2kW of solar panels onto the roof of a motorhome/caravan of the size I'm looking at, as well as another 2 - 4 kW tucked away or underneath them (eg. slide-out arrangement). The latest flexible panels using polycarbonate rather than glass are thin and lightweight (though less scratch resistant, and lower UV resistance).

If I can get an efficient enough vehicle arrangement, along with say 30kWh of lithium, this is enough to get me travelling at least 100-150km per day on solar power alone (most days!).

So when I'm reading through these aerodynamic discussions, I'm considering not just the aero side, but also the impact on solar panel mounting and output.

This is planning for a long term relaxed travelling lifestyle, that is completely free of typical constraints re fuel stations and the like. A slow meander through the Aussie outback, wherever we feel like going.

Sure, it's probably going to cost me somewhere around $60-90k to do this, as well as a fair bit of my labour, but people routinely spend more than that for equivalent gas guzzling machines (or just an ordinary caravan). And I love the engineering challenges associated with building something myself - something that will most definitely be unique :D

We used to have a slide in camper that goes in the bed of a long bed full size pickup. A motorhome would be even easier to get on the move, but we did love that. My big plus was my wife and daughter are slow to get moving in the morning. A few times in Yellowstone park I would just up and start driving while they were still in pajamas sipping coffee in back. Then stop and get breakfast 30 mins later at a new spot, then again with lunch, etc. Sure there are $10 crappy burgers here and there in Yellowstone but only where the crowds are. Last trip we had the camper, nicer to sleep in, but we were on the road later every morning and usually stopping back by at some point during the day at least once. Not as good as having everything on your back, but it was easier to go more places without the heavy camper in the bed.

(definitive design criteria)

Here is the motorhome design I showed earlier as a class-C.

http://ecomodder.com/forum/member-fr...56-aerobug.png

While my donor chassis of choice was the Beetle, this could easily be a Sprinter van. The defining characteristic would be a second door frame cut into a wedge and added to the existing frame (aligned on the hinge line) so the door seals to it. This eliminates the dogleg most Class-Cs have.

You'll notice the geodesic layout can be read as diamonds, hexagons or triangles. The diamond layout define bands that run around the body. If the altitude of the diamond equals the width of your flexible solar panel they would integrate well.

Instead of folding solar panels, perhaps lightweight reflectors that would concentrate on the [minimal] panels?

Sounds like some nice holidaying :thumbup:

Hadn't thought of creating a second door frame, that angles the door out gently! If it weren't for the window, it would be easier to just put an outer skin over existing door, tapering in thickness. But if I can get enough living area (in plan view) from longer length, then might not need to widen the body beyond the existing cab/vehicle width (lower frontal area).

Understand re the bands. The best is never easy is it - why can't the optimal shape be a rectangular prism :D though even that isn't too bad with some relatively simple mods according to the Cd figures in that NASA Dryden truck study!

from: http://ecomodder.com/forum/showthrea...c-9287-67.html
This design would give more floor space in which we could stand up. Perhaps greater volume too. But less floor area, which is a disadvantage in fitting everything in.

However, I want to spend some time looking at the possibilities a bit more closely. Maintaining max height has some advantages I might be able to utilise (eg. Murphy bed, or bed that stores up flat against the ceiling). You've also got me thinking about possibilities relating to designs where the rear vertical walls rotate outwards to 'open up' the rear of the motorhome when camped (to create close to a square rear). I've got some questions about the aero effect of keeping the roof and floor in a rectangular shape (to the back), extending out past the sides which are tapering in at the back, but I'll have to sketch it to explain.

I found the following presentation useful in reinforcing some of the concepts I've read on this forum (though got a bit lost in the last quarter or so).

http://www.bakker.org/dartmouth06/engs150/11-bl.pdf

In other threads I've seen comments about achieve better milage figures after a clean/detail. How sensitive is turbulence and separation to surface finish, in objects of car/motorhome size? Roughly what sized Reynolds number is involved?

Whatever I build is going to be 'homebuilt', and I haven't been planning on having to get it mirror smooth! Motorhomes/caravans have windows on the sides, which involves surface irregularities of a few mm. I'm planning polycarbonate sealed solar panels on the top, which are of about 3mm thickness (1/8"). I was just planning on gluing them on as is, leaving the 3mm vertical edge around each panel.

Will these kind of surface imperfections have no/little effect, or be beneficially tripping the boundary layer and causing minor turbulence that assists laminar flow beyond that (like a golf ball, or slide 20 in that presentation link above), or just destroy laminar flow and mess up aero properties?

Even a Bonneville Salt Flats where there are smooth shiny finishes they still use duct tape as needed.

With a 1/8" step, maybe a bead of caulk to fillet the edge?

how sensitive
 

The mechanism which rules the transition from a laminar,to,turbulent boundary layer,occurs at a scale smaller than the surface roughness of glossy paint.
It's called 'critical surface roughness' and there's nothing to be done about it.
Even on a Messerschmitt fighter aircraft,the difference between a polished surface and flat paint makes no difference to performance.
On a laminar airfoil it would,but only at flight conditions,at very high distance from the ground,in calm air.
You won't have any laminar boundary layer on your vehicle,and that's actually a good thing.
Minor surface defects may be embedded within the thickness of the boundary layer,not affecting the laminar inviscid flow outside the boundary layer.
For the hard edge on the PVs,you could soften that with caulk.

Trailers might have some advantages, but I still believe a small motorhome might not turn into an exercise of gross excess all-year round. Assuming that you could get a pop-up roof to keep the overall height contained while driving, it would already decrease the overall drag. When it comes to interior arrangements, you could eventually attempt to make it easily reconfigured from campervan to cargo (even though some structures would require a more permanent or semi-permanent fitment such as the washroom and the kitchen).

Yes, I figure I can drive this to work each day, on electric power only. 2kW of panels on the roof is easily enough to fully recharge the batteries even if its cloudy - it is only a 50km round trip (25km each way).

Been a while since I posted here - I ended up looking at all those original plans/concepts I've drawn up, and realising that they were still all inferior to the Vixen, which is a 30 year old design. It had lower profile (1.8m high), good Cd, fairly low weight and good space/usability as a motorhome. Not available here in Australia though.

So I've been looking at ideas to actually advance/progress on past products/solutions. These include:

- a frame/body design that allows the floor of the motorhome to be very low to the ground (which means the roof profile can be correspondingly lower). The floor of most motorhomes is 0.8m or more above ground, which is very inefficient.

- A section of drop down flooring (rather than pop top roof). Just the main isle, which is also where you stand when at the kitchen. That way the solar panels on the roof are less affected, and I think sealing is easier too.

- adjustable height suspension, via airbags. Low for highway aerodynamics, medium for around town (sufficient clearance to ground) and high for offroad.

- all electric, very low Cd.

I've worked out a lot of the design concepts. The main problem is that this is now a completely custom vehicle- would have to be built as an ICV (Individually Constructed Vehicle). Which means an incredibly painful and expensive process to get approved and registered. Also a huge amount of design and construction work. I'm currently looking into those issues and considering if I want to take it on or not.

Not sure about it being easier to seal. But it actually sounds quite interesting to say the least, even though it might not be so easily doable without any compromise to the structural rigidity.

To provide a bit of an update, I ended up working on the motorhome option, as I think that suits our intended use better, and I want offroad capability (to get to many parts of Australia we haven't seen). Whilst starting from a donor vehicle would be preferable (and considerably easier), there is very little (nothing!) in the way of aerodynamic 4x4 motorhomes (or even suitable base 4x4 vehicles) in Australia. The 4x4 mercedes sprinter van is probably the closest. So I did a fair bit of looking at various concepts, and developed a preferred one.

But first, to skip to the conclusion, I've put this project on hold for 3-5 years. Decided that now isn't the right time to be putting a huge amount of effort and money into building a custom vehicle. Hopefully in several years time I'll be able to take this up again, though I think registration/roadworthy requirements (red tape) are only going to get more difficult as time goes on.

I designed a rigid centre tube (RHS) chassis to act as a structural backbone for the whole vehicle. Similar concept to Tatra trucks. Not fully detailed, but it checks out well in FEA analysis. Custom independent air bag suspension on all 4 corners. Approx 450mm of wheel travel (max for CV joints), and airbags make it height adjustable to optimise for road (low and aerodynamic) or offroad (lifted for clearance) driving.

Two electric motors, one for front, one for rear. Motors, gearboxes and diffs (two 'rear' diffs) are actually contained within the large RHS frame (flanged top surface). Gearbox are a small custom design, just to get low range for offroad use (ie. 2 speed, no clutch). A number of different motor options; Parker make some nice ones for example. Because of the offset middle part of the chassis, can get some cooling air flowing through the RHS pieces containing the motors and diffs.

Another main reason for the offset RHS tube in the middle is to limit vehicle height - the floor for the kitchen/walkway will be beside and lower than the top of the RHS chassis. With the floor of the standing part of the motorhome also forming the underside of the body, overall vehicle height can be kept at around 2.1m with standing room inside (when air bags fully lowered). Planned an almost flat underbody, just the centre RHS tube (but not cross beams) sticking down 50-100mm or so, and there will need to be cut-outs/slots for suspension arms to move up and down past floor pan. That centre part of the chassis will also act as a water tank.

Considered a monocoque frame, and yes it could probably be done a little lighter that way, but construction would be significantly more complicated and involved (especially considering I have very little practical experience in composites, and it would be critical to get that right). Also the state of the structure would be more difficult to assess, monitor and repair (remembering this is an offroad vehicle).

For the cab, plan to graft the front part of a Renault Trafic, or possible Ford Transit Custom, onto the frame (just the cab, not including drive, suspension, wheels etc). Using the already approved windscreen, doors, seats etc would help a lot in meeting local roadworthy/registration requirements.

There are lots of custom features in this, and whilst none of them are individually overly difficult, put together it is a huge amount of work. In the meantime I am going to improve the aero performance of my box shaped homebuilt camper/caravan! (some pics in here: From Here to There | Around Aus)

For now, hopefully this discussion helps trigger design ideas for other people. Thanks to those who contributed and provided advise in the thread. If all goes well I'll revive it in several years time :D


http://www.aroundaus.com/wp-content/...18/04/side.jpg

http://www.aroundaus.com/wp-content/...lan_layout.jpg

http://www.aroundaus.com/wp-content/...04/chassis.jpg

http://www.aroundaus.com/wp-content/...8/04/front.jpg

http://www.aroundaus.com/wp-content/.../04/strain.jpg

1. Looks like a good van to start with.

2. I'm always shocked when some of the from scratch vehicle costs are listed.

3. Back to Ecomodding verses from scratch eco-design, the reason this forum exists. :)

Most of the sketches I've done on a similar topic is converting my 8'x12' flat snowmobile trailer into an aero-camper. The main feature is a pop-up roof like a VW van camper with the front edge hinged.

I have an alternate design with full height rear entry but it is very narrow at the tail end. Fewer moving parts and the simplicity makes me lean in that direction - sort of a Box Fish.

https://www.greencarreports.com/news...economy-record

Article about an attempt to set a cross country fuel efficiency record for a semi truck haul freight cross country.

More like flashbacks, really. :)

http://ecomodder.com/forum/member-fr...1396746884.jpg

Section your Forester at the top of the wheelwells and graft a Vanagon body on 6-8" above the original floor level. Put sliding drawers in the gap for jacks and wrenches.

Don't forget the Hello Kitty mascot.
________

I think kach22i is trying to steal my idea:

http://ecomodder.com/forum/member-fr...17-trdrp1a.jpg

Good on him. More people need to do that.

http://ecomodder.com/forum/member-fr...56-aerobug.png

I’m late to the party.

I prefer (and forty-five years of experience tells me) that for lowest overall cost (as fuel is but one part; generally half) drivetrain separate from RV works best. Why? Majority of hours of use are ALWAYS when parked. The right trailer can last decades. The motive power can be changed as desired.

Building new looks great on paper. But years go by. Even an non- extensive renovation can be two plus years given a working man’s available hours.

Get something that will re-sell quickly. That should be focus (in my opinion) while your boys are still at home. (Can take six months or more to sell a nice RV at a fair price in USA). So whatever the choice of type, get the brand in demand. This removes the end of ownership pains.

Good luck!!

Sounds pretty accurate, but it's too hard (and expensive) to find a trailer suitable to tougher off-roading conditions, which looks like a top-tier priority in this project.

Not in his country. They’ve plenty. Purpose-built.

https://1.bp.blogspot.com/-AQd52imxN...s640/ewgwe.JPG
http://justacarguy.blogspot.com/2018/04/did-you-know-that-u-haul-got-into.html

Find one of these. Ship it to Oz-tralia.

I have a nice new RV and previously had something more MPG friendly the Hi-Lo, but my last trip was in a tent. Now don't go thinking some confined dome thing, I'm talking 12x15 circus style 10oz flame/marine canvas from Reliable Tent and Tepee in Montana. I have a wood stove for it and a XL cot with memory foam pad. Me and a 13 year old set it all up in about 20 mins and it takes about that long to tear down and pack. It was under $1000 with shipping during g the winter sale and I love it. It is so airy inside with a 13' peak and dry and warm with the stove going. I like to cook outside even with the fancy new camper, i the weather looks bad I'll set up a tarp cookshack. All I need now is a dry aeroshell for the 8' bed of the pickup to store everything. My wife like the camper, so we will still use that, but I think I'm a wall tent guy now.

https://c1cleantechnicacom-wpengine....ving-tents.jpg
https://cleantechnica.com/2013/03/24/army-energy-efficiency-goals-lead-to-new-energy-saving-tents/

This place is out sort of by you, Greys Harbour, Wa. SingCore | Warp-free, lightweight honeycomb panels and door cores
This is what I wanted to make my camper out of but they sell a lightweight AFrame shelter that sets up easy with just a few panels.

It's an interesting product.

So it's an egg-crate with EPS foam infill. One could order custom triangles with a thick edge then rip them to the proper angles to make a geodesic dome. Hmm.....

And they can do any metal exterior, and wood interior combo.

Aluminum/Birch plywood would be retro-cool.

Yes, that applies in many situations, perhaps including my own right now! If only using it for a few weeks a year, its some expensive machinery to sit unused the majority of the time.

I'll add a few 'buts' though, which help to mitigate this:
- I planned to drive the electric motorhome to work and back each day. So it would take the place of an ICE vehicle. Overkill for commuting, yes, but at least not sitting idle from months.
- it makes sense when thinking about regularly spending long periods travelling, as we did for 6 months in 2014.
- during our around Australia trip in 2014, fuel was our single biggest expense (and as far as motorhomes go, it was quite fuel efficient). Fuel costs completely dwarfed depreciation of the motorhome on resale.

True, there are plenty to choose from. But made in low volumes, so they charge an arm and a leg...

For now I'll stick with the homemade camper/caravan/RV. Whilst it doesn't have all the comforts of commercial ones, and our existing tow vehicles can't get us everywhere we might want to go, we'll manage using it for a few more years and then re-assess.

Interesting - haven't seen SIP cores like that, with ply providing the shear strength. More similar to traditional building techniques of timber frame and insulation infill, but at a smaller scale and in a pre-packed core type arrangement. Also brings back the risks of moisture damage though, which common SIP panels avoid.

Motorhome or trailer, if it has a rear entry and it's parking spot is backed up to a porch, then it becomes an extension of the [more] permanent structure.

http://ecomodder.com/forum/member-fr...2-p1010018.jpg