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Old 07-15-2017, 03:55 PM   #1 (permalink)
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RV / Square box leading edge interesting stuff...

Stumbled on two interesting tidbits relative to leading edges on square boxes, particularly RVs and box trucks.

First is the first attached image. Just wrapped up a 2 week RV trip. Saw a few late model Jayco Class C motorhomes on our trip. Didn't take picture myself (should have) but googled a random image and attached it. Google says a Jayco Redhawk or Greyhawk has the feature. It's the leading edge/face treatment of the box near the cab doors. The rounded bubble is only part of the face piece. There's a"normal" square edge beneath it. I can't say that it is an aerodynamic device, but it obvious to me it would help. Maybe it was done to improve the wind noise or something.

Second is a document I found searching for some information on frontal area tow ratings from the manufacturers (yes, some do spec frontal area in addition to weight for tow ratings). It's Ford service bulletin about the frontal area and weight ratings for the 2017 Ford cutaway van.

https://www.fleet.ford.com/truckbbas...ml/Q-246R5.pdf

Note that it specs a 2.4" leading edge radius for box type trucks over a certain frontal area. We know 4% of the width is a minimum, that'd be slightly more than 2.4", but very interesting to see something like this directly from a manufacturer. I suspect there's no engineering behind the number (too bad). I'd guess 2.4" is probably what someone like U-Haul uses on their boxes, and Ford grabbed that as a working minimum. U-Haul does claim aerodynamic leading edges on the advertising of some of their trucks, so the story seems to fit.

There's also some ridiculous 1700mm spec for the stripped chassis. But this too appears to be a number grabbed from a manufacturer. The only RV built on the stripped chassis right now is the Thor Axis/Vegas. It's been around a couple years now. Note its shape below...

So maybe the Jayco devices help them meet the new Ford restrictions? I've only seen it on a Jayco, so far.

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Last edited by ennored; 07-15-2017 at 09:41 PM..
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Old 07-16-2017, 06:29 AM   #2 (permalink)
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Frontal area as context for towing recommendations is older than I remember, and that's fifty years ago.

Degree of radius of vehicle edges is 1920s, IIRC

Entry aero is nice. But means little without trailing.

Sidewall edge radius is what distinguishes an aero RV from one non-aero.
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Old 07-16-2017, 07:57 AM   #3 (permalink)
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I vaguely recall some Ford vehicles specified frontal area in the tow rating. Ironic as it was on a forum for molded fiberglass "egg" campers and a Ford lover dinged the spec for not appling to a Scamp with such generous edge radii. Cd missing, yes, and CdA would be better but depending on the tug, which sets up the flow field that trailers follow, it still falls short of the whole picture. My point is complexity of aero. Many (most?) lay folks probably don't understand or care so maybe "why bother" is the conclusion auto manufacturers came to long ago. (shrug)

As for the Axis, it's side profile may help keep bugs off the windscreen and rooftop A/C unit. My GF has a class A (older, boxier) and solar gain while traveling can be a problem. That Axis better have one heckuva dash A/C system and/or excellent glass treatment to reflect the heat rays.

My 2c
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Old 07-16-2017, 11:45 AM   #4 (permalink)
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http://ecomodder.com/forum/member-he...117-154543.jpg
It makes me feel better about trying this on my Hi-Lo. That edge really stuck out into the wind being an 8' wide box. It only cost about $20 to add as well and has held up now 3 years although the tape could use a few new pieces.
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Old 07-17-2017, 02:59 PM   #5 (permalink)
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Quote:
Originally Posted by slowmover
Sidewall edge radius is what distinguishes an aero RV from one non-aero.
Also one that will not rot out since the edge won't leak.

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Old 07-22-2017, 12:10 PM   #6 (permalink)
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leading edge

Quote:
Originally Posted by ennored View Post
Stumbled on two interesting tidbits relative to leading edges on square boxes, particularly RVs and box trucks.

First is the first attached image. Just wrapped up a 2 week RV trip. Saw a few late model Jayco Class C motorhomes on our trip. Didn't take picture myself (should have) but googled a random image and attached it. Google says a Jayco Redhawk or Greyhawk has the feature. It's the leading edge/face treatment of the box near the cab doors. The rounded bubble is only part of the face piece. There's a"normal" square edge beneath it. I can't say that it is an aerodynamic device, but it obvious to me it would help. Maybe it was done to improve the wind noise or something.

Second is a document I found searching for some information on frontal area tow ratings from the manufacturers (yes, some do spec frontal area in addition to weight for tow ratings). It's Ford service bulletin about the frontal area and weight ratings for the 2017 Ford cutaway van.

https://www.fleet.ford.com/truckbbas...ml/Q-246R5.pdf

Note that it specs a 2.4" leading edge radius for box type trucks over a certain frontal area. We know 4% of the width is a minimum, that'd be slightly more than 2.4", but very interesting to see something like this directly from a manufacturer. I suspect there's no engineering behind the number (too bad). I'd guess 2.4" is probably what someone like U-Haul uses on their boxes, and Ford grabbed that as a working minimum. U-Haul does claim aerodynamic leading edges on the advertising of some of their trucks, so the story seems to fit.

There's also some ridiculous 1700mm spec for the stripped chassis. But this too appears to be a number grabbed from a manufacturer. The only RV built on the stripped chassis right now is the Thor Axis/Vegas. It's been around a couple years now. Note its shape below...

So maybe the Jayco devices help them meet the new Ford restrictions? I've only seen it on a Jayco, so far.
That technology was patented by Dr. Paul MacCready of Aerovironment.
Access for the door swing may have limited the useful radius.
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Old 07-22-2017, 05:05 PM   #7 (permalink)
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Were it I, the inner face would have a radius defined by the door-swing +1/2". Basically a quarter-round and a reverse curve.

Has anyone added blisters to the door skins yet?
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Old 07-23-2017, 11:20 AM   #8 (permalink)
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I think just making the box better is the answer.
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Old 07-25-2017, 10:57 AM   #9 (permalink)
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Quote:
Originally Posted by Hersbird View Post
It makes me feel better about trying this on my Hi-Lo. That edge really stuck out into the wind being an 8' wide box. It only cost about $20 to add as well and has held up now 3 years although the tape could use a few new pieces.
I'm curious...did you notice any difference in fuel mileage? I know it might be hard to tell unless you took the same road on different trips with the trailer.
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Old 07-26-2017, 06:11 AM   #10 (permalink)
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Quote:
Originally Posted by Hersbird View Post
I think just making the box better is the answer.
With Orbywans boat tail, you'd be right.

Except it's still a Moho. A distinctly space and weight inefficient RV. Fuel burn will still be higher than an aero trailer with more living space and well chosen tow vehicle.

Having to tow another vehicle behind it is the "answer" to its shortcoming for local touring or grocery runs.

"Efficiency" with an RV starts with the premise that re-supply (of every kind; and water, electric & sewer are included) is the denominator: How many nights in one location for how many people? That's where to start.

Thus, fresh water supply is king.

A Moho gives this up in favor of having been designed as a commercial vehicle with a service life X10 what the owner of the RV will utilize. A penalty that can't be overcome.

All this is before price comparison.
.

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