Community Forums

ROADOG · 08-31-2012, 09:51 AM

Does anyone have either good personal experience, or professional expertise that can answer whether the front end shape of a large Class A coach has any noticeable effect on fuel mileage?
My understanding is that the two factors that control wind resistance, and thereby mileage (let's not worry about hills and weight just now as it only adds unnecessary complication), are frontal area and speed. Despite this, a lot of designers, particularly back in the earlier days of large coaches, felt they had to angle the front of the coach to give it an aero-wedge look. Even in the more modern designs, manufacturers are going to a lot of trouble and expense to "round" the front of the vehicle.
Given the acceptance of all the protrusions: mirrors, awnings, steps and grab handles, air horns, spot lights, roof ACs and vents, TV antennas, satellite receivers, solar panels, etc.; not to mention towed vehicles (for good or ill), can a little slope or rounding on the front end of the Rig really make any difference? Or is it all about STYLE?

Midniteoyl · 08-31-2012, 10:13 AM

On tablet right now, but, there is a NASA? test using a van that shows the best way to reduce air resistance of a square front is to round the front corners. Same test showed a partial boattail and enclosed undercarriage did even better.

RickO · 08-31-2012, 10:52 AM

Hi and welcome to the forum.

It's a really good question. We love our Winnie, but I've never been crazy about their external styling lines as, until recently, IMO were very flat and boxy looking. I've often wondered if sleeker looking rigs like Monacos or the Fleetwood Revolution would get better mileage.

I'll be interested in the responses.

Rick

Glenn and Kathy · 08-31-2012, 01:22 PM

I think most of the newer coaches with the rounded front ends would be sufficiently slippery and agree that best results would be from enclosing the underside and breaking the vacuum in the rear. But then what do I know

Glenn

Jack1234 · 08-31-2012, 01:35 PM

The people with the most innovative "aero" design(s) at the moment are Rexhall. If a slippery body actually improves mpg then their designs should be studied.

clyon51 · 08-31-2012, 02:03 PM

I would bet if you took a 10 ton Class A MH and retro fit it with 10 different front ends of your choosing, the diff in mpg might be 0.1. Certainly front end shape would be on the bottom of the list for me.

doc · 08-31-2012, 02:10 PM

We can say the west wind crossing S Dakota caused less milage when going into it that having it from the rear on another trip. I have no numbers, but you can certainly tell the engine was working harder into the wind than with it.

Midniteoyl · 08-31-2012, 04:38 PM

Quote:

Originally Posted by clyon51

I would bet if you took a 10 ton Class A MH and retro fit it with 10 different front ends of your choosing, the diff in mpg might be 0.1. Certainly front end shape would be on the bottom of the list for me.

Actually, more than that... I'll try to find the .pdf with the exact numbers, but heres a summary.

Quote:

During a decade spanning the 1970s and 1980s, Dryden researchers conducted tests to deter-mine the extent to which adjustments in the shape of trucks reduced aerodynamic drag and improved efficiency. During the investigation of truck aerodynamics, the techniques honed in flight research proved highly applicable.

During the tests, the vehicle's sides were fitted with felt tufts, or strings, that showed airflow. The investigators concluded that rounding the vertical corners front and rear reduced drag by 40 percent, yet decreased the vehicle's internal volume by only 1.3 percent. Rounding both the vertical and horizontal corners cut drag by 54 percent, while losing only three percent of internal volume.

The first phase involved a cab-over tractor-trailer, modified by rounding all of its front corners and edges. In addition, technicians attached sheet metal fairings over the cab's roof and sides as far back as the trailer. This covering closed the open space between the cab and trailer. During the phase one research, researchers found that in highway driving at 55 miles per hour, rounding the corners and fairing the space to close the gap between the truck's cab and trailer resulted in a significant reduction in aerodynamic drag, resulting in 20 to 25 percent lower fuel consumption than the standard vehicle. The improvements soon spurred design changes in production trucks, as many truck manufacturers subsequently incorporated similar modifications on their products.

NASA - Aerodynamic Truck Studies

Midniteoyl · 08-31-2012, 04:42 PM

Here..

http://www.nasa.gov/centers/dryden/p...main_H-831.pdf

http://www.nasa.gov/centers/dryden/p...ain_163113.pdf

http://www.nasa.gov/centers/dryden/p...main_H-977.pdf

http://ntrs.nasa.gov/archive/nasa/ca...1999074877.pdf

Jack1234 · 08-31-2012, 04:57 PM

If, by design, you could get a 20% mpg advantage, at today's prices for diesel, it would save you $1000 every 10,000 miles.

Everything being pretty much equal, I'd choose the aero design over the "drag" queen!

winnie32v · 08-31-2012, 05:18 PM

Just a practical approach.
I had a GMC 2500 Crew Cab, and pulled a 31 ft. 5th wheel from Pa. to Fla.
I was getting 7 - 8 mpg while traveling ~60 mph.
I traded for a Winnebago, Class A, 33 ft. On the way back to Pa. from Fla. I was getting 7 - 8 mpg, driving the same roads, same speed.
So, I would expect that this comparison of two actual units with totally different shapes may answer some questions.

I also would like to know who hasn't got the knowledge that when 10% ethanol was mandated, the mpg went down....where is the savings there....both in energy use as well as environmental advantages???

RickO · 08-31-2012, 05:22 PM

Quote:

Originally Posted by winnie32v

I also would like to know who hasn't got the knowledge that when 10% ethanol was mandated, the mpg went down....where is the savings there....both in energy use as well as environmental advantages???

Timely question because I've been traveling in Canada for the past week and I'm getting better MPG. Is there a difference in Canadian diesel?

Rick

Scarab0088 · 08-31-2012, 05:23 PM

Quote:

Originally Posted by ROADOG

Does anyone have either good personal experience, or professional expertise that can answer whether the front end shape of a large Class A coach has any noticeable effect on fuel mileage?
My understanding is that the two factors that control wind resistance, and thereby mileage (let's not worry about hills and weight just now as it only adds unnecessary complication), are frontal area and speed....can a little slope or rounding on the front end of the Rig really make any difference? Or is it all about STYLE?

I saw a Discover Channel show about this. The show focused on OTR Trucks with lots of wind tunnel tests, and has direct applicability to RV's...The order of impact are:
1. Speed
2. Frontal area.
3. Surface Turbulance
4. Bottom and Rear Suction.

1. Decreased speed has the highest impact on MPG (that was why the whole Carter era 55MPH thing).
2-4. Wind resistance (AKA Drag Coefficient) were the next highest points for improvement. Picture an airliner body to visualize the best design for this.

Ironically, the TV show concluded by saying the biggest issue with OTR trucks is the buyers mental image for what a truck must look like. The designers would be selling 737 shaped rigs, if truckers would buy them.

For the RV world, the same holds true...buyers love the "BUS" style Class "A", so the aerodynamic shapes of the 70's-80's are gone.

ROADOG · 09-04-2012, 10:32 AM

I'd like to thank everyone who replied to my question, there was lots of good information, but no one had much to say about the rearward slope that was used on a lot of coaches back in the early nineties; London Aire would be a prime example, though most of the other manufacturers did the same thing to some degree. As far as I can tell all of the companies have now moved to a more vertical front end.
Anyone care to comment?