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Old 12-28-2020, 08:29 AM   #1
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Motorhome Aerodynamics

Sitting here drinking my morning coffee and thinking back over our 1800 mile journey from Pennsylvania to South Padre Island Texas. We averaged 6.5 mpg in our 2020 FR3 30DS towing our Jeep Wrangler.
As a retired engineer I notice things such as architecture and the method other vehicles on the road are designed. The automobile industry including, the trucks that pull the 53 foot trailers, have made great strides in designing aerodynamic features into their product. Many 5th wheel trailers and Class C motorhomes are designed with aerodynamics in mind.
What about the Class A motorhome? There seems to be no effort by the RV industry to apply aerodynamics to these RVs. The majority are square flat front with little consideration to aerodynamics. In our age of computer simulation it seems to me the industry could put more effort into engineering the front of the Class A motorhome. Maybe an increase in fuel economy due to better aerodynamics would increase motorhome sales.
So now my coffee and thinking are done for today You all be safe and have a great day.
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Old 12-28-2020, 08:36 AM   #2
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If you want a motorized accommodation of the size offered in Class A motorhomes, they have a solution already.

They’re called Super C chassis units, and are front end designed and powered by the same motors as the transport trucks.

They’re big and not cheap.

Just sayin’.
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Old 12-28-2020, 08:40 AM   #3
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Class A Motorhomes have actually gone backwards in aerodynamics, if you check out many older motorhomes they where much more streamlined and not as tall.

Cost cutting and space maximization has reverted them to boxes and made them taller, which has also raised their center of gravity leading to worse handling especially on the Ford chassis. This year Ford finally increased the sway bar diameter to compensate for the much higher COG to go along with the new engine.

I get about the same mileage as you with my Jeep, would like a more streamlined diesel for my next MH to maximize range and fuel economy, perhaps a super C, however you definitely sacrifice living space to get a more streamlined front end. Gas is cheap and I am no hurry, but the engineer in me has a hard time looking at the shape of my MH, even just a little less hard angles front and back would significantly improve aerodynamics while barely impacting interior volume, but it would cost more to make.

A really cool MH was the Monaco Vesta / Holiday Rambler Trip, they did some wind tunnel design on it, looked cool, got decent mileage, was not cheap and hampered by a bad ecomomy and the Navistar engine that was doing all sorts of EGR and other things to avoid going DEF for EPA regs. Wish they would bring that one back with a cummins and a bunk model.
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Old 12-28-2020, 08:45 AM   #4
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I’ve often thought this and wondered why the roof mounted items and sides of a motorhome are so obviously poorly integrated from a aerodynamic point of view. Seems like such a no-brainer. The only thing I’ve come up with is that perhaps it would make too little improvement to bother with.

Manufacturers just tack on anything they want to the exterior of the box willy nilly. Like awning legs and arms, non-flush slides, etc.
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Old 12-28-2020, 09:07 AM   #5
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I think our old 2002 Monaco Windsor is more aerodynamically clean than most of the new models!
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Old 12-28-2020, 09:12 AM   #6
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If you use space (length) to accommodate the aerodynamics, you loose living space in the same length of coach. People would rather have the living space than gain another MPG.
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Old 12-28-2020, 09:20 AM   #7
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The bottom line is aerodynamic considerations really don't matter on the typical class A motorhome, for myriad reasons. Manufacturers figured it out a long time ago and stopped trying.

Some of the reasons include:
  1. The single most important feature of a motorhome is usable space. The most efficient way, in terms of cost/cubic foot, to create usable space is a rectangular box. Unfortunately, the rectangular box has the aerodynamic efficiency of a brick.

  2. To meaningfully alter the aerodynamic efficiency of a rectangular box requires trading usable space for said efficiency. Think turning your motorhome into something that looks more like a passenger airliner. Much more aerodynamic than the box, but much less usable space per dollar.

    Some manufacturers tried kind of in-between designs - remember the original Airstream and GM motorhomes? They didn't catch on.

  3. If you accept that space is the most important requirement, then you are stuck with the box and limited to small things that you could do to provide small, incremental improvements to aerodynamics. Unfortunately the changes tend to be costly, and/or the efficiency improvements very small making them economically questionable.

  4. Motorhomes, unlike over-the-road trucks, aren't really driven anywhere. Some motorhomes may put 100,000 miles on the odometer in their lifetime, but the majority will never reach anywhere near that number. Contrast that with a semi tractor that may put on a million or more miles in it's 10 to 15 year useful lifespan.

    Small efficiency improvements over the 50,000 or 60,000 miles a motorhome may travel in it's lifespan will save a small amount of fuel compared to the same improvements over the 1,000,000 miles the truck will move. It makes sense to make even small improvements to the semi; not so much to the RV.

  5. When folks ask about aerodynamics they are usually really interested in fuel consumption. While major improvements are not possible, there are several things that can be done to reduce fuel consumption.

    The first is to simply slow down. Air resistance is the the biggest variable in the complex equation that determines fuel efficiency. 5 mph can make a noticeable difference. Second, reduce rolling resistance. Keep your tires properly inflated for your coach's weight. And lastly, keep it light. The less you're trying to move, the less fuel you'll burn to move it.

Regards,

Randy
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Old 12-28-2020, 09:45 AM   #8
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The Monaco Vesta was designed to be aerodynamic. It's still a more or less square box. All that is needed is a little rounding at the front, and some taper at the rear to make it MUCH better than a square edged box.

Sadly, Monaco/Navistar never reported actual wind tunnel numbers.
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Old 12-28-2020, 09:49 AM   #9
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Quote:
Originally Posted by groswald View Post
The bottom line is aerodynamic considerations really don't matter on the typical class A motorhome, for myriad reasons. Manufacturers figured it out a long time ago and stopped trying.

Some of the reasons include:
  1. The single most important feature of a motorhome is usable space. The most efficient way, in terms of cost/cubic foot, to create usable space is a rectangular box. Unfortunately, the rectangular box has the aerodynamic efficiency of a brick.

  2. To meaningfully alter the aerodynamic efficiency of a rectangular box requires trading usable space for said efficiency. Think turning your motorhome into something that looks more like a passenger airliner. Much more aerodynamic than the box, but much less usable space per dollar.

    Some manufacturers tried kind of in-between designs - remember the original Airstream and GM motorhomes? They didn't catch on.

  3. If you accept that space is the most important requirement, then you are stuck with the box and limited to small things that you could do to provide small, incremental improvements to aerodynamics. Unfortunately the changes tend to be costly, and/or the efficiency improvements very small making them economically questionable.

  4. Motorhomes, unlike over-the-road trucks, aren't really driven anywhere. Some motorhomes may put 100,000 miles on the odometer in their lifetime, but the majority will never reach anywhere near that number. Contrast that with a semi tractor that may put on a million or more miles in it's 10 to 15 year useful lifespan.

    Small efficiency improvements over the 50,000 or 60,000 miles a motorhome may travel in it's lifespan will save a small amount of fuel compared to the same improvements over the 1,000,000 miles the truck will move. It makes sense to make even small improvements to the semi; not so much to the RV.

  5. When folks ask about aerodynamics they are usually really interested in fuel consumption. While major improvements are not possible, there are several things that can be done to reduce fuel consumption.

    The first is to simply slow down. Air resistance is the the biggest variable in the complex equation that determines fuel efficiency. 5 mph can make a noticeable difference. Second, reduce rolling resistance. Keep your tires properly inflated for your coach's weight. And lastly, keep it light. The less you're trying to move, the less fuel you'll burn to move it.

Regards,

Randy
Randy,

You nailed it!

Take care,
Stu
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Old 12-28-2020, 10:17 AM   #10
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The best way to improve MPG is to slow down and always drive with a tail wind.
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Old 12-28-2020, 10:18 AM   #11
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Stu,

Thanks.

I'll be really glad when we can get back on the road instead of sitting here at the computer wishing we could get back on the road and writing long-winded posts...



Regards,

Randy
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Old 12-28-2020, 10:19 AM   #12
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I figure I can either buy a tent and a fuel efficient car, or buy a motorhome. There are trade offs for everything. Just gotta decide which is more important. Just dont see getting the aerodynamics out of a square box, because in the end, thats what we want, a wide open feeling with a wall for fireplaces. I'd be tickled with 10MPG, but 8 is what it is for me. I suppose I could drive 55 and might find that mileage, but I also want to get there. Its that trade off thing again
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Old 12-28-2020, 10:33 AM   #13
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Quote:
Originally Posted by tderonne View Post
The Monaco Vesta was designed to be aerodynamic. It's still a more or less square box. All that is needed is a little rounding at the front, and some taper at the rear to make it MUCH better than a square edged box.
Yep They are one of the few that has some effort put into a taper at the rear, the front is significantly more sloped than most modern Class A's. Also armless awnings and low profile a/c's all help a little. I remember owners reporting 10-11 mpg with them even with the Navistar Maxxforce. I bet it could do better with a DEF based Cummins B series.

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Old 12-28-2020, 10:53 AM   #14
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With all the technology already developed in the aerospace industry I've often wondered why they couldn't add equipment and sensors that would "trim" the MH so that it wouldn't be as affected by outside influences, like cross winds and altitude changes.

Aerodynamics aside, it seems nobody considers the total package when units are designed or what the final package performance will be like. It seems more a word of mouth, a tribal knowledge kind of thing, when shopping for a MH. Like you said earlier, you buy the inside space and then learn what it's capable of doing as you go along. Fuel mileage is half the battle. The other half is the driver's experience in getting there.
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