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Old 08-31-2012, 12:11 PM   #1
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Aerodynamics of it all.

My gas mileage goes from 24 mpg(hwy) to 10 mpg when I tow my 154BH. As we all know, the Jayco is not a very aerodynamic unit and I was thinking about putting a roof top carrier on my Santa Fe in hopes of breaking the air resistance while doing 65 on the highway. Any thought would be appreciated
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Old 08-31-2012, 12:20 PM   #2
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Try driving 55mph. There are a lot of other things you could try, but nothing will improve your mileage like driving slower.
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Old 08-31-2012, 01:05 PM   #3
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it is a drag - not much difference you can make other than leave the rv at home,
or find an understanding flat bed driver to haul you,
or draft within 3 feet of an large box trailer _ NOT !
but what fun is that ?
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Old 08-31-2012, 01:08 PM   #4
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On our way home from camping last time I drove with a lighter foot than ever b4.
MPG went from about 10.5 to about 11.7.
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Old 08-31-2012, 01:16 PM   #5
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24 down to 10, wow! More than 1/2 drop. Speed is the biggest factor for fuel consumption.

My experience with my GMC and 27' toyhauler;
Solo I get 14.5 MPG keeping it under 70. 13 mpg doing 75.
Towing the TH home the first time, I was in a hurry and did about 72 most of the way home - 7.6 MPG.
Second trip I slowed to 67/68 and managed 8.2 MPG.
After reading a bunch here, I slowed to 62/63 and MPG picked up to about 10.

Each trip the trailer gained weight. I brought it home empty and fast. Slowed down because I wasn't in a hurry for the second trip with bike and a weekends worth of supplies for 2. While researching different things here, I read about fuel savings increasing with lower speeds. Tried an experiment and loaded with bike and 4 days supplies for 6, I slowed even more and realized better fuel economy. Resistance in air flow increases dramatically as you get above 60 MPH.
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Old 08-31-2012, 07:28 PM   #6
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Thanks folks,

guess I'll take the lead out of my right shoe. Travel safe.
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Old 09-01-2012, 06:50 AM   #7
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Quote:
Originally Posted by SaltyoftheAL
guess I'll take the lead out of my right shoe. Travel safe.
I too had to lift the right foot, 9.9 to 12.1. But you might check into a roof top deflector, I tied one on my 2012 Explorer, it wors great, and it's adjustable angles make it versatile for any TV. Fuel economy using the device 16.9 to 17.5. New vehicles are much aerodynamically balanced, so all that air dumps right on the front of the trailer. Just like big rigs that have one on the cab roof, it deflects the air over the barn door we pull. My 2 cents. I find the link and repost again.

P n J

Here you go, http://www.icondirect.com/aeroshield...-wd500-series/
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Old 09-01-2012, 09:09 AM   #8
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Thanks for the input.

Quote:
Originally Posted by 10Boomer View Post
I too had to lift the right foot, 9.9 to 12.1. But you might check into a roof top deflector, I tied one on my 2012 Explorer, it wors great, and it's adjustable angles make it versatile for any TV. Fuel economy using the device 16.9 to 17.5. New vehicles are much aerodynamically balanced, so all that air dumps right on the front of the trailer. Just like big rigs that have one on the cab roof, it deflects the air over the barn door we pull. My 2 cents. I find the link and repost again.

P n J

Here you go, Truck & RV wind deflector for towing trailers
I already have the top carrier, will try it first and let all kno how it goes. Peace.
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Old 09-02-2012, 01:59 AM   #9
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Originally Posted by 10Boomer View Post
I too had to lift the right foot, 9.9 to 12.1. But you might check into a roof top deflector, I tied one on my 2012 Explorer, it wors great, and it's adjustable angles make it versatile for any TV. Fuel economy using the device 16.9 to 17.5. New vehicles are much aerodynamically balanced, so all that air dumps right on the front of the trailer. Just like big rigs that have one on the cab roof, it deflects the air over the barn door we pull. My 2 cents. I find the link and repost again.

P n J

Here you go, Truck & RV wind deflector for towing trailers
4-5mpg increase? WOW is all I can say. I know several who didn't get anything but a lighter wallet by buying and trying 1...including myself.
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Old 09-02-2012, 02:15 PM   #10
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If you double the speed, the air resistance or drag force goes up by a factor of FOUR. So speed is the enemy.

The roof top air deflectors can help, but it is very critical of the height of the deflector, the angle and the distance from the trailer. Unless you have access to a wind tunnel to do the design, it is doubtful that you would get the right combination for any benefit in fuel economy.

I tried an air deflector on an S10 Blazer and an 18' Prowler TT. The ONLY thing it helped was the selling dealers profit.It did keep a lot of the bugs off the front of the trailer...but then I had to clean the3 deflector.

Ken
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Old 09-04-2012, 07:50 AM   #11
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Originally Posted by TXiceman
If you double the speed, the air resistance or drag force goes up by a factor of FOUR. So speed is the enemy.

The roof top air deflectors can help, but it is very critical of the height of the deflector, the angle and the distance from the trailer. Unless you have access to a wind tunnel to do the design, it is doubtful that you would get the right combination for any benefit in fuel economy.

I tried an air deflector on an S10 Blazer and an 18' Prowler TT. The ONLY thing it helped was the selling dealers profit.It did keep a lot of the bugs off the front of the trailer...but then I had to clean the3 deflector.

Ken
I agree in some cases it may not help, the drag numbers for the 2011-2013 Explorer are very low, so thus a lot of air hits the middle to front of TT. My decision was based on a small experiment using a homemade wind deflector, last trip. Yes the position is very critical, and assist in pushing air over TT. I can see maybe an S10 you may not see a benefit ( windshield slope) already puts the air up higher. The key is that 40sq feet of exposure in the TT, to minimize that the better.
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Old 09-04-2012, 07:51 AM   #12
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Originally Posted by 10Boomer

I agree in some cases it may not help, the drag numbers for the 2011-2013 Explorer are very low, so thus a lot of air hits the middle to front of TT. My decision was based on a small experiment using a homemade wind deflector, last trip. Yes the position is very critical, and assist in pushing air over TT. I can see maybe an S10 you may not see a benefit ( windshield slope) already puts the air up higher. The key is that 40sq feet of exposure in the TT, to minimize that the better.
Speed also for sure, 55mph
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Old 09-16-2012, 11:26 AM   #13
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The RV world has problems, overall, which can make comparisons difficult for one to another as compared to, say, commuters on a daily driver with a small range of vehicles. Thus, quantifying an aero aid also has to do with qualifying the vehicle. And, okay, not all are interested in drilling down for the details, but some are. So, for them:

Records: All miles, all gallons. And some notes about

I] Vehicle Spec
II] Climate
III] Terrain
IV] Use

Highs or lows don't mean anything, it's about the average mpg.

Improvements to the average are the only thing to quote, and then, in terms of percentage gain or loss.

"I gained 1.7-mpg using this gizmo last summer" is a meaningless statement, overall.

If that "gain" was from an average of 9.3-mpg (all towing miles since purchase) then the real gain was 18%. Which, in terms of paying for fuel, is more than significant.

My towing mpg is at 25-cpm at present. An [unlikely] 18% gain would drop that by nearly 2.5-cpm. (See both EDMUNDS and AAA on cents-per-mile [cpm] calculations as this is the direct method of understanding fuel cost as a part of overall vehicle economy). IOW, my fuel burn per 100-miles of travel on average would be reduced by one gallon. Same coin, both showing numbers I can use: percentage gain, on one side; and fuel cost reduction per 100-miles of travel, on the other.

"Will it apply to my rig?" Well, with percentage gains stated (and enough info about the other rig, conditions, etc to make meaningful comparisons from Jones to Smith) . . maybe. Driving smarter means more than any device (and is far more extensive in practice than just driving slower).

All other things constant for a given rig, about the only way to improve mpg is via:

A] Gearing
B] Reductions in aerodynamic resistance

And the latter has the broadest range of possibility over varying conditions. We may focus too much on weight, yet it is aerodynamic resistance that ought to be our first consideration for an RV. (Applying aero aids to a non-aero rig is cart-before-horse, but we'll all start somewhere along a spectrum of best to worst).

Anyone who (like me) tended to think lightly of rooftop deflectors can conduct a simple test:

Install the device and record mpg while solo. And without the device in the same way, again, solo (non-towing). (The drag is substantial).

It is doing something . . so the question is down to, "how much / how well"?

One ought to, IMO, get rid of the noise in the system to be able to filter for the effects (how much, versus yes/no). So, after lack of effective reporting (records) we should eliminate that which, when corrected, could show a higher gain than an aero device (assume nothing is correct; verify):

1] Zero brake drag on both vehicles (perfect bearing adjustment on TT)
2] Axle alignment on both vehicles
3] Zero steering wander on the TV (rare with recirc ball and/or 4WD)
4] Hitch rigging that, at a minimum, restores TV FA weight value while hitched to equal the unhitched value
5] Ideal tire pressure (max sidewall on TT tires; pressure versus load on TV within vehicle manufacturer guidelines); as well as wheel balance
6] Regular scale weighings to establish best range of hitch rigging adjustments, TV tire pressures, and to keep an eye on overall weights.

Put the dollars here first (this is proving the minimum . . one can also improve mpg via mechanical changes even if indirect). Past driver skill, MPG is mainly small changes to the vehicle when considering mechanical condition.

Next are a few notes about a particular (representative) trip:

a] Number of engine-on starts
b] Number of acceleration events (from stop; maybe from an extra-low speed to a higher one)
c] Number of braking events (all types)
d] Target travel speed and average speed (engine run time against miles travelled).
e] Actual time spent above 50-mph (not miles)
f] Difficult to record, but important to consider (per truck industry): number of steering corrections per 100-miles of travel

Average mph is an important tool. The same rig, the same trip can show substantial differences (that "change" mpg results). This is engine hours versus miles travelled. For aero aids: time spent at/above 50-mph is key.

This all seems like a lot. But after over ten years of RV Internet board reports on mileage, the noise in the system still predominates in discussions about mpg aids (the "quality" of reporting is so low -- due to lack of quantification -- that one cannot make meaningful comparisons).

Those who attend to those mechanical issues, keep extensive records, and make a few notes are most likely to be able to determine whether or not a particular device is worthwhile. But not otherwise.

Discussions elsewhere online about aero aids are adamantly in favor of them according to those who know . . . but making things work for us means being able to separate the wheat from the chaff. We many of us (perhaps most of us) don't travel enough (total miles), or in similar conditions (different climate, terrain, etc) to make good comparisons against ourselves.

Thus a few records, notes and the rest for others as well as for ourselves is likely, IMO, to be of help. Collaborative efforts that pay off for those so interested. (Yes, one may not "pay back" the installed cost, but the indirect benefits of reduced fuel burn on the whole of the vehicle -- not to mention the driver [wind effects] -- should also be considered as part of that payback).

Fuel economy is, after all, only a subset of Economy: all the dollars in consideration short & long term. And not the most important, IMO, as purchase price versus depreciation against nights of use is the predominant equation. (RV's are not cheaper than hotels, folks)

Aero aids do have their place.

But maybe not for the "commuter" trundling back and forth to Florida from Ohio every year (this is where attention to reducing to a minimum the number of steering wheel corrections will pay [per KENWORTH and CUMMINS]) as an examination of actual time (not just miles) spent at above 50-mph over 5k annual miles is not quite enough to warrant an expensive aero aid (versus "mechanical perfection" dollars; a sway-eliminating hitch would be in this category, too, as a fuel saver even though that is not the main reason people buy them).

For those who think they know how much time they spend on the highway, again, keep a log of actual time above 50-mph (the period between any acceleration and braking events at target travel speed). As a truck driver I can assure you it is lower than you "think" it is (surprised me once I started to note it) when it stands out against the full day of time in transit (from before the pre-trip to the post-trip inspection -- breaking camp and then setting camp for RV'ers). IOW, the "period" in which an aero aid is at work for us is rather small, overall, and mainly benefits those traveling more than, say, an above average number of RV'ng miles annually.

Notes & records. The RV'er will have to calculate the cost-effective point along with the indirect benefits as above to make a determination. Use tools, and share results. Numbers talk. Fuel ain't getting cheaper (and won't). Collaboration just might keep a few more of us on the road (driving smarter, smarter condition of vehicles, and smart aids) if enough of us concentrate on what works and what doesn't and try to determine "why . . . .?

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Old 09-17-2012, 12:07 PM   #14
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Wow Rednax !!!

Are you related to AL gore? (just kidding !!!) that's some write up !

In my line of work, some users want me to put 800 variables in my coding to 'get it right' when the changes cost more than the 0.005% difference in accuracy.... for banking and bridge building, that's important, for most other things - swag is good enough usually

but you are right, for most statistics, say like unemployment figures,
there's too much 'fuzzy math' that skews the numbers from the truth...
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