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03-07-2006, 04:45 PM
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#21
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3 Rivet Member
Commercial Member
1950 25' Cruiser
Currently Looking...
Currently Looking...
Lebanon
, Ohio
Join Date: May 2004
Posts: 238
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Exactly....
Quote:
Originally Posted by Silvertwinkie
You know, I watched something similar to this on Mythbusters a few months back.
They had two identical trucks one with the tailgate down, and one with it up.
The one that had it up got better economy than the one that had it down.
I forget the term, but when it's closed, it makes the air shoot over the bed, but when open, that air pocket is gone.
Anyway, to answer your question, yes, having one would be better than not having one when towing. The problem is that some caps weigh a lot, so any economy gain you might get from the aerodynamics could be lost if you get a heavy cover.
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Independent research has shown that there is no discernable difference to adding these to your vehicle.....
In fact, air drag is increased because more mass is introduced, regardless of the design.
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03-07-2006, 05:24 PM
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#22
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Rivet Master
Vintage Kin Owner
1977 31' Excella 500
Berkeley Springs
, West Virginia
Join Date: Sep 2005
Posts: 1,638
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In regard to the Mythbusters episode:
The only way that could be true would be if the tailgate being up formed a "barrier" similar to a tonneau cover would. Essentially, it'd form a cushion of air inside the enclosed portion of the bed now. So essentially, you'd only have a vortex form from the depth of the top of the cab to the top of the bed. With the tailgate down, I'm assuming they're saying that the vortex would be larger because now the air is trying to converge from the top of the cab to the bottom of the bed. The larger the vortex, the greater the drag.
A perfect example is the '68 Dodge Charger. While the car was very sleek and racy looking, it had the flying buttress rear roof line where the C-pillars extended further back than did the rear glass. The rear glass was rather steep, actually. When they raced them in NASCAR, they said the suction formed by that vortex forming off that backlight would suck potato chip bags onto the track. In '69 Dodge lost to the Torino Talledega of Ford, which was a true fast back. Dodge responded with the Charger 500 where they made the rear glass flush with the C-pillars. That helped a lot. They then put a bullet nose on it and that made the car incredibly fast, only there was too much downforce at the front and it made the rear light, so they added the wing (not a spoiler but a real wing, a NACA 2412, mounted upside down to provide downward "lift) and that car was truly unbeatable. It was so that NASCAR outlawed them after one season. But I digress...
Again, think of a tear drop. That is nature's low drag shape and we've not been able to beat it.
There was an airplane brought out some years ago called the Questair Venture. I always thought it was ugly, but it was very fast. It looks like an egg. The better looking planes are more streamlined and elongated looking (like a Swearingen SX-300 for example), but that elongation actually increases drag. The ideal ratio for aerodynamic drag reduction is about 3 to 1. That is, the length of the body should be triple the diameter. When you get higher than that, you're increasing the surface area excessively, even though it looks a lot better.
A Venture will cruise at 300mph on a 300hp engine. The SX-300 is probably 15mph slower, but (in my opinion) looks a lot better.
Anyway, pickups and SUVs are bricks and there's not much of any aerodynamic aids that will really make a huge difference. The boat idea is a good one. You probably can't do a whole lot better than that.
__________________
- Jim
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03-07-2006, 05:46 PM
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#23
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_
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Join Date: Dec 2004
Posts: 8,812
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hi air flow, air foil, air head, airstreamers......
the tail gait up/down issues just won't die....
and like the myth busters demo, i've seen several demos/reports debunking the value of tail gait down suggestions.....
now as for the cap issue on a truck pulling an airstream....
clearly the cap lowers mpg on the truck sans trailer....more weight, more drag, blunt butt...
while towing.....
i'd like to think it helps.....but i'm not so sure....
1. i use a hensley...so the gap betweeen truck and trailer is way more than drafting cars/bikes and so on...
2. the trailer is much taller than my 4/4 superduty with is pretty tall...
3. during rain or bugs...the front on my trailer (rockguards, upper segments and lower panels still get hit by lots of stuff...water/bugs don't flow past the front end of the trailer....as expected in a draft....
4. ford, gm, dodge, and all of the other towing vehicle guides mention what FRONTAL AREA (in sq.ft.) size limitations exist....just like weight load/towing or hitch weight limits....with no mention of improvements using a shell
5. the van versions of ford/gmc do not get better mpg than the trucks while towing....at least i've not seen anyone report better mpgs...
6. bike racks mounted between the truck/cap and the trailer still get really dirty....air does not flow past this stuff but all around it....
utilimately the full caps let us carry more crap and keep our crap from flying out of the truck...... but i really don't thing sheels/caps help fuel economy while towing our silver bullets...
show me the data!
cheers
2air'
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03-07-2006, 08:18 PM
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#24
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We can tow it!
1996 28' Excella
Where the water tastes like wine
, Michigan
Join Date: Jan 2005
Posts: 1,625
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Here's the data!
Wow I put on my tin foil thinking cap and remembered that I saw something about this on tv, it was great, and it boiled down to something about Strouhal numbers and simplified flight waveforms. So the formula is that you take the airspeed velocity of an unladen swallow and…wow, Here’s the data!
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03-08-2006, 06:06 AM
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#25
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Naysayer
1968 24' Tradewind
Russellville
, earth
Join Date: Jun 2005
Posts: 4,961
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I can tell you this about air flow between my 91 F150 with camper shell and 68 trade wind: if the window on the shell isnt LOCKED the air pressure pulls it to an open position every time.
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03-08-2006, 06:13 AM
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#26
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Aluminut
2004 25' Safari
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, Illinois
Join Date: Feb 2003
Posts: 10,477
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Quote:
Originally Posted by JimGolden
In regard to the Mythbusters episode:
The only way that could be true would be if the tailgate being up formed a "barrier" similar to a tonneau cover would. Essentially, it'd form a cushion of air inside the enclosed portion of the bed now. So essentially, you'd only have a vortex form from the depth of the top of the cab to the top of the bed. With the tailgate down, I'm assuming they're saying that the vortex would be larger because now the air is trying to converge from the top of the cab to the bottom of the bed. The larger the vortex, the greater the drag.
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I think thats what they were referrring to. It was one of the more interesting shows they've had.....most of them are kind of fluffy, but I really liked this one.
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03-13-2006, 06:45 PM
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#27
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2 Rivet Member
Join Date: Jan 2004
Posts: 52
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Many thanks for the good advice.
Sam
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09-25-2007, 04:10 PM
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#28
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2 Rivet Member
2001 31' Excella
Bluffton
, South Carolina
Join Date: Oct 2005
Posts: 62
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Pick Up with Cover for storage.
I purchased a ARE MX series with a lift rear door for my CC because my 30' SO trailer has such limited storage and weight restrictions. It was necessary for me also to get a canoe on top of the PU and bikes off the end of the cab. Too much stuff to travel without having that extra room that a camper back has to offer. When it's fully loaded, it handles great with Hensley managing my back side too. I use a cover for the bikes to keep them clean.
And for storage, it's fully lockable too!
Happy camping
__________________
Fred Bartlett
30' S/O 01 Excella, maple floors in Galley, HA Hitch, Hyd. Disc Brakes, P3 Controller, SS Kit. Bk Splash, LED RL's, 2K/Tri Fuel Gen's & RV on TT
04 Chevy HD, CC, D/A, Curt Magnum V Rec., Full Amsoil Syn.,Dual Filters, ARE, MX Topper with lift Gate Door, Bed Rug, TPS & Bk.up Camera.
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09-25-2007, 05:31 PM
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#29
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Rivet Master
1992 34' Excella
Austin
, Texas
Join Date: Mar 2007
Posts: 563
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I saw an episode of Motorweek (I think it was) where they featured a professional racing pickup at a closed track, and they did 5 timed trials of the truck with the tailgate open and 5 with the tailgate closed and the pickup attained the same maximum speed regardless of configuration. This seems to debunk the theory that tailgate open creates less drag.
__________________
Crusty
"If you come to a fork in the road, take it."
Lake Travis, TX
"Rancho Deluxe"
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09-25-2007, 06:02 PM
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#30
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4 Rivet Member
Currently Looking...
Yakima
, Washington
Join Date: Sep 2006
Posts: 381
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NASCAR has the areo thing down. They know what and what does not cause drag. The closer the back of your pickup is to the front on your airstream of course the less drag and the more important haveing a cap or not comes into play. We are talking about drag and fuel mileage in the end. If you truly want to increase fuel mileage while towing the Airstream of course is the way to tow....not only because of the curves front and rear but because its lower to the ground. The closer to the ground you get with a vehilce the less belly drag. That one reason a 2wd pickup gets better fuel mileage than a 4x4 besides the extra weight of the 4x4 there is less drag underneath. If you want to mess up fuel mileage real bad just put a lift on your truck. ITS NOT just the bigger tires but the increases drag because of the height of the truck. I saw a demonstration of this in 4X4 magazine a few years back. The put a 6 inch lift on a pickup and a set of 37's on the truck...the truck lost over 3 mpg because of the chassis and tire modifications....soo....they then put the STOCK tires and wheels on the truck but left the lift kit on and they only gained back half a mile per gallon. The lowered the truck...and holly crap....it grained the full 3 mpg. NOW it don't take a genious to understand it was the drag sucking the truck to the ground when it was wayyyy up in the air to know where the fuel mileage went.
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09-25-2007, 09:35 PM
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#31
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4 Rivet Member
2005 25' International CCD
1960 18' "Footer"
1959 26' Overlander
Riverside
, California
Join Date: Oct 2006
Posts: 339
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We can weigh in on this issue. We have a 2005 Dodge Ram 2500 with the Cummings diesel. In fairness I can't tell the difference in fuel mileage. Towing on level freeway at 55 to 65 and under good conditions we typically average 13.5 to 14.5. Head winds and quarter winds, speed, weather and terrain can cause towing mileage to vary between 10 mpg to an ocasional 17 mpg both with and without the shell. However I pulled out the five trips we did in two subsequent 12 month periods.
We pulled our first Airstream, a 22' CCD and then a 25' CCD for about 11,000 miles before we got our cab high LEER shell. Five trips in that time period were:
772 miles, 13.2 mpg (22')
6765 miles, 13.3 mpg (traded 22' for 25' on trip)
1800 miles, 13.9 mpg (25')
622 miles, 12.6 mpg (25')
1037 miles, 13.0 mpg (25'); average for the five trips 13.3.
Once we got our shell we again towed our 25' CCD about 11,000 miles:
6626 miles; 12.4 mpg
650 miles; 12.9 mpg
2126 miles; 14.9 mpg
806 miles; 13.3 mpg
795 miles; 14.2 mpg; 13.1 mpg average for these five trips.
It looks too close to call for me and no significant difference with the upgrade from the 22' to the 25' CCD. However, the shell does give us more security for our bikes, generator, chairs and other belongings in the back of the pickup.
__________________
Don (KD6UVT) & Gail Williams
What do you want to be in life, a spectator or a participant?
SNU #157
FCU #004
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09-26-2007, 03:57 AM
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#32
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Rivet Master
1973 Argosy 26
Norristown
, Pennsylvania
Join Date: Sep 2006
Posts: 645
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Quote:
Originally Posted by JimGolden
F-105's are cool too. heaviest single seater we had.
My personal favorite is the F-104 Starfighter. The Missile with a Man in it. Stall speed of about 230mph; you have to go faster to take off than a Cessna goes wide open straight down. That's cool! Who needs wings...just put some directional vanes on there and light the burners!
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Hey Jim; This human missile almost put me in the grave in 1965. "Boatdoc"
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09-26-2007, 04:30 AM
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#33
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3 Rivet Member
1986 25' Sovereign
Allegan
, Michigan
Join Date: Oct 2005
Posts: 146
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Aerodynamic Drag
Had to jump in with a few observations. Before retirement I was occassionally involved with vehicle road load issues working with various manufacturers. It is NOT a simple issue and sometimes things will suprise you.
One time we evaluated a vehicle with "flip up" headlights that caused a significant increase in road load at 50 mph - other sporty cars were not so bad. In discussing this with an engineer from a car company he said one of the vehicles he tested did better with the lights up.
I have not tested a truck with the tailgate up and down. But another engineer told me they tested a truck with an "air gate" (holes to "reduce" drag but keep stuff in) and it increased road load. I believe that SEMA tested various bed covers and they did improve fuel economy - but their members may have an interest in the outcome.
Several years ago I recall that one truck manufacturer was offering a cover that did reduce drag. Also, another manufacturer had a special version of its half ton trucks that had a lower ride height to reduce drag. They would not have taken those steps if they were not effective.
Finally, aerodynamic drag increases with the square of vehicle speed while tire rolling resistance increases only slightly. Slowing down does help - I think that the current Trailer Life did a test confirming this.
Interesting thread - hope this helps.
Whit Nash
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09-26-2007, 05:33 AM
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#34
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Retired.
Currently Looking...
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, At Large
Join Date: Jun 2003
Posts: 21,276
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The "wall" is about 52 mph. I've forgotten exact figures, but horsepower needed to move at 60 is about 30% more than at 50, and there is a percentage increase per mile per hour over that.
Moral is, if you want better gas mileage, drive slower.
Also, our trailer trails better with the cap on the bed, versus no cap.
__________________
Meddle not in the affairs of dragons, for you are crunchy, and taste good with ketchup.
Terry
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01-05-2012, 04:34 AM
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#35
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Vintage Kin
Fort Worth
, Texas
Join Date: Nov 2006
Posts: 8,014
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The short answer is . . maybe.
The utility of the pickup truck bed topper is of benefit in and of itself for closed covered storage.
So the shape of the topper is the first item to be covered: Should be "form fitting", that is, follows the contour of cab roof height and bed lines. As if one had built a Suburban-like vehicle, windows or not.
Does a traditional square pickup topper help fuel economy?
But to make the truck "aerodynamic" one needs to follow the "Streamlining Template which leaves a clean wake behind the TV, as with:
Those with a deeper interest:
Tonneau Cover or Cab Extension?
And a topper that shows an improvement of 20%:
Commercially Produced Aerodynamic Pickup Bed Cap
We've more recently covered this topic in Truck Shells and MPG
My best understanding is that one should treat both vehicles for aerodynamics separately as the gap between TV and TT is too large for a combined effect. Big trucks want just 24" between tractor and trailer or less (per aero aids) to reduce or eliminate wind turbulence between them. With a Hensley, the distance between tailgate skin and trailer front skin is 60".
So, an optimized truck will show better mpg while towing, not just
solo. As with the A/S itself: reduced HP demand shows a mpg benefit. And going to trouble to improve solo mpg by driving changes, and some truck spec changes, can underwrite a heckuva lot of vacation travel fuel via savings.
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