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Old 06-30-2022, 01:06 PM   #1
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RV Drag And Affect On MPG

With each high fuel cost cycle there is interest in improving fuel economy. This is nothing new. Lots of things are tried to improve MPG, but since RVs are mostly driven at highway speeds, the elephant in the room is aerodynamic drag.

Obviously other things help some also, like lower weight that reduces rolling resistance, and more efficient engines, but for “big improvements” in fuel economy, aerodynamic drag is as important as it gets. Ignoring this will cost at the pump.

Unfortunately, aerodynamic drag (which is based mostly on combination of frontal area, coefficient of drag, velocity squared, and air density) does not seem as important to buyers as is fuel economy; though the two are closely related when it comes to RVs (because of mostly highway driving). Quote below from an RV engineer’s technical blog:

“Most RV manufacturers generally DO NOT use wind-tunnel testing to evaluate the aerodynamics of their RV designs. Curb appeal and layout drive the shape, not aerodynamics.”

In this light, how flexible would you be on frontal area size and RV shape in order to reduce aerodynamic drag to improve fuel economy? Driving slower is also an obvious option, but has been discussed sufficiently already.

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Old 06-30-2022, 02:06 PM   #2
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Quote:
Originally Posted by Chance View Post
With each high fuel cost cycle there is interest in improving fuel economy. This is nothing new. Lots of things are tried to improve MPG, but since RVs are mostly driven at highway speeds, the elephant in the room is aerodynamic drag.

Obviously other things help some also, like lower weight that reduces rolling resistance, and more efficient engines, but for “big improvements” in fuel economy, aerodynamic drag is as important as it gets. Ignoring this will cost at the pump.

Unfortunately, aerodynamic drag (which is based mostly on combination of frontal area, coefficient of drag, velocity squared, and air density) does not seem as important to buyers as is fuel economy; though the two are closely related when it comes to RVs (because of mostly highway driving). Quote below from an RV engineer’s technical blog:

“Most RV manufacturers generally DO NOT use wind-tunnel testing to evaluate the aerodynamics of their RV designs. Curb appeal and layout drive the shape, not aerodynamics.”

In this light, how flexible would you be on frontal area size and RV shape in order to reduce aerodynamic drag to improve fuel economy? Driving slower is also an obvious option, but has been discussed sufficiently already.

THOR just bought the design and patent rights for this. Coming to a Camping World near you soon!
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Old 06-30-2022, 02:32 PM   #3
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We do not have to get out in left field to discuss a subject seriously based on facts and data. It may actually help educate a few prospective buyers to better understand RV choices they will make.


https://www.rvlifemag.com/andy-thoms...v-hitch-hints/


Above is not a scientific test but makes the point nonetheless. A larger and heavier trailer can be more fuel efficient by simply being more aerodynamic.

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Old 06-30-2022, 02:44 PM   #4
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Quote:
Originally Posted by Chance View Post
... how flexible would you be on frontal area size and RV shape in order to reduce aerodynamic drag to improve fuel economy? Driving slower is also an obvious option, but has been discussed sufficiently already.
Honestly, not flexible at all, unless it gives us more space internally or more functionality and is more beautiful.

There is a technical reason why aerodynamic drag is not the focus of the manufacturers: Because the customer doesn't care for that (most of them at least).

We put somewhere around 5K miles in our rigs every year and we spend more time at campsites than on the road.
Curb appeal will always be important (needs to be beautiful to our eyes because is our HOME on the road) and internal space is a must since we spend more time inside it than the time we spent on the road.

In our case a drop down bed up front that can sleep two adults is a must and a sloped front would just kill it.

Then it comes to cost:
Fuel consumption is only a part of the operating cost of the MH so in our case we have this:
5000 miles @ 7.5mpg @ $5.15/gallon is ~$3400.00 in gasoline per year.
If you can reduce the fuel consumption by 20% ( a huge reduction) that will be a savings of ~$690/year.
Compare that with depreciation, insurance, registration, etc and you have the picture....

That cost reduction is not significant for us to give up internal space and curb appeal so no, we are not interested.

We are also not interested in pay 20% or more on a RV so they can electrify it in order to save that kind of $....

I can get 20% improvement in fuel consumption myself:
I just need to drive at 55mpg instead of 65mpg...

but I don't want to...
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Old 06-30-2022, 02:56 PM   #5
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Originally Posted by Chance View Post
Above is not a scientific test but makes the point nonetheless. A larger and heavier trailer can be more fuel efficient by simply being more aerodynamic. .
I know Andy take on this and He helped me a lot when I started towing campers.

His test is absolutely right and I had the opportunity to test this on my own so his result is right and validated BUT have you seeing the bedroom inside a camper with a sloped front like that?

In my experiment my friend, with a rig sloped like that, got 1mpg better than me (thats a good 12% improvement over my baseline) BUT his bedroom was cramped, didn't have a wardrobe on a slide (because you don't have space at the side wall for that) and in general, it looked to have 1/2 the internal space we had with an almost vertical front cap.

So our response in that case is still NO.

BTW, my motorhome towing our SUV burns just 1mpg more than my F150 towing a camper and just 2mpg more than my friend with his F250 towing a camper with a sloped front so to me the MH is way more efficient than those rigs.
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Old 06-30-2022, 03:01 PM   #6
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Gas mileage is a hobby, maybe even a derangement.
A hobby incongruent with RV.

You knew the job was dangerous when you took it, or you shouldn't be allowed in polite society.

Some take this speculation of the future and change to nauseating lengths.


Live for today a bit.
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Old 06-30-2022, 03:47 PM   #7
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I don't think the loss of internal space would near offset the gain in micro fuel mileage to make it worth it. The only thing you can improve on would be the A's and all you could do is point the front end out further (significantly extend dead space and bury engine further) or drop and slant the stand up space over the front seats. That's a big reason people buy A's...the additional, usable space up front. What would be interesting is how much people would sacrifice (space) and pay (camping dollar) for the extra mileage...it it really would increase. I don't think people care enough about it. As I've said...other than Class B's, who really cares about mileage when buying a large C or A? The C's have already been addressed by the chassis manufacturer and the overhead cap (added space) so all that's left is the A's...which is where we start over at the top of this post.
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Old 06-30-2022, 04:01 PM   #8
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Quote:
Originally Posted by Chance View Post
We do not have to get out in left field to discuss a subject seriously based on facts and data. It may actually help educate a few prospective buyers to better understand RV choices they will make.


https://www.rvlifemag.com/andy-thoms...v-hitch-hints/


Above is not a scientific test but makes the point nonetheless. A larger and heavier trailer can be more fuel efficient by simply being more aerodynamic.

.
Just FYI for anyone here new to towing rvs!
That vehicle in these photos is probably not adequate for either of the TTs shown, regardless of their aerodynamics. Those vans are great people mover grocery getters but were never designed to tow big heavy rectangular boxes where a WDH is mandatory.
Sorry for the hijack! Now back to our regularly scheduled program.
If you want better aerodynamics get an Airstream! All aluminum, lightweight, shaped like a giant Twinky, next to no outdoor storage, indoor storage is all rounded making difficult to store square boxes, if taller than about 5'10" it's difficult to walk end to end without a few bumps & VERY pricey.
If you think rving is going to be cheap travel with relaxing time spent at a campground, you'd better rethink rv ownership! It's expensive, not only for fuel for a motorhome or tow vehicle after the initial purchase, there's ALWAYS something you need, or think you do, & ALWAYS something to repair, replace or redo, it's a home on wheels suffering from earthquake like conditions every mile you travel.
There's 2 major requirements for rv ownership, tools & a credit card, these 2 items will get more use than anything else you may carry with you!
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Old 06-30-2022, 04:08 PM   #9
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People buying NEW motorhomes are generally affluent with an appreciable disposable income... or they wouldn't be dropping a minimum of $100k+ on the largest depreciating non-asset known to mankind.

Therefore, the economics of the whole picture are WAY down the list. The folks who do prioritize fuel economy don't seem to be doing it for economic (cost) reasons... their incentive seems to be more environmentally motivated - whether it is or not is another debate.

So... is someone plunking down over $200k for #vanlife concerned about fuel COST? Or is it another motivation? I STILL cannot see why the smaller vans are priced so high when using basically the same components as the stripped chassis... and obviously less labor to build??? Another topic...

I think Thor has analyzed the new purchaser to create a profile... and their products sitting on RV dealers lots reflects that research. Until attitudes change about fuel economy for COST sake, we're going to see more of the same.
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Old 06-30-2022, 04:19 PM   #10
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Honestly, not flexible at all, unless it gives us more space internally or more functionality and is more beautiful.

There is a technical reason why aerodynamic drag is not the focus of the manufacturers: Because the customer doesn't care for that (most of them at least).

......cut......

Appreciate your honesty and well thought out opinion. I agree a rectangular box is most space efficient, yet least aerodynamic at same time; but I don’t necessarily exclude the possibility of a larger “odd-shaped” RV that has as much or more total space and yet lowers drag to improve fuel economy.

Granted, cost of sleek designs are usually higher than a box and the added cost may not be justified solely from a monetary standpoint. That compromise becomes much more subjective based on one’s likes and budget.

Talking about more “aerodynamic” designs, classic Airstream trailers have represented that mindset for a very long time. More recently affordable trailers in many sizes have started taking aerodynamics more seriously. Even so, one of my favorites that I personally find “beautiful” in part because of its shape is the Airstream Base Camp.
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Old 06-30-2022, 04:19 PM   #11
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You can't make chicken salad out of chicken poop. Until the RV industry designs their own specific purpose chassis and not depend on commercially powered frame rails to build around, nothing will ever change. Unless people quit standing in line to purchase what's available, there's no need for a change. Fuel mileage is at the bottom of the list. I think the school bus chassis (somewhat aerodynamic) with the engine out front would be the best fit they could ever come up with...and it could roll down a hill and stay intact...if they built it like a school bus body is built. They run forever and everything stays intact.
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Old 06-30-2022, 04:31 PM   #12
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Just FYI for anyone here new to towing rvs!
That vehicle in these photos is probably not adequate for either of the TTs shown, regardless of their aerodynamics.
I don't want to deviate from the post subject but the ONLY two reasons that towing in America the rigs Andy put together are:
1 - Legal issues in case of accident
2 - Almost nobody in America really understands what ball angle in a hitch is for.

Andy proved over and over again that a minivan and a sedan can be more stable towing a 9000lbs camper than a truck and I tested that towing a 6000lbs camper with a BMW X5 for 22000 miles and until today I haven't found a more stable rig than that.

They show you a 1 ton truck flipping over on highway and scare the hell out of people so they will buy a more expensive vehicle and obviously if you question, the legal argument is there... and since people in America don't know that payload and towing capacity for the same vehicles outside America is higher (the same BMW X5 I had here in US have a towing capacity 1000lbs higher in Europe) everybody keeps profiting off Americans ignorance on towing....
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Old 06-30-2022, 04:38 PM   #13
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but I don’t necessarily exclude the possibility of a larger “odd-shaped” RV that has as much or more total space and yet lowers drag to improve fuel economy.
.
It doesn't need to be odd shaped!
I really like the appearance of the GMC Motorhome and I think that a modern version of that with the Ford 3.5L ecoboost engine would be beautiful and consume less fuel BUT it would probably cost more than an Axis, for example, AND offer less features.
I, for example, would not buy it besides finding it beautiful.
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Old 06-30-2022, 04:42 PM   #14
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.....cut....

So... is someone plunking down over $200k for #vanlife concerned about fuel COST? Or is it another motivation? I STILL cannot see why the smaller vans are priced so high when using basically the same components as the stripped chassis... and obviously less labor to build??? Another topic...

I think Thor has analyzed the new purchaser to create a profile... and their products sitting on RV dealers lots reflects that research. Until attitudes change about fuel economy for COST sake, we're going to see more of the same.
Vans cost more for their size due to requiring more labor. Building vans is more labor intensive. Off topic but you deserve to know.

Buyers seem to prioritize based on what is happening or just happened; and have very short memories. Looking back the RV industry has changed after each economic and or fuel-cost cycle. The Axis/Vegas was meant to offer a cheaper and more fuel efficient Class A option following hard times (allowing for design and development period) when gas got expensive.

Class B vans offer not only fuel economy but also car-like use. I honestly don’t know which is more important to “B” buyers.
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Old 06-30-2022, 05:37 PM   #15
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Going to Memphis TN from Central Florida in Oct 2020 my buddy pulling his open trailer vette with e450 class c v-10 and me in e450 v-10 based vegas pulling my car on open trailer. Pretty close same weight overall. Every gas stop were within tenths of gallons to fill. Running 65 to 68 MPH swapping lead

Would have thought class C overhead air brake drag could hurt MPG. If it did, was minimal.
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Old 06-30-2022, 06:23 PM   #16
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1968 UltraVan - 8 x 8 x 22 ft class A, 140 hp Corviar drive train under the bed. 22 mpg @ 60 mph. Four abreast seating up front, all aluminum semi-monocoque construction (no frame), 4 wheel independent coil spring suspension, net weight 3,400 lbs, four tanks (fuel, fresh water, black water and gray water), 30 gallon tanks. Macerator with 50 ft of garden hose installed in the closet wall. Accessed from the outside. 40 ft of 10 awg shoreline installed in the closet wall. Full car type trunk 4 x 8 x 1.5 ft. Coleman Mach 10,000 btu. I had one for 17 years and still miss it.
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Old 06-30-2022, 06:51 PM   #17
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Beau,
Thanks for proof that it has always been available.

Sometimes it's about the
I want mpg at any cost
Vs
What I said should not be used against me.

If any of these mileage guys wanted mileage, truly, they'd search out the corvair or it's bmw cousin.

It's about self-torment at some point.
We're well into torment phase according to some posts.
Nothing will ever fit their False need/want.
If it were 50mpg, the dull roar would be for 52mpg.
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Old 06-30-2022, 08:05 PM   #18
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1968 UltraVan - 8 x 8 x 22 ft class A, 140 hp Corviar drive train under the bed. 22 mpg @ 60 mph. Four abreast seating up front, all aluminum semi-monocoque construction (no frame), 4 wheel independent coil spring suspension, net weight 3,400 lbs, four tanks (fuel, fresh water, black water and gray water), 30 gallon tanks. Macerator with 50 ft of garden hose installed in the closet wall. Accessed from the outside. 40 ft of 10 awg shoreline installed in the closet wall. Full car type trunk 4 x 8 x 1.5 ft. Coleman Mach 10,000 btu. I had one for 17 years and still miss it.

Thanks Beau for sharing.

Your motorhome can be seen as a predecessor of popular modern high-roof Euro-van-based Class Bs. Apparently large cargo vans needed to catch up with demand for motorhomes that size.

Standard vans in the background, including the Ford, puts their small size in perspective. Your Corvair was not much different in length or height compared to present vans, just much wider than Bs.

It’s easy to forget or ignore that B-vans are outselling Class As, and that doesn’t even count DIY units.
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Old 07-01-2022, 03:20 PM   #19
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Originally Posted by RACarvalho View Post
It doesn't need to be odd shaped!
I really like the appearance of the GMC Motorhome and I think that a modern version of that with the Ford 3.5L ecoboost engine would be beautiful and consume less fuel BUT it would probably cost more than an Axis, for example, AND offer less features.
I, for example, would not buy it besides finding it beautiful.

The old GMC...we had one when I was a kid. Beautiful orange and white with 3 tone "shag carpet". Compared to today's floor plans, you couldn't "stand up" in the front. You had to sort of scoot across the engine cover and slide into the seat. It was very low profile up front.
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Old 07-01-2022, 04:15 PM   #20
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Thanks Beau for sharing.

Your motorhome can be seen as a predecessor of popular modern high-roof Euro-van-based Class Bs. Apparently large cargo vans needed to catch up with demand for motorhomes that size.

Standard vans in the background, including the Ford, puts their small size in perspective. Your Corvair was not much different in length or height compared to present vans, just much wider than Bs.

It’s easy to forget or ignore that B-vans are outselling Class As, and that doesn’t even count DIY units.
Modern B vans are much heavier and not rear engine, thus do not have the four abreast seating up front. There was seldom a need to run the A/C when on the road as all the engine heat and noise was behind you where the bed added more sound dreading in the rear. The front wheel cut is 60 degrees allowing the coach to make a U-turn in 23 ft, the width of a residential street. With 14 x 5.5 rims and needing tires with a load rating of 1,800 lbs, it was easy to re-shoe the coach for $400 at Walmart. Try that in your class B.

The only reason I sold it is, I was getting old and moving to a Life Car Village. Like any 1960's car, it required constant maintenance much more than a modern RV and I no longer had a place to keep the spare parts and to work on the coach in an heated, air conditioned shop. Remenber GM went bankrupt so spare parts were not available.
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