Class A motorhomes - aerodynamics of a brick

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snackdog
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Class A motorhomes - aerodynamics of a brick

Post by snackdog »

Class A motorhomes are shaped like a bus (photo below). The front is typically a massively tall and flat grill and windshield which is basically a huge non-aerodynamic slab pushing fuel efficiency on these rigs into the toilet (typically well under 10 mpg). A symmetric rear end is set up to maximize wind drag.

My question is WHY? When was the last time you saw a cabover semi-tractor? They don't make them anymore instead favoring long-nosed conventional designs with all sorts of aerodynamic features on the front, type, sides, bottom and even the back of the trailer. An 18-wheeler towing 80,000 pounds gets about the same MPG as many motorhomes (around 7 mpg).

I get that people like the extra space in the Class A motorhome but the designs are extreme with apparently ZERO effort towards efficiency. Even the old Class As from the 70s (Airstream, GMC) were more rounded on the front and rear. What's the problem?

Has anyone modified their Class A to reduce drag and increase MPG?

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Re: Class A motorhomes - aerodynamics of a brick

Post by mkc »

They make them on such a truck chassis. They are popular with the auto racing crowd thanks to their towing capacity.

Super C on a class 8 tractor chassis:

https://www.renegaderv.com/rvmodel/2022-renegade-ikon/

https://www.showhauler.com/motorhomes/


Class A's basically start as a purchased lower rail frame, power plant, and running gear. The house box, including the front area, is built entirely by the RV builder.

Gasser front engine chassis https://www.ford.com/commercial-trucks/ ... motorhome/

Typical diesel pusher chassis https://www.fcccrv.com/chassis/xc/
Last edited by mkc on Thu Sep 22, 2022 8:23 am, edited 3 times in total.
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Re: Class A motorhomes - aerodynamics of a brick

Post by 7eight9 »

snackdog wrote: Thu Sep 22, 2022 7:52 am When was the last time you saw a cabover semi-tractor? They don't make them anymore instead favoring long-nosed conventional designs with all sorts of aerodynamic features on the front, type, sides, bottom and even the back of the trailer. An 18-wheeler towing 80,000 pounds gets about the same MPG as many motorhomes (around 7 mpg).
Last week I saw (for the first time) a Nikola TRE --- https://nikolamotor.com/tre-bev

But I do take your point.
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Re: Class A motorhomes - aerodynamics of a brick

Post by David Jay »

The flat back end of the vehicle is the biggest drag, the front end is secondary.

Without getting deep into the aerodynamics, look at commercial airliners. They have a big, relatively blunt nose but always taper to a fine tail.

Those “tails” on the back of semi trailers are a step in the right direction, but drivers don’t like the hassle and leave them folded up most of the time. I have considered creating such a device for motor homes and travel trailers but I am convinced that the education curve is too big for this old guy to overcome. Everybody wants something that is “slope-ey” on the front, so the industry gives the customers what they want and builds these things, which are exactly backwards from an aerodynamic standpoint:

Image
Last edited by David Jay on Thu Sep 22, 2022 8:49 am, edited 2 times in total.
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Re: Class A motorhomes - aerodynamics of a brick

Post by Jack FFR1846 »

It's marketing against competitors. Say you decide you want to buy one of these busses. What do you do first? Decide how much interior room you want. Then you look at vehicles with this interior space. With aerodynamic features, interior space is carved away and it becomes quite significant. So the RV maker has less sales because the class vehicle they are selling is competing with the smaller square box that has the same interior space. MPG is just a number until you start driving thousands of miles and paying for fuel.
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Re: Class A motorhomes - aerodynamics of a brick

Post by snackdog »

mkc wrote: Thu Sep 22, 2022 8:16 am They make them on such a truck chassis. They are popular with the auto racing crowd thanks to their towing capacity.

Super C on a class 8 tractor chassis:

https://www.renegaderv.com/rvmodel/2022-renegade-ikon/

https://www.showhauler.com/motorhomes/


Class A's basically start as a purchased lower rail frame, power plant, and running gear. The house box, including the front area, is built entirely by the RV builder.

Gasser front engine chassis https://www.ford.com/commercial-trucks/ ... motorhome/

Typical diesel pusher chassis https://www.fcccrv.com/chassis/xc/
$780,000 for a truck-based "more aerodynamic" 40,000 pound Class C motorhome that still gets under 10 mpg? Holy cow.
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Re: Class A motorhomes - aerodynamics of a brick

Post by snackdog »

7eight9 wrote: Thu Sep 22, 2022 8:20 am
snackdog wrote: Thu Sep 22, 2022 7:52 am When was the last time you saw a cabover semi-tractor? They don't make them anymore instead favoring long-nosed conventional designs with all sorts of aerodynamic features on the front, type, sides, bottom and even the back of the trailer. An 18-wheeler towing 80,000 pounds gets about the same MPG as many motorhomes (around 7 mpg).
Last week I saw (for the first time) a Nikola TRE --- https://nikolamotor.com/tre-bev

But I do take your point.
Would love to see some on the road. But with a max range of 350 miles I'm not sure the long haul truckers flock to them. Most of those guys average 600 miles per day. I guess with a two-hour lunch break to eat and take a nap while recharging they could make sense.
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Re: Class A motorhomes - aerodynamics of a brick

Post by snackdog »

David Jay wrote: Thu Sep 22, 2022 8:29 am The flat back end of the vehicle is the biggest drag, the front end is secondary.

Without getting deep into the aerodynamics, look at commercial airliners. They have a big, relatively blunt nose but always taper to a fine tail.

Those “tails” on the back of semi trailers are a step in the right direction, but drivers don’t like the hassle and leave them folded up most of the time. I have considered creating such a device for motor homes and travel trailers but I am convinced that the education curve is too big for this old guy to overcome. Everybody wants something that is “slope-ey” on the front, so the industry gives the customers what they want and builds these things, which are exactly backwards from an aerodynamic standpoint:

Image
Maybe someone should sell a snap-on fiberglass tail cone for RVs? Better yet, an inflatable one that could be deflated when parked?
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Re: Class A motorhomes - aerodynamics of a brick

Post by homebuyer6426 »

They're running into a legal length limit somewhere, right? So if the engineers make it more aerodynamic, they're going to be giving up storage space. When your legal limits for vehicle dimensions are a rectangular prism, a box becomes the most space-efficient shape. So maybe they believe the RV owners are more interested in space than MPG.
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Re: Class A motorhomes - aerodynamics of a brick

Post by NHRATA01 »

I figured it was a combination of getting as much interior space for the length, and also giving people who aren't quite expert semi-truck drivers (I don't think you even need a CDL for them?) the best view forward possible so they don't run the rest of us over.
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Re: Class A motorhomes - aerodynamics of a brick

Post by BrooklynInvest »

Having spent a lot of time on highways I'd have to think the other issue with the pointy rear end is what happens when someone runs into the back of the motorhome?
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Re: Class A motorhomes - aerodynamics of a brick

Post by RetiredAL »

Cab-overs were to maximize cargo space when the trucking reg's limited the truck's length. When the reg's were changed to allow longer lengths, cab-overs went away. Cab-overs were a PIA for maintenance and gave poor driver comfort.

Class A's want to maximize the inside space while keeping to a given length. Hence the box shape.
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Re: Class A motorhomes - aerodynamics of a brick

Post by mkc »

NHRATA01 wrote: Thu Sep 22, 2022 9:03 am I figured it was a combination of getting as much interior space for the length, and also giving people who aren't quite expert semi-truck drivers (I don't think you even need a CDL for them?) the best view forward possible so they don't run the rest of us over.
No CDL, unless you're paid to drive it (RV driver-for-hire) or it's used for business. It's state-specific, but many states require an exempt class B if it's over 26,000 lbs GVWR or exempt class A if you're pulling more than 10,000 lbs. Air brake endorsement if it has air brakes.
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Re: Class A motorhomes - aerodynamics of a brick

Post by David Jay »

snackdog wrote: Thu Sep 22, 2022 7:52 amHas anyone modified their Class A to reduce drag and increase MPG?
The problem is that the aerodynamics approximates (cross-section x drag coefficient x speed squared). Based on a given cross-section (you can’t really modify your Class A to make it shorter or narrower), all you can do is reduce the drag coefficient or reduce speed. Since speed is squared in the equation, reducing speed will provide a much bigger reduction in fuel consumption than any aerodynamic modifications.

A 5 MPH speed reduction will likely save more in fuel than aerodynamic mods. For truckers, trailer skirts and aero “tails” may make sense because of the time-sensitive nature of their costs. But for a recreational user it probably doesn’t make sense.

I evaluated this issue at one time. Let’s say I could increase your fuel economy by a half mile per gallon at 70 MPG by aerodynamic modification. The mods will cost $5000. What is your payback period? How many miles do you drive your motorhome per year?

By contrast, you can probably get more than a half mile per gallon increase by slowing from 70 to 65 MPH. And in the real world, that will extend your driving day by about 28 minutes if you are planning to cover 400 miles a day.
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Re: Class A motorhomes - aerodynamics of a brick

Post by David Jay »

snackdog wrote: Thu Sep 22, 2022 8:54 amMaybe someone should sell a snap-on fiberglass tail cone for RVs? Better yet, an inflatable one that could be deflated when parked?
That’s exactly what I evaluated.
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Re: Class A motorhomes - aerodynamics of a brick

Post by galawdawg »

Owning a Class A is a lifestyle choice and is often more costly than many travel alternatives. Class A owners, as a general proposition, are financially independent with more disposable income than many. You frequently will see comments on RV forums to prospective Class A owners that if you have to ask what the fuel economy is on a Class A rig, you probably can't afford one. Plus Class A owners are not traveling tens of thousands of miles a year like those who drive commercial trucks and buses. Most Class A owners travel under 10,000 miles per year in their RV.

So from a financial standpoint, the MPG on a Class A and the price of diesel fuel is not going to make or break Class A owners whose rigs run anywhere from $100k for a nice pre-owned but vintage model to several million for a new Newell or other high-end coach.
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Re: Class A motorhomes - aerodynamics of a brick

Post by smitcat »

BrooklynInvest wrote: Thu Sep 22, 2022 9:03 am Having spent a lot of time on highways I'd have to think the other issue with the pointy rear end is what happens when someone runs into the back of the motorhome?
Not really a consideration at all.
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Re: Class A motorhomes - aerodynamics of a brick

Post by nhs76 »

Idle thought: I wonder how many investors stress about miniscule differences in bid/ask spreads on their securities and then buy a motorhome, which has a bid/ask spread of... :oops:
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Re: Class A motorhomes - aerodynamics of a brick

Post by quantAndHold »

snackdog wrote: Thu Sep 22, 2022 7:52 am My question is WHY?
People shop for class A motorhomes like they’re shopping for a home. Gas mileage is much less important than the size of the bedroom and whether or not it has a washer/dryer and how big the TV is. This is because people spend a lot more time living in them than they do driving them. Manufacturers know this, and design accordingly.

People that care about gas mileage get smaller rigs.
Yes, I’m really that pedantic.
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Re: Class A motorhomes - aerodynamics of a brick

Post by jumppilot »

galawdawg wrote: Thu Sep 22, 2022 9:49 am
So from a financial standpoint, the MPG on a Class A and the price of diesel fuel is not going to make or break Class A owners whose rigs run anywhere from $100k for a nice pre-owned but vintage model to several million for a new Newell or other high-end coach.
Exactly.

Let’s say you drive your Class A 10,000 miles a year and pay $4,500 in gas. Even if you double fuel economy it isn’t going to have a measurable impact on what the lifestyle costs or why people get into it.
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Re: Class A motorhomes - aerodynamics of a brick

Post by IowaFarmBoy »

I'm guessing there is a big difference in the average miles per day a truck is driven vs a motorhome. I'd guess most motorhomes only make a few long trips a year. Our friends who recently sold their house to live the RV lifestyle plan to stay in each new location for a few weeks.
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Re: Class A motorhomes - aerodynamics of a brick

Post by mkc »

It's been a while since I looked at the data, but in the past 7,000 miles was used (insurance, evaluating used units) as the average annual mileage of a recreational vehicle. We typically spend several months total on the road, and our average over 22 years is around 6,000 miles per year. The only year we topped 10,000 was going from Texas to Maine, then Michigan, then back to Texas, and then to Santa Fe, NM and back (plus a couple of 400 mile round trip service visits).
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Re: Class A motorhomes - aerodynamics of a brick

Post by Sprucebark »

snackdog wrote: Thu Sep 22, 2022 7:52 am When was the last time you saw a cabover semi-tractor? They don't make them anymore instead favoring long-nosed…
Cab over trucks are still used. It’s mostly use to length restrictions. Sometimes with really long loads (like specialized poles or slabs) you would need to use a cab over. It would be state specific. School buses are like that too- some have the nose in the front, and some are just a big rectangle (usually with the engine in back). In tight spaces the big rectangle will be easier to navigate.

If you go to Europe nearly 100% of the heavy trucks on the road are cab overs. It’s easier to get around tight spots without that big nose sticking out.

Looks like Freightliner used to make cab overs in the US until two years ago-
https://en.m.wikipedia.org/wiki/Freightliner_Argosy
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Re: Class A motorhomes - aerodynamics of a brick

Post by Stinky »

If you need to be concerned about the fuel economy of a Class A motorhome, you're not wealthy enough to own one.
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Re: Class A motorhomes - aerodynamics of a brick

Post by snackdog »

jumppilot wrote: Thu Sep 22, 2022 11:06 am
galawdawg wrote: Thu Sep 22, 2022 9:49 am
So from a financial standpoint, the MPG on a Class A and the price of diesel fuel is not going to make or break Class A owners whose rigs run anywhere from $100k for a nice pre-owned but vintage model to several million for a new Newell or other high-end coach.
Exactly.

Let’s say you drive your Class A 10,000 miles a year and pay $4,500 in gas. Even if you double fuel economy it isn’t going to have a measurable impact on what the lifestyle costs or why people get into it.
If you drive 10,000 miles at 7.5 mpg, that is 1,333 gallons. At the current average diesel price of $4.90/gal that is $6,500 per year. I read on RV forums about people reducing their annual travel due to the current high prices. It would seem if they could have a more aerodynamic rig that could make the same difference. And many quite wealthy people are also quite frugal. Bogelheads!
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Re: Class A motorhomes - aerodynamics of a brick

Post by CascadiaSoonish »

galawdawg wrote: Thu Sep 22, 2022 9:49 am ... to several million for a new Newell or other high-end coach.
I didn't believe this until I Googled it. Found a 2019 for sale for $1.8M that looks like a $150/night hotel out by the airport.
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Re: Class A motorhomes - aerodynamics of a brick

Post by hicabob »

Sprucebark wrote: Thu Sep 22, 2022 12:42 pm
If you go to Europe nearly 100% of the heavy trucks on the road are cab overs. It’s easier to get around tight spots without that big nose sticking out.
A truck driver told me that "you always get to the scene of an accident first if you are in a cabover".
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Re: Class A motorhomes - aerodynamics of a brick

Post by Northern Flicker »

RetiredAL wrote: Thu Sep 22, 2022 9:20 am Cab-overs were to maximize cargo space when the trucking reg's limited the truck's length. When the reg's were changed to allow longer lengths, cab-overs went away. Cab-overs were a PIA for maintenance and gave poor driver comfort.
They also were much less safe for the driver, who would be sitting much closer to the most common point of impact in a collision.
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Re: Class A motorhomes - aerodynamics of a brick

Post by quantAndHold »

CascadiaSoonish wrote: Thu Sep 22, 2022 9:54 pm
galawdawg wrote: Thu Sep 22, 2022 9:49 am ... to several million for a new Newell or other high-end coach.
I didn't believe this until I Googled it. Found a 2019 for sale for $1.8M that looks like a $150/night hotel out by the airport.
I’ve never understood why the aesthetics on so many RVs, even really expensive ones, are straight out of 1986.
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Re: Class A motorhomes - aerodynamics of a brick

Post by onourway »

Stinky wrote: Thu Sep 22, 2022 12:57 pm If you need to be concerned about the fuel economy of a Class A motorhome, you're not wealthy enough to own one.
There are plenty of people wealthy enough to own one who would also prefer to see their efficiency improved. I suspect that at the moment, most of those people are driving much smaller units, in part because they are more versatile, and in part because they are (modestly) more efficient.

I expect that like the rest of the transportation industry, these will be radically changed in the coming decade by battery electrics. If what people say here is true, that these are driven low mileage by most owners, the vehicle will become orders of magnitude better as a BEV. Cheaper to operate, better driving, capable of supporting extended stays off the grid in comfort, easily rechargeable with the existing 240v RV park infrastructure, etc.
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Re: Class A motorhomes - aerodynamics of a brick

Post by Wanderingwheelz »

quantAndHold wrote: Thu Sep 22, 2022 10:28 am
snackdog wrote: Thu Sep 22, 2022 7:52 am My question is WHY?
People shop for class A motorhomes like they’re shopping for a home. Gas mileage is much less important than the size of the bedroom and whether or not it has a washer/dryer and how big the TV is. This is because people spend a lot more time living in them than they do driving them. Manufacturers know this, and design accordingly.

People that care about gas mileage get smaller rigs.
That’s not necessarily true. Why? Because people with “touring coaches” like the #1 selling Winnebago Travato Class B motorhome, often put 40,000 or more miles per year on them- especially if they’re “full-timers”. We owned one for about 18 months and put almost 30,000 miles on the odometer, achieving about 15 mpg on most trips (gas engine), which dropped significantly in the mountains.

MPG becomes far more important when the owner is touring (staying somewhere only a night or two and moving onto the next place), rather than driving from Virginia to Florida and staying parked for 3 months like many Class A owners. By contrast, the longest my wife and I ever stayed somewhere was 3 days and that was only once.
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Re: Class A motorhomes - aerodynamics of a brick

Post by just frank »

First, if somebody built a more aerodynamic motorhome, they would up the MSRP to the point it would be hard for the owner to recoup the savings.

Second, rolling friction is a large part of drag, and for a heavy weight motorhome, the savings from a perfect front and back end nose cone might be smaller than you think (and would make the vehicle 10-20' longer). The 'perfect' shape would be egg-shaped and both hard to build and very impractical (companies have started marketing egg-shaped jet aircraft that have 2X the fuel eff of conventional airliners). Those little add on's on semi-trailers DO work, but make only a marginal improvement to eff.

Third, efficiency is always a cost benefit analysis. Why not make a hybrid drivetrain to get better mpg instead of a nosecone? Wouldn't that achieve the same effect? The real answer is probably that most buyers of motorhomes do not consider fuel costs much in the total cost of ownership, compared to the upfront (and thus depreciation) costs.
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Re: Class A motorhomes - aerodynamics of a brick

Post by JoMoney »

I think people driving the beasts like the (relatively) unobstructed view looking down on the road.
mkc wrote: Thu Sep 22, 2022 8:16 am ... They are popular with the auto racing crowd ...
:D I thought that was going to be a joke about discussing the "aerodynamics" of these things. The punch-line left me realizing it was a serious comment
...thanks to their towing capacity.
Thanks for the laugh, and the information :beer
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Re: Class A motorhomes - aerodynamics of a brick

Post by smitcat »

Wanderingwheelz wrote: Fri Sep 23, 2022 5:30 am
quantAndHold wrote: Thu Sep 22, 2022 10:28 am
snackdog wrote: Thu Sep 22, 2022 7:52 am My question is WHY?
People shop for class A motorhomes like they’re shopping for a home. Gas mileage is much less important than the size of the bedroom and whether or not it has a washer/dryer and how big the TV is. This is because people spend a lot more time living in them than they do driving them. Manufacturers know this, and design accordingly.

People that care about gas mileage get smaller rigs.
That’s not necessarily true. Why? Because people with “touring coaches” like the #1 selling Winnebago Travato Class B motorhome, often put 40,000 or more miles per year on them- especially if they’re “full-timers”. We owned one for about 18 months and put almost 30,000 miles on the odometer, achieving about 15 mpg on most trips (gas engine), which dropped significantly in the mountains.

MPG becomes far more important when the owner is touring (staying somewhere only a night or two and moving onto the next place), rather than driving from Virginia to Florida and staying parked for 3 months like many Class A owners. By contrast, the longest my wife and I ever stayed somewhere was 3 days and that was only once.
"That’s not necessarily true. Why? Because people with “touring coaches” like the #1 selling Winnebago Travato Class B motorhome, often put 40,000 or more miles per year on them- especially if they’re “full-timers”"

I would live to see data that says folks put anywhere near 40,000+ miles per year on their RV's. Every article I have read has the average mileage at closer to 1/8th of that baseline - about 5,000 or so miles per year.
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Re: Class A motorhomes - aerodynamics of a brick

Post by snackdog »

Lots of good comments here.

One shapes, one needn't go full cone on the front and rear, just shave off the corners a bit. It was possible in the 70s (see below) but somehow has gone out of style.

Image

Image

Image
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Re: Class A motorhomes - aerodynamics of a brick

Post by Stinky »

snackdog wrote: Fri Sep 23, 2022 7:40 am
Image
I love this one!
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Re: Class A motorhomes - aerodynamics of a brick

Post by smitcat »

snackdog wrote: Fri Sep 23, 2022 7:40 am Lots of good comments here.

One shapes, one needn't go full cone on the front and rear, just shave off the corners a bit. It was possible in the 70s (see below) but somehow has gone out of style.

Image

Image

Image


Is there any data and/or studies indicating which designs affect fuel economy and at what speeds? Often it is not so clear which 'design' will actually yield a more fuel efficient result.
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Re: Class A motorhomes - aerodynamics of a brick

Post by JoMoney »

Stinky wrote: Fri Sep 23, 2022 8:04 am
snackdog wrote: Fri Sep 23, 2022 7:40 am ...
I love this one!
:thumbsup :D
Agreed! Although built from older parts, despite the comment it's not an invention of the 70's, rather something assembled somewhat recently:

https://www.autoevolution.com/news/the- ... 74506.html
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Re: Class A motorhomes - aerodynamics of a brick

Post by Wanderingwheelz »

smitcat wrote: Fri Sep 23, 2022 7:32 am
Wanderingwheelz wrote: Fri Sep 23, 2022 5:30 am
quantAndHold wrote: Thu Sep 22, 2022 10:28 am
snackdog wrote: Thu Sep 22, 2022 7:52 am My question is WHY?
People shop for class A motorhomes like they’re shopping for a home. Gas mileage is much less important than the size of the bedroom and whether or not it has a washer/dryer and how big the TV is. This is because people spend a lot more time living in them than they do driving them. Manufacturers know this, and design accordingly.

People that care about gas mileage get smaller rigs.
That’s not necessarily true. Why? Because people with “touring coaches” like the #1 selling Winnebago Travato Class B motorhome, often put 40,000 or more miles per year on them- especially if they’re “full-timers”. We owned one for about 18 months and put almost 30,000 miles on the odometer, achieving about 15 mpg on most trips (gas engine), which dropped significantly in the mountains.

MPG becomes far more important when the owner is touring (staying somewhere only a night or two and moving onto the next place), rather than driving from Virginia to Florida and staying parked for 3 months like many Class A owners. By contrast, the longest my wife and I ever stayed somewhere was 3 days and that was only once.
"That’s not necessarily true. Why? Because people with “touring coaches” like the #1 selling Winnebago Travato Class B motorhome, often put 40,000 or more miles per year on them- especially if they’re “full-timers”"

I would live to see data that says folks put anywhere near 40,000+ miles per year on their RV's. Every article I have read has the average mileage at closer to 1/8th of that baseline - about 5,000 or so miles per year.
Join some of the Class B Facebook groups and read how people are using their class B vans. They’re routinely going to Alaska, for example. 40,000 miles is definitely on the high side and I was by no means trying to imply that that’s anywhere close to the average. Not counting the people who buy them and find out it’s not their thing (leaving them parked), I suspect the average active Class B owner puts @ 15,000 miles on it per year. What’s the point of keeping it parked?

For us, 5,000 miles would be exceeded on some trips. We live a mile from the right coast, so to get to the other one and back rolls the odometer. There were several times we drove for more than 12 hours in a single day, which would be in the upper hundreds of miles. I just saw a 2 year old van listed for sale with 90,000 miles on it.

The Class B touring lifestyle isn’t the same thing as what you’re reading about on the internet. There’s no way I’d want an RV if my use case was to stay close to home, and many of the van buyers typically don’t either.
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Re: Class A motorhomes - aerodynamics of a brick

Post by smitcat »

Wanderingwheelz wrote: Fri Sep 23, 2022 10:57 am
smitcat wrote: Fri Sep 23, 2022 7:32 am
Wanderingwheelz wrote: Fri Sep 23, 2022 5:30 am
quantAndHold wrote: Thu Sep 22, 2022 10:28 am
snackdog wrote: Thu Sep 22, 2022 7:52 am My question is WHY?
People shop for class A motorhomes like they’re shopping for a home. Gas mileage is much less important than the size of the bedroom and whether or not it has a washer/dryer and how big the TV is. This is because people spend a lot more time living in them than they do driving them. Manufacturers know this, and design accordingly.

People that care about gas mileage get smaller rigs.
That’s not necessarily true. Why? Because people with “touring coaches” like the #1 selling Winnebago Travato Class B motorhome, often put 40,000 or more miles per year on them- especially if they’re “full-timers”. We owned one for about 18 months and put almost 30,000 miles on the odometer, achieving about 15 mpg on most trips (gas engine), which dropped significantly in the mountains.

MPG becomes far more important when the owner is touring (staying somewhere only a night or two and moving onto the next place), rather than driving from Virginia to Florida and staying parked for 3 months like many Class A owners. By contrast, the longest my wife and I ever stayed somewhere was 3 days and that was only once.
"That’s not necessarily true. Why? Because people with “touring coaches” like the #1 selling Winnebago Travato Class B motorhome, often put 40,000 or more miles per year on them- especially if they’re “full-timers”"

I would live to see data that says folks put anywhere near 40,000+ miles per year on their RV's. Every article I have read has the average mileage at closer to 1/8th of that baseline - about 5,000 or so miles per year.
Join some of the Class B Facebook groups and read how people are using their class B vans. They’re routinely going to Alaska, for example. 40,000 miles is definitely on the high side and I was by no means trying to imply that that’s anywhere close to the average. Not counting the people who buy them and find out it’s not their thing (leaving them parked), I suspect the average active Class B owner puts @ 15,000 miles on it per year. What’s the point of keeping it parked?

For us, 5,000 miles would be exceeded on some trips. We live a mile from the right coast, so to get to the other one and back rolls the odometer. There were several times we drove for more than 12 hours in a single day, which would be in the upper hundreds of miles. I just saw a 2 year old van listed for sale with 90,000 miles on it.

The Class B touring lifestyle isn’t the same thing as what you’re reading about on the internet. There’s no way I’d want an RV if my use case was to stay close to home, and many of the van buyers typically don’t either.
There was no intention to say you or I were not ones to put larger amounts of miles on our cars/boats/RV's - likely we did and are not an indicator of the average. Whenever we looked for RV's or boats it became really obvious that the average mileage put on these things in like maybe 5,000 miles or so per year.
If that is really more like the average then the builders have very little motivation to reach fir a mileage increase of say 10% when those costs savings would be minor compared to the total cost of ownership.
Just curious if anyone has any data on these things based on average resale mileages and years.
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Re: Class A motorhomes - aerodynamics of a brick

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mkc wrote: Thu Sep 22, 2022 9:29 am
NHRATA01 wrote: Thu Sep 22, 2022 9:03 am I figured it was a combination of getting as much interior space for the length, and also giving people who aren't quite expert semi-truck drivers (I don't think you even need a CDL for them?) the best view forward possible so they don't run the rest of us over.
No CDL, unless you're paid to drive it (RV driver-for-hire) or it's used for business. It's state-specific, but many states require an exempt class B if it's over 26,000 lbs GVWR or exempt class A if you're pulling more than 10,000 lbs. Air brake endorsement if it has air brakes.
Whether a CDL or NCDL is needed is dependent on the state which issued the license to drive; those which have caveats usually honor the licensing state. For example, in Arkansas if the weight is over 26,000 lbs a CDL is required. In NC, a CDL is required if the vehicle and whatever is being towed is over 26,000 lbs. MD (other states too) has a NCDL for vehicles weighing over 26,000 lbs.and so on and so forth.
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Re: Class A motorhomes - aerodynamics of a brick

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smitcat wrote: Fri Sep 23, 2022 8:09 amIs there any data and/or studies indicating which designs affect fuel economy and at what speeds? Often it is not so clear which 'design' will actually yield a more fuel efficient result.
Yes, this is all well known in the aerodynamics community. Any good Fluid Dynamics package can model aero quite effectively. See my crude formula above (crude because it does not include any factor for the roadway interface), the most important factor at highway speeds is [velocity2].

BTW - the DC-3 fuselage is cool, but as noted above is not particularly efficient due to the "chopped-off" rear end. While we are posting cool vehicles, check out the 1938 Dymaxion designed by Buckminster Fuller (father of the geodesic dome):
Image
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Re: Class A motorhomes - aerodynamics of a brick

Post by smitcat »

David Jay wrote: Fri Sep 23, 2022 6:29 pm
smitcat wrote: Fri Sep 23, 2022 8:09 amIs there any data and/or studies indicating which designs affect fuel economy and at what speeds? Often it is not so clear which 'design' will actually yield a more fuel efficient result.
Yes, this is all well known in the aerodynamics community. Any good Fluid Dynamics package can model aero quite effectively. See my crude formula above (crude because it does not include any factor for the roadway interface), the most important factor at highway speeds is [velocity2].

BTW - the DC-3 fuselage is cool, but as noted above is not particularly efficient due to the "chopped-off" rear end. While we are posting cool vehicles, check out the 1938 Dymaxion designed by Buckminster Fuller (father of the geodesic dome):
Image
Interesting - I would have expected much better improvement than 1/2mpg (5%??) when reducing speeds from 70 to 65mph with that as a general formula. I know from boats that reducing speeds increases fuel economy in a really big way based on experience.
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Re: Class A motorhomes - aerodynamics of a brick

Post by JoeRetire »

snackdog wrote: Thu Sep 22, 2022 7:52 am My question is WHY?
Maximize interior space.
Oh, noooooo! I'm so sorry, it's the moops! The correct answer is 'the moops'.
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Re: Class A motorhomes - aerodynamics of a brick

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smitcat wrote: Sat Sep 24, 2022 6:32 am
Interesting - I would have expected much better improvement than 1/2mpg (5%??) when reducing speeds from 70 to 65mph with that as a general formula. I know from boats that reducing speeds increases fuel economy in a really big way based on experience.
For every 5 miles per hour faster than 60 that you drive, your gas mileage decreases by up to 7 percent so that works out about right for a 10 mpg marble-floored behemoth. Optimal speed is about 60, so going 70 will cost you a bit.
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Re: Class A motorhomes - aerodynamics of a brick

Post by smitcat »

snackdog wrote: Sat Sep 24, 2022 7:12 am
smitcat wrote: Sat Sep 24, 2022 6:32 am
Interesting - I would have expected much better improvement than 1/2mpg (5%??) when reducing speeds from 70 to 65mph with that as a general formula. I know from boats that reducing speeds increases fuel economy in a really big way based on experience.
For every 5 miles per hour faster than 60 that you drive, your gas mileage decreases by up to 7 percent so that works out about right for a 10 mpg marble-floored behemoth. Optimal speed is about 60, so going 70 will cost you a bit.
Is there any data on testing these RV's anywhere at various speeds? Would not the best mpg be much less than 65 mph? Are there any hp vs MPH charts available for these types of units similar to boats?
It was fairly easy with boats - the fuel use was able to be displayed in real time and compared to the current speed, should be similar with these RV's since many utilize the same or very similar engines.
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Re: Class A motorhomes - aerodynamics of a brick

Post by nisiprius »

I could ask our daughter-in-law's folks who are full-time RV-ers, but I think I'll settle for what Google searches can tell me.

Class A motorhome apparently get 6-10 mpg typically, the most efficient 13. They cost $350,000 to $500,000. I would guess that even for wealthy people that's a purchase they would research carefully, if only for interior features etc. so they are probably aware of fuel efficiency. The average RV is driven 4,000-5,000 miles per year, I can't seem to find numbers for Class A motorhomes specifically but a couple of postings from full-time RV-ers suggests that true for class A motorhomes, too. Our DIL's folks drive from place to place but then stay in one place for a couple of months.

So the first answer is that unlike trucks, busses, etc. they aren't driven that much so fuel efficiency isn't a big factor. When I was commuting to work I was putting 20,000 miles/year in a car that got 30 mpg, so total annual fuel consumption wasn't that different between my car and their motor home.

For a motorhome, if we assume 8 mpg, 5,000 miles/year, and, I dunno, $3/gallon gas, that's less than $2,000 a year for gas. For someone who can afford a motorhome, that... you know... same ballpark as many annual homeowner expenses.

In other words... not to be too much of an apologist for the lifestyles of the somewhat-wealthy... in the grand scheme of things I don't think class A motorhomes collectively, in toto, are an environmental disaster. As a matter of public policy it doesn't sound as if improving the fuel efficiency of motorhomes would be where you'd get the most bang for the buck environmentally.

If someone who can spend $500,000 says "We considered fuel efficiency, but it wasn't worth compromising having our motorhome just the way we want it just to save a thousand or so a year, which we can afford," I wouldn't argue the point.

Also I would guess that a boxy construction is best for having a comfortable interior with lots of storage space.

My DIL's mom is one of the few people I know to own a Microsoft Surface, and it's because everything they buy is chosen with a view to occupying as little space as possible.
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Re: Class A motorhomes - aerodynamics of a brick

Post by JoMoney »

FWIW, many school buses have the same design, and probably have more mileage than motor homes of similar styling.
Image

And on that note, I found the below site, which kind of alludes to what I mentioned above that many drivers preferring the view of the road they get with that styling
https://busconversion101.com/other/why- ... front.html
Why are busses so square?
The reason buses are designed with a flat front is that it maximizes volume and a flat front and window allow the driver to see better, especially when turning. The increased space and visibility are more important than any small increase in efficiency from a more aerodynamic shape.

Why are buses not aerodynamic?
Buses don’t move at very high speeds, like 60-70 kmph max( on the highway) and 30-35 kmph (in city). Air drag is proportional to speed. Since the speed is not very high, there is very low value of air drag , which can easily be neglected.
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Re: Class A motorhomes - aerodynamics of a brick

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Re: Class A motorhomes - aerodynamics of a brick

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Zeno wrote: Sat Sep 24, 2022 7:59 am ...

I do not believe, but am not entirely certain, that GHG emission from RV's are yet regulated. They may be, but I doubt it. And if they aren't, they likely will be shortly. RV's likely look the way they do, however, because their GHG emissions are not yet regulated, or the RV manufacturers are controlling the GHG emissions some other way.
...
It is not clear but I believe new EPA Phase 2 regs began for model year 2021 includes most RVs. That is mostly emissions, not MPG. The RV industry opposed them, of course, saying that RVing was all about enjoying the environment outdoor but that standards could reduce their profit margin, haha.
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