Aerodynamics acording to Myth Busters!

[cavelamb himself[_2_]]

It is more fuel efficient to drive a pick-up truck with its tailgate
down, rather than up.

Busted

Driving with the tailgate down actually increased drag on the pick-up
and caused it to consume fuel faster than the identical truck driven
with the tailgate up. It was later revealed that the closed tailgate
creates a locked vortex flow that created a smoother flow of air over
the truck. With the tailgate down, the trapped vortex was dissipated and
the drag increased.




http://en.wikipedia.org/wiki/MythBusters_%28season_3%29#Blown_Away

[Peter Dohm]

"cavelamb himself" > wrote in message
link.net...
>
> It is more fuel efficient to drive a pick-up truck with its tailgate
> down, rather than up.
>
> Busted
>
> Driving with the tailgate down actually increased drag on the pick-up
> and caused it to consume fuel faster than the identical truck driven
> with the tailgate up. It was later revealed that the closed tailgate
> creates a locked vortex flow that created a smoother flow of air over
> the truck. With the tailgate down, the trapped vortex was dissipated and
> the drag increased.
>
>
>
>
> http://en.wikipedia.org/wiki/MythBusters_%28season_3%29#Blown_Away

Somehow, that is even harder to accept, as a general statement, than the
original assertion.

It has not been proven either way, from my experience.
When I had a 1977 Toyota pick-up, I tried it and could not prove any
difference at all; so I went back to driving with the gate closed--because I
felt that leaving it open defeated that purpose of having a rear bumper.
OTOH, a know two owners of late 1990s full sized Chevy pick-up
trucks who both swear that it adds between 1 and 2 mpg. One of them, who
only occasionally loads cargo in the bed, simply leaves the tailgate at home
when not in use.

I must suggest that different trucks may have different results, even when
tested at the same speed, and that the biggest improvement of all might be
from driving the other car when there is nothing to haul. Then again, if
you have only one vehicle, there is a damned good case to be made in favor
of the truck!

Peter

[cavelamb himself[_2_]]

Peter Dohm wrote:

> "cavelamb himself" > wrote in message
> link.net...
>
>>It is more fuel efficient to drive a pick-up truck with its tailgate
>>down, rather than up.
>>
>>Busted
>>
>>Driving with the tailgate down actually increased drag on the pick-up
>>and caused it to consume fuel faster than the identical truck driven
>>with the tailgate up. It was later revealed that the closed tailgate
>>creates a locked vortex flow that created a smoother flow of air over
>>the truck. With the tailgate down, the trapped vortex was dissipated and
>>the drag increased.
>>
>>
>>
>>
>>http://en.wikipedia.org/wiki/MythBusters_%28season_3%29#Blown_Away
>
>
> Somehow, that is even harder to accept, as a general statement, than the
> original assertion.
>
> It has not been proven either way, from my experience.
> When I had a 1977 Toyota pick-up, I tried it and could not prove any
> difference at all; so I went back to driving with the gate closed--because I
> felt that leaving it open defeated that purpose of having a rear bumper.
> OTOH, a know two owners of late 1990s full sized Chevy pick-up
> trucks who both swear that it adds between 1 and 2 mpg. One of them, who
> only occasionally loads cargo in the bed, simply leaves the tailgate at home
> when not in use.
>
> I must suggest that different trucks may have different results, even when
> tested at the same speed, and that the biggest improvement of all might be
> from driving the other car when there is nothing to haul. Then again, if
> you have only one vehicle, there is a damned good case to be made in favor
> of the truck!
>
> Peter
>
>

I personaly can't argue either way.

Both answers seem to have merit - and then not...

But those webbed tailgate thingies have GOT to increase drag.

Don't they???

Richard

[John Clear]

In article . net>,
cavelamb himself > wrote:
>
>I personaly can't argue either way.
>
>Both answers seem to have merit - and then not...
>
>But those webbed tailgate thingies have GOT to increase drag.
>
>Don't they???

They re-visited the tailgate issue in a later episode, and re-busted
tail gate down being more efficient, but found that the mesh thingie
increased mileage. Still questionable since they were doing the
tests on public roads with traffic.

http://en.wikipedia.org/wiki/MythBusters_%28season_4%29#Tailgate_Up_vs._Tailgat e_Down


Necessary aviation content: They also tested the 'jealous wife
shredded the plane with a chain saw' myth.

http://kwc.org/resources/2006/mbnew/sp.photo.jpg

http://en.wikipedia.org/wiki/MythBusters_%28season_3%29#Shredded_Plane

John
--
John Clear - http://www.clear-prop.org/

[Matt Whiting]

cavelamb himself wrote:
>
> It is more fuel efficient to drive a pick-up truck with its tailgate
> down, rather than up.
>
> Busted
>
> Driving with the tailgate down actually increased drag on the pick-up
> and caused it to consume fuel faster than the identical truck driven
> with the tailgate up. It was later revealed that the closed tailgate
> creates a locked vortex flow that created a smoother flow of air over
> the truck. With the tailgate down, the trapped vortex was dissipated and
> the drag increased.
>
>
>
>
> http://en.wikipedia.org/wiki/MythBusters_%28season_3%29#Blown_Away

I remember seeing a wind tunnel test on this many years ago (I think in
the late 70s when I was an aerospace engineering student) long before
Mythbusters existed. However, there is no way you can convince some
people that this is a myth. They will argue to the death that they gain
mileage with the tailgate down, however, the wind tunnel tests were very
conclusive. A rotating vortex is established in the truck once you get
above a fairly slow speed (I think 30 MPH or so) and this acts almost
like a large balloon in the bed of the truck and directs the airflow
over the tailgate. You can see this pretty easily from the smoke in the
wind tunnel and the drag change was noticeable also.

Not only do you not get better mileage with the tailgate down ... you
actually get worse mileage! Then again, most people can't even check
their MPG correctly...


Matt

[Peter Dohm]

>
> I personaly can't argue either way.
>
> Both answers seem to have merit - and then not...
>
> But those webbed tailgate thingies have GOT to increase drag.
>
> Don't they???
>
> Richard

My feeling about the web tailgates is the same as yours; but I really don't
know.

Peter

[Harry K]

On Jul 23, 6:42 pm, (John Clear) wrote:
> In article . net>,
> cavelamb himself > wrote:
>
>
>
> >I personaly can't argue either way.
>
> >Both answers seem to have merit - and then not...
>
> >But those webbed tailgate thingies have GOT to increase drag.
>
> >Don't they???
>
> They re-visited the tailgate issue in a later episode, and re-busted
> tail gate down being more efficient, but found that the mesh thingie
> increased mileage. Still questionable since they were doing the
> tests on public roads with traffic.
>
> http://en.wikipedia.org/wiki/MythBusters_%28season_4%29#Tailgate_Up_v...
>
> Necessary aviation content: They also tested the 'jealous wife
> shredded the plane with a chain saw' myth.
>
> http://kwc.org/resources/2006/mbnew/sp.photo.jpg
>
> http://en.wikipedia.org/wiki/MythBusters_%28season_3%29#Shredded_Plane
>
> John
> --
> John Clear - http://www.clear-prop.org/

Yes, it has been proven more than once that tailgate up is better but
IIANM the difference is too small to worry about. Minor change in
driving behavior from one day to the next would wipe it out.

Harry K

[Richard Isakson]

"Matt Whiting" wrote ...
> cavelamb himself wrote:
> >
> > It is more fuel efficient to drive a pick-up truck with its tailgate
> > down, rather than up.
> >
> > Busted
> >
> > Driving with the tailgate down actually increased drag on the pick-up
> > and caused it to consume fuel faster than the identical truck driven
> > with the tailgate up. It was later revealed that the closed tailgate
> > creates a locked vortex flow that created a smoother flow of air over
> > the truck. With the tailgate down, the trapped vortex was dissipated and
> > the drag increased.
> >
> >
> >
> >
> > http://en.wikipedia.org/wiki/MythBusters_%28season_3%29#Blown_Away
>
> I remember seeing a wind tunnel test on this many years ago (I think in
> the late 70s when I was an aerospace engineering student) long before
> Mythbusters existed. However, there is no way you can convince some
> people that this is a myth. They will argue to the death that they gain
> mileage with the tailgate down, however, the wind tunnel tests were very
> conclusive. A rotating vortex is established in the truck once you get
> above a fairly slow speed (I think 30 MPH or so) and this acts almost
> like a large balloon in the bed of the truck and directs the airflow
> over the tailgate. You can see this pretty easily from the smoke in the
> wind tunnel and the drag change was noticeable also.
>
> Not only do you not get better mileage with the tailgate down ... you
> actually get worse mileage! Then again, most people can't even check
> their MPG correctly...
>
>
> Matt

YMMV!

Rich

[Stealth Pilot[_2_]]

On Tue, 24 Jul 2007 01:42:48 +0000 (UTC),
(John Clear) wrote:


>Necessary aviation content: They also tested the 'jealous wife
>shredded the plane with a chain saw' myth.
>
>http://kwc.org/resources/2006/mbnew/sp.photo.jpg
>
that photo is of an aircraft parked on parafield in south australia.
a chap with a flat battery in another aircraft hand propped the
aircraft, engine started and roared into life, the aircraft jumped the
chocks and graunched along the fuselage of the twin until the meatier
parts around the wing stopped the engine.
photo taken after the aircraft were pulled apart.

Stealth Pilot

[wright1902glider]

On Jul 24, 3:46 am, Stealth Pilot >
wrote:
> On Tue, 24 Jul 2007 01:42:48 +0000 (UTC),
>
I tested this one on my own after reading a NASA report about that
locked vortex thingie. Using cruise control only, at 70 mph, out
accross I-70 in western Kansas, both ways to account for headwinds,
etc., ad-infinitum, my 2002 Dakota 4-door averages 1/2 mpg better with
the tailgate up. woohoo. big deal.

The vortex is not a myth though. Wanna guess what happens when a house
painter opens the sliding rear window of his Chevy and then flicks his
butt out of the driver's window? Non-smoker's revenge! Yep, that so-
called vortex grabbed hold of the forest-fire inducing, bar polluting
cancer stick and shot it back inside the truck, under the seat. Where
it did its thing and set his truck on fire a few minutes later in the
parking lot of the local strip joint.

Now here's the real question. If Jamie was using an "aircraft" fuel-
flow meter and it had been "calibrated" to his truck (also a Dodge
Dakota, but a 2-door and I'm guessing a 3.9L V-6), why did it show
something like 5.4. ...5.4 what? They never did say how many mpg
they got with either method. And I've never been able to work out the
math on that number. If its gph, then at 55 mph, thats 5.4 gallons,
devided by 55 miles, equals 9.163 mpg? In a Dak? Only if he's towing a
5k trailer into a 30mph headwind uphill. The 3.9L Dakota should be
somewhere around 18-21 mpg at 55 mph. I can get 19-20 with my V-8.
Could it be pounds per hour? That doesn't sound right either. A gallon
of unleaded is roughly 6 lbs according to BP, or so sez the internet
page I just Googled. So Jamie's truck is getting nearly 60 mpg at 55
mph?

Anyone have any more insight on this?

Harry "saws arm off and hands to gas station clerk" Frey
'02 Dak pusher

[Denny]

On Jul 24, 12:20 am, "Richard Isakson" > wrote:
> "Matt Whiting" wrote ...
>
>
>
>
>
> > cavelamb himself wrote:
>
> > > It is more fuel efficient to drive a pick-up truck with its tailgate
> > > down, rather than up.
>
> > > Busted
>
> > > Driving with the tailgate down actually increased drag on the pick-up
> > > and caused it to consume fuel faster than the identical truck driven
> > > with the tailgate up. It was later revealed that the closed tailgate
> > > creates a locked vortex flow that created a smoother flow of air over
> > > the truck. With the tailgate down, the trapped vortex was dissipated and
> > > the drag increased.
>
> > >http://en.wikipedia.org/wiki/MythBusters_%28season_3%29#Blown_Away
>
> > I remember seeing a wind tunnel test on this many years ago (I think in
> > the late 70s when I was an aerospace engineering student) long before
> > Mythbusters existed. However, there is no way you can convince some
> > people that this is a myth. They will argue to the death that they gain
> > mileage with the tailgate down, however, the wind tunnel tests were very
> > conclusive. A rotating vortex is established in the truck once you get
> > above a fairly slow speed (I think 30 MPH or so) and this acts almost
> > like a large balloon in the bed of the truck and directs the airflow
> > over the tailgate. You can see this pretty easily from the smoke in the
> > wind tunnel and the drag change was noticeable also.
>
> > Not only do you not get better mileage with the tailgate down ... you
> > actually get worse mileage! Then again, most people can't even check
> > their MPG correctly...
>
> > Matt
>
> YMMV!
>
> Rich- Hide quoted text -
>
> - Show quoted text -

Paper material in the bed of my truck at 55mph rotates once or trwice
in that standing vortex then vanishes over the tailgate - never to be
seen again...
Any empty, plastic gas can left against the tailgate slides up against
the back of the cab - again that standing vortex...
Now, that the gate up has less drag overal still amazes me, in spite
of knowing what is theoretically happening...

denny

[Peter Dohm]

"Denny" > wrote in message
ups.com...
> On Jul 24, 12:20 am, "Richard Isakson" > wrote:
> > "Matt Whiting" wrote ...
> >
> >
> >
> >
> >
> > > cavelamb himself wrote:
> >
> > > > It is more fuel efficient to drive a pick-up truck with its tailgate
> > > > down, rather than up.
> >
> > > > Busted
> >
> > > > Driving with the tailgate down actually increased drag on the
pick-up
> > > > and caused it to consume fuel faster than the identical truck driven
> > > > with the tailgate up. It was later revealed that the closed tailgate
> > > > creates a locked vortex flow that created a smoother flow of air
over
> > > > the truck. With the tailgate down, the trapped vortex was dissipated
and
> > > > the drag increased.
> >
> > > >http://en.wikipedia.org/wiki/MythBusters_%28season_3%29#Blown_Away
> >
> > > I remember seeing a wind tunnel test on this many years ago (I think
in
> > > the late 70s when I was an aerospace engineering student) long before
> > > Mythbusters existed. However, there is no way you can convince some
> > > people that this is a myth. They will argue to the death that they
gain
> > > mileage with the tailgate down, however, the wind tunnel tests were
very
> > > conclusive. A rotating vortex is established in the truck once you
get
> > > above a fairly slow speed (I think 30 MPH or so) and this acts almost
> > > like a large balloon in the bed of the truck and directs the airflow
> > > over the tailgate. You can see this pretty easily from the smoke in
the
> > > wind tunnel and the drag change was noticeable also.
> >
> > > Not only do you not get better mileage with the tailgate down ... you
> > > actually get worse mileage! Then again, most people can't even check
> > > their MPG correctly...
> >
> > > Matt
> >
> > YMMV!
> >
> > Rich- Hide quoted text -
> >
> > - Show quoted text -
>
> Paper material in the bed of my truck at 55mph rotates once or trwice
> in that standing vortex then vanishes over the tailgate - never to be
> seen again...
> Any empty, plastic gas can left against the tailgate slides up against
> the back of the cab - again that standing vortex...
> Now, that the gate up has less drag overal still amazes me, in spite
> of knowing what is theoretically happening...
>
> denny
>
I, for one, am not convinced that a p/u has less drag with the gate
closed--although I concede that many trucks may gain a trivial benefit at
some particular speed.

Their are just too many variables; including the shape of the cab, shape of
the nose, slope of the windshield, and length of the bed; to draw any
meaningful concludions from just one or two tests. In addition, the vortex
grows in size with increasing speed--so that the forward moving portion will
be nearer the back of the cab at lower speeds and may be aft of the tailgate
at very high speeds. However, a radically sloped nose and windshield may
cause the vortex to be much longer front to back at all speeds; the the
lengths of the cab and bed will have an effect.

If I had to wager on the outcome of a *real* test of this issue, I would
guess that there would be a slight drag reduction (on average) with the
tailgate down or removed--especailly if there is any cross wind--and that a
bed cover (a/k/a tonneau) would do at least as well under *all* conditions.

In any case, if you choose to run with the gate open or off, be sure to
secure your load! About 20 years ago, I personally watched a driver lose a
P&W Wasp Jr when he forgot it was there and mashed the accelerator. That
was a major "ah, darn"!

Just my $0.02
Peter

[OldPhart]

"Peter Dohm" > wrote in message
...
|
| "Denny" > wrote in message
| ups.com...
| > On Jul 24, 12:20 am, "Richard Isakson" > wrote:
| > > "Matt Whiting" wrote ...
| > >
| > >
| > >
| > >
| > >
| > > > cavelamb himself wrote:
| > >
| > > > > It is more fuel efficient to drive a pick-up truck with its
tailgate
| > > > > down, rather than up.
| > >
| > > > > Busted
| > >
| > > > > Driving with the tailgate down actually increased drag on the
| pick-up
| > > > > and caused it to consume fuel faster than the identical truck
driven
| > > > > with the tailgate up. It was later revealed that the closed
tailgate
| > > > > creates a locked vortex flow that created a smoother flow of
air
| over
| > > > > the truck. With the tailgate down, the trapped vortex was
dissipated
| and
| > > > > the drag increased.
| > >
| > > >
>http://en.wikipedia.org/wiki/MythBusters_%28season_3%29#Blown_Away
| > >
| > > > I remember seeing a wind tunnel test on this many years ago (I
think
| in
| > > > the late 70s when I was an aerospace engineering student) long
before
| > > > Mythbusters existed. However, there is no way you can convince
some
| > > > people that this is a myth. They will argue to the death that
they
| gain
| > > > mileage with the tailgate down, however, the wind tunnel tests
were
| very
| > > > conclusive. A rotating vortex is established in the truck once
you
| get
| > > > above a fairly slow speed (I think 30 MPH or so) and this acts
almost
| > > > like a large balloon in the bed of the truck and directs the
airflow
| > > > over the tailgate. You can see this pretty easily from the
smoke in
| the
| > > > wind tunnel and the drag change was noticeable also.
| > >
| > > > Not only do you not get better mileage with the tailgate down
.... you
| > > > actually get worse mileage! Then again, most people can't even
check
| > > > their MPG correctly...
| > >
| > > > Matt
| > >
| > > YMMV!
| > >
| > > Rich- Hide quoted text -
| > >
| > > - Show quoted text -
| >
| > Paper material in the bed of my truck at 55mph rotates once or
trwice
| > in that standing vortex then vanishes over the tailgate - never to
be
| > seen again...
| > Any empty, plastic gas can left against the tailgate slides up
against
| > the back of the cab - again that standing vortex...
| > Now, that the gate up has less drag overal still amazes me, in spite
| > of knowing what is theoretically happening...
| >
| > denny
| >
| I, for one, am not convinced that a p/u has less drag with the gate
| closed--although I concede that many trucks may gain a trivial benefit
at
| some particular speed.
|
| Their are just too many variables; including the shape of the cab,
shape of
| the nose, slope of the windshield, and length of the bed; to draw any
| meaningful concludions from just one or two tests. In addition, the
vortex
| grows in size with increasing speed--so that the forward moving
portion will
| be nearer the back of the cab at lower speeds and may be aft of the
tailgate
| at very high speeds. However, a radically sloped nose and windshield
may
| cause the vortex to be much longer front to back at all speeds; the
the
| lengths of the cab and bed will have an effect.
|
| If I had to wager on the outcome of a *real* test of this issue, I
would
| guess that there would be a slight drag reduction (on average) with
the
| tailgate down or removed--especailly if there is any cross wind--and
that a
| bed cover (a/k/a tonneau) would do at least as well under *all*
conditions.
|
| In any case, if you choose to run with the gate open or off, be sure
to
| secure your load! About 20 years ago, I personally watched a driver
lose a
| P&W Wasp Jr when he forgot it was there and mashed the accelerator.
That
| was a major "ah, darn"!
|
| Just my $0.02
| Peter


This issue came up on the Toyota PU site some time back. One of the
references was a university study that clearly showed that the mileage
was better with the tail gate up. Here is a link showing no meaningful
difference- http://www.scangauge.com/support/tailgate.shtml

The university study also tested bed covers and bed caps. The best
mileage was obtained with a cap that was cab high and had a rounded rear
end that blended into the tailgate.

While searching I found that in some states running with the tailgate
down is technically against the law.

--
OldPhart



----== Posted via Newsfeeds.Com - Unlimited-Unrestricted-Secure Usenet News==----
http://www.newsfeeds.com The #1 Newsgroup Service in the World! >100,000 Newsgroups
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[Capt. Geoffrey Thorpe]

"John Clear" > wrote in message
...
> In article . net>,
> cavelamb himself > wrote:
>>
>>I personaly can't argue either way.
>>
>>Both answers seem to have merit - and then not...
>>
>>But those webbed tailgate thingies have GOT to increase drag.
>>
>>Don't they???
>
> They re-visited the tailgate issue in a later episode, and re-busted
> tail gate down being more efficient, but found that the mesh thingie
> increased mileage. Still questionable since they were doing the
> tests on public roads with traffic.
>

It's hard to measure small differences in fuel economy even when you are
running a vehicle over the same cycle on a chassis dyno with a million
dollars worth of analyzers- trust me on that one - I've looked at the
results of thousands of emission tests over the years.

Trying to see the difference on public roads in traffic? You might as well
throw some dice for anything less than 10 or 20 percent.

Doing coast-downs is a much better method - it gives a reasonably reliable
results for finding changes in aero drag.


--
Geoff
The Sea Hawk at Wow Way d0t Com
remove spaces and make the obvious substitutions to reply by mail
When immigration is outlawed, only outlaws will immigrate.

[Ernest Christley]

Peter Dohm wrote:

> I, for one, am not convinced that a p/u has less drag with the gate
> closed--although I concede that many trucks may gain a trivial benefit at
> some particular speed.
>

Why is it so hard to believe. You have seperated flow, and seperated
flow equals high drag. Put the tailgate down and you have one huge drag
chute in the form of the cab. Put the gate up, and you get an air dam
in front of the gate that sort of creates an airfoil from the top of the
cab to the top of the gate. You've still got a big drag chute, just not
as big as before.

Someone smarter than me said that how you meet the air isn't as
important as how you leave it.

[Peter Dohm]

"Capt. Geoffrey Thorpe" <The Sea Hawk at wow way d0t com> wrote in message >
> It's hard to measure small differences in fuel economy even when you are
> running a vehicle over the same cycle on a chassis dyno with a million
> dollars worth of analyzers- trust me on that one - I've looked at the
> results of thousands of emission tests over the years.
>
> Trying to see the difference on public roads in traffic? You might as well
> throw some dice for anything less than 10 or 20 percent.
>
> Doing coast-downs is a much better method - it gives a reasonably reliable
> results for finding changes in aero drag.
>
>
> --
> Geoff
> The Sea Hawk at Wow Way d0t Com
> remove spaces and make the obvious substitutions to reply by mail
> When immigration is outlawed, only outlaws will immigrate.
>
>
Exdellent point.

Peter

[Peter Dohm]

"Ernest Christley" > wrote in message
...
> Peter Dohm wrote:
>
> > I, for one, am not convinced that a p/u has less drag with the gate
> > closed--although I concede that many trucks may gain a trivial benefit
at
> > some particular speed.
> >
>
> Why is it so hard to believe. You have seperated flow, and seperated
> flow equals high drag. Put the tailgate down and you have one huge drag
> chute in the form of the cab. Put the gate up, and you get an air dam
> in front of the gate that sort of creates an airfoil from the top of the
> cab to the top of the gate. You've still got a big drag chute, just not
> as big as before.
>
> Someone smarter than me said that how you meet the air isn't as
> important as how you leave it.

It is not hard to believe at all--except for the proposition that either
trick (gate open or gate closed) works on all trucks at all speeds.

[Morgans[_2_]]

"Peter Dohm" > wrote
>
> It is not hard to believe at all--except for the proposition that either
> trick (gate open or gate closed) works on all trucks at all speeds.

Trucks are not that different. Except for something like the old El Camino, thy
are mostly boxes on wheels, to the wind.

Sure, they have a small styling curve here and there, but most of them are
square chopped off behind the cab, and have tailgates about the same height.
--
Jim in NC

[Blueskies]

"Denny" > wrote in message ups.com...
>
> Paper material in the bed of my truck at 55mph rotates once or trwice
> in that standing vortex then vanishes over the tailgate - never to be
> seen again...
> Any empty, plastic gas can left against the tailgate slides up against
> the back of the cab - again that standing vortex...
> Now, that the gate up has less drag overal still amazes me, in spite
> of knowing what is theoretically happening...
>
> denny
>

I forget which brand truck we were looking at (Ford?), but it had a sculpted shape to the top of the tailgate about 8"
wide. The salesman said it helped to improve gas mileage...

[Morgans[_2_]]

"Blueskies" > wrote
>
> I forget which brand truck we were looking at (Ford?), but it had a sculpted
> shape to the top of the tailgate about 8" wide. The salesman said it helped to
> improve gas mileage...

He was wrong. It is to provide extra clearance for people towing 5th wheel or
gooseneck trailers.
--
Jim in NC

[Blueskies]

"Morgans" > wrote in message ...
>
> "Blueskies" > wrote
>>
>> I forget which brand truck we were looking at (Ford?), but it had a sculpted shape to the top of the tailgate about
>> 8" wide. The salesman said it helped to improve gas mileage...
>
> He was wrong. It is to provide extra clearance for people towing 5th wheel or gooseneck trailers.
> --
> Jim in NC

No, it was not a 5th wheel clearance thing. I did not describe it very clearly. Regular rectangular tailgate with a wide
upper (top when closed) edge...

[Ernest Christley]

Blueskies wrote:
> "Morgans" > wrote in message ...
>> "Blueskies" > wrote
>>> I forget which brand truck we were looking at (Ford?), but it had a sculpted shape to the top of the tailgate about
>>> 8" wide. The salesman said it helped to improve gas mileage...
>> He was wrong. It is to provide extra clearance for people towing 5th wheel or gooseneck trailers.
>> --
>> Jim in NC
>
> No, it was not a 5th wheel clearance thing. I did not describe it very clearly. Regular rectangular tailgate with a wide
> upper (top when closed) edge...
>
>

You see it on a lot of trucks. It used to be that a box van was just
that, a box. A decade or so ago, they started rounding the front edges
of the box. Can you say "fairing". Some started rounding the back
corners also. I've seen SUV's with what looks like reverse scoops that
could have to catch the air passing over the top of the vehicle and blow
it down the back. Counterintuitive until you realize that attached flow
offers less drag.

The thick tailgate with the rounded edge keeps the air in attached flow
longer. Even a small amount, just a few inches, offers major
improvements over the previous generations 90 degree chop offs.

[Peter Dohm]

"Morgans" > wrote in message
...
>
> "Peter Dohm" > wrote
> >
> > It is not hard to believe at all--except for the proposition that either
> > trick (gate open or gate closed) works on all trucks at all speeds.
>
> Trucks are not that different. Except for something like the old El
Camino, thy
> are mostly boxes on wheels, to the wind.
>
> Sure, they have a small styling curve here and there, but most of them are
> square chopped off behind the cab, and have tailgates about the same
height.
> --
> Jim in NC
>
IMHO, they vary a great deal, and radically sloped windshields and extended
cabs both influence the size, intensity, and placement of the vortex. The
length of the cab will also influence the width of the vortex and crosswinds
will cause an offset.

OTOH, aerodynamic side mirrors (if available), or the presence or absence of
a front air dam, would probably make much more difference than the position
of the tail gate.

There are a lot of aerodynamic tricks, such as belly pans which can make a
huge difference, that just won't work for owners who really use their
trucks.

Peter

[cavelamb himself[_2_]]

Morgans wrote:

>
> "Peter Dohm" > wrote
>
>>
>> It is not hard to believe at all--except for the proposition that either
>> trick (gate open or gate closed) works on all trucks at all speeds.
>
>
> Trucks are not that different. Except for something like the old El
> Camino, thy are mostly boxes on wheels, to the wind.
>
> Sure, they have a small styling curve here and there, but most of them
> are square chopped off behind the cab, and have tailgates about the same
> height.


Weeeel, shoot. Princeton seems to have changes their web site.
They had one of the best on line aerodynamics sites I ever saw.
Looks like it turned into a book.
All that I could find is the lecture slides...


For visualization, right side top of page - Wind Rotor.
http://www.princeton.edu/~stengel/MAE331Lecture20.pdf

[Peter Dohm]

>
> You see it on a lot of trucks. It used to be that a box van was just
> that, a box. A decade or so ago, they started rounding the front edges
> of the box. Can you say "fairing". Some started rounding the back
> corners also. I've seen SUV's with what looks like reverse scoops that
> could have to catch the air passing over the top of the vehicle and blow
> it down the back. Counterintuitive until you realize that attached flow
> offers less drag.
>
You have been tricked! The attached downward flow has nothing to do with
drag; but is to keep exhaust and also road dirt away from the rear of the
vehicle. Similar ducts became a standard feature of tull sized station
wagons during the '60s and '70s.

> The thick tailgate with the rounded edge keeps the air in attached flow
> longer. Even a small amount, just a few inches, offers major
> improvements over the previous generations 90 degree chop offs.
>
I have yet to examine the rounded tail gate, but have my doubts. Suffice it
to say that some salesmen are even more creative than the brochure
writers--so you might compare the company litterature to the salesman's
assertion.

Peter

[Morgans[_2_]]

"Peter Dohm" > wrote

> IMHO, they vary a great deal, and radically sloped windshields and extended
> cabs both influence the size, intensity, and placement of the vortex. The
> length of the cab will also influence the width of the vortex and crosswinds
> will cause an offset.
>
> OTOH, aerodynamic side mirrors (if available), or the presence or absence of
> a front air dam, would probably make much more difference than the position
> of the tail gate.
>
> There are a lot of aerodynamic tricks, such as belly pans which can make a
> huge difference, that just won't work for owners who really use their
> trucks.

The underbody aerodynamics is probably the best thing you can do on a truck, but
like you say, then they couldn't work them.

Someone else said, the way you break the wind is not as important as how you
leave the wind. There is a lot of truth in that, but not completely true. A
truck breaks the wind a lot better than a short nosed van, I'll bet.

Still, I think all the above stuff you mentioned is mainly there for styling.
Sure, it helps a little, but to what degree would a slick looking van improve on
a boxy on from the 70's, say? 2%? 5%?

The vortex still has a chance to form, be it a short bed, long bed, slick cab,
boxy cab. How it is shaped is not all that important. It is still a vortex.
All the trucks still have a straight, square drop-off from the cab to the bed.
The vortex just fills in whatever space is left over.
--
Jim in NC

[Morgans[_2_]]

"Blueskies" > wrote in message
et...
>
> "Morgans" > wrote in message
> ...
>>
>> "Blueskies" > wrote
>>>
>>> I forget which brand truck we were looking at (Ford?), but it had a sculpted
>>> shape to the top of the tailgate about 8" wide. The salesman said it helped
>>> to improve gas mileage...
>>
>> He was wrong. It is to provide extra clearance for people towing 5th wheel
>> or gooseneck trailers.
>> --
>> Jim in NC
>
> No, it was not a 5th wheel clearance thing. I did not describe it very
> clearly. Regular rectangular tailgate with a wide upper (top when closed)
> edge...

I guess I still don't understand. I'll mosey over to the Ford site in a while
and look at it.
--
Jim in NC

[Morgans[_2_]]

"Ernest Christley" > wrote

> I've seen SUV's with what looks like reverse scoops that could have to catch
> the air passing over the top of the vehicle and blow it down the back.
> Counterintuitive until you realize that attached flow offers less drag.

That scoop helps drag, but more importantly, it prevents carbon monoxide from
entering in the back window when it is open and driving at highway speeds.

When a chopped off SUV has the window open, that low pressure area in the back
gathers up the exhaust, and the swirling motion of the vortex allows some to
enter in though the open window. Not good. The scoop supplies fresh air
flowing down from the top, and keeps the bad air down lower than the open
window.

They have been doing that since the 60's, on old station wagons, before they
cared about mileage and aerodynamics.
--
Jim in NC

[Blueskies]

"Morgans" > wrote in message ...
>
> "Blueskies" > wrote in message et...
>>
>> "Morgans" > wrote in message ...
>>>
>>> "Blueskies" > wrote
>>>>
>>>> I forget which brand truck we were looking at (Ford?), but it had a sculpted shape to the top of the tailgate about
>>>> 8" wide. The salesman said it helped to improve gas mileage...
>>>
>>> He was wrong. It is to provide extra clearance for people towing 5th wheel or gooseneck trailers.
>>> --
>>> Jim in NC
>>
>> No, it was not a 5th wheel clearance thing. I did not describe it very clearly. Regular rectangular tailgate with a
>> wide upper (top when closed) edge...
>
> I guess I still don't understand. I'll mosey over to the Ford site in a while and look at it.
> --
> Jim in NC

Like I said, not sure if it was Ford or not... We looked at the Dodge also and ended up with the Toy(ota)...

dd

[Bill Daniels]

You guys haven't mentioned the biggie - roof racks. Removing the cross bars
from my Grand Cherokee added 2MPG. Glider pilots look at roof racks and see
open air brakes. I can't believe the number of vehicles I see in Denver
with ski racks in July.

Then, of course, you can park the truck. I understand that trucks have very
low drag when parked.

If the vehicle has a MPG computer, you can see what speed does to MPG. At
65, my Jeep gets 19MPG. At 55, it gets 24MPG. At 45, it gets 36MPG. If
you do the numbers, saving two minutes by driving fast costs big time.
Those are REALLY expensive minutes - more than the rental on some airplanes.

Bill Daniels


"Morgans" > wrote in message
...
>
> "Ernest Christley" > wrote
>
>> I've seen SUV's with what looks like reverse scoops that could have to
>> catch the air passing over the top of the vehicle and blow it down the
>> back. Counterintuitive until you realize that attached flow offers less
>> drag.
>
> That scoop helps drag, but more importantly, it prevents carbon monoxide
> from entering in the back window when it is open and driving at highway
> speeds.
>
> When a chopped off SUV has the window open, that low pressure area in the
> back gathers up the exhaust, and the swirling motion of the vortex allows
> some to enter in though the open window. Not good. The scoop supplies
> fresh air flowing down from the top, and keeps the bad air down lower than
> the open window.
>
> They have been doing that since the 60's, on old station wagons, before
> they cared about mileage and aerodynamics.
> --
> Jim in NC

[Alan Baker]

In article >,
"Bill Daniels" <bildan@comcast-dot-net> wrote:

> You guys haven't mentioned the biggie - roof racks. Removing the cross bars
> from my Grand Cherokee added 2MPG. Glider pilots look at roof racks and see
> open air brakes. I can't believe the number of vehicles I see in Denver
> with ski racks in July.
>
> Then, of course, you can park the truck. I understand that trucks have very
> low drag when parked.
>
> If the vehicle has a MPG computer, you can see what speed does to MPG. At
> 65, my Jeep gets 19MPG. At 55, it gets 24MPG. At 45, it gets 36MPG. If
> you do the numbers, saving two minutes by driving fast costs big time.
> Those are REALLY expensive minutes - more than the rental on some airplanes.


Real world case; let's use your figures for simplicity's sake:

I want to drive from Vancouver to Kelowna. A little check with Google
Maps tells us that that's a distance of...

....393 kilometers, or about 245 miles.

At 65, that takes me about 3 hours, 45 minutes and I use about 13
gallons of gas.

At 55, it takes 4 hours, 30 minutes and I use about 10 gallons of gas.

Even at Canadian gas prices, those 10 litres of fuel cost me only about
$11.50. Is that really too much to pay to save 45 minutes of my precious
time? 25 cents a minute seem pretty cheap to me.



>
> Bill Daniels
>
>
> "Morgans" > wrote in message
> ...
> >
> > "Ernest Christley" > wrote
> >
> >> I've seen SUV's with what looks like reverse scoops that could have to
> >> catch the air passing over the top of the vehicle and blow it down the
> >> back. Counterintuitive until you realize that attached flow offers less
> >> drag.
> >
> > That scoop helps drag, but more importantly, it prevents carbon monoxide
> > from entering in the back window when it is open and driving at highway
> > speeds.
> >
> > When a chopped off SUV has the window open, that low pressure area in the
> > back gathers up the exhaust, and the swirling motion of the vortex allows
> > some to enter in though the open window. Not good. The scoop supplies
> > fresh air flowing down from the top, and keeps the bad air down lower than
> > the open window.
> >
> > They have been doing that since the 60's, on old station wagons, before
> > they cared about mileage and aerodynamics.
> > --
> > Jim in NC

--
Alan Baker
Vancouver, British Columbia
"If you raise the ceiling four feet, move the fireplace from that wall
to that wall, you'll still only get the full stereophonic effect if you
sit in the bottom of that cupboard."

[cavelamb himself[_2_]]

Bill Daniels wrote:
> You guys haven't mentioned the biggie - roof racks. Removing the cross bars
> from my Grand Cherokee added 2MPG. Glider pilots look at roof racks and see
> open air brakes. I can't believe the number of vehicles I see in Denver
> with ski racks in July.
>
> Then, of course, you can park the truck. I understand that trucks have very
> low drag when parked.
>
> If the vehicle has a MPG computer, you can see what speed does to MPG. At
> 65, my Jeep gets 19MPG. At 55, it gets 24MPG. At 45, it gets 36MPG. If
> you do the numbers, saving two minutes by driving fast costs big time.
> Those are REALLY expensive minutes - more than the rental on some airplanes.
>
> Bill Daniels
>

Can't argue against the open drag brake idea.

I have a nice pair on my Blazer.
My impression is that they are there to enhance roll-over protection.

I don't have the MPG computer, but I just drove down to Centerville
last weekend. 120 miles alnost exactly.

V6 4 speed automatic - with highay gearing, it's NOT a tow truck.
(and when I hook the boat on the back the Blazer whines and whimpers)

22 MPG going down at 75-85 mph.
26 coming back ar 65.

But about 18 around town...

Bringing the boat home from Khema - in stop and go Houston traffic -
climbing the ovepasses from a standing stop -
first 100 miles at 10 to 25 MPH took 10 gallons.

After that, at about 50 MPH I got more like 15-16 MPG.

(The boat is 18 ft long, but over 1500 pounds - plus a heavy trailer)

As they say, YMMV.

Sometimes a LOT...

Richard

[Matt Whiting]

Peter Dohm wrote:
> "Ernest Christley" > wrote in message
> ...
>> Peter Dohm wrote:
>>
>>> I, for one, am not convinced that a p/u has less drag with the gate
>>> closed--although I concede that many trucks may gain a trivial benefit
> at
>>> some particular speed.
>>>
>> Why is it so hard to believe. You have seperated flow, and seperated
>> flow equals high drag. Put the tailgate down and you have one huge drag
>> chute in the form of the cab. Put the gate up, and you get an air dam
>> in front of the gate that sort of creates an airfoil from the top of the
>> cab to the top of the gate. You've still got a big drag chute, just not
>> as big as before.
>>
>> Someone smarter than me said that how you meet the air isn't as
>> important as how you leave it.
>
> It is not hard to believe at all--except for the proposition that either
> trick (gate open or gate closed) works on all trucks at all speeds.

I agree that we must always be careful when extrapolating. However,
having said that, what little real data I've seen has showed a benefit
with the tailgate up. I have yet to see a single test show a benefit
with the tailgate down.

Matt

[Harry K]

On Jul 27, 7:49 pm, Alan Baker > wrote:
> In article >,
> "Bill Daniels" <bildan@comcast-dot-net> wrote:
>
> > You guys haven't mentioned the biggie - roof racks. Removing the cross bars
> > from my Grand Cherokee added 2MPG. Glider pilots look at roof racks and see
> > open air brakes. I can't believe the number of vehicles I see in Denver
> > with ski racks in July.
>
> > Then, of course, you can park the truck. I understand that trucks have very
> > low drag when parked.
>
> > If the vehicle has a MPG computer, you can see what speed does to MPG. At
> > 65, my Jeep gets 19MPG. At 55, it gets 24MPG. At 45, it gets 36MPG. If
> > you do the numbers, saving two minutes by driving fast costs big time.
> > Those are REALLY expensive minutes - more than the rental on some airplanes.
>
> Real world case; let's use your figures for simplicity's sake:
>
> I want to drive from Vancouver to Kelowna. A little check with Google
> Maps tells us that that's a distance of...
>
> ...393 kilometers, or about 245 miles.
>
> At 65, that takes me about 3 hours, 45 minutes and I use about 13
> gallons of gas.
>
> At 55, it takes 4 hours, 30 minutes and I use about 10 gallons of gas.
>
> Even at Canadian gas prices, those 10 litres of fuel cost me only about
> $11.50. Is that really too much to pay to save 45 minutes of my precious
> time? 25 cents a minute seem pretty cheap to me.
>
>
>
>
>
>
>
> > Bill Daniels
>
> > "Morgans" > wrote in message
> ...
>
> > > "Ernest Christley" > wrote
>
> > >> I've seen SUV's with what looks like reverse scoops that could have to
> > >> catch the air passing over the top of the vehicle and blow it down the
> > >> back. Counterintuitive until you realize that attached flow offers less
> > >> drag.
>
> > > That scoop helps drag, but more importantly, it prevents carbon monoxide
> > > from entering in the back window when it is open and driving at highway
> > > speeds.
>
> > > When a chopped off SUV has the window open, that low pressure area in the
> > > back gathers up the exhaust, and the swirling motion of the vortex allows
> > > some to enter in though the open window. Not good. The scoop supplies
> > > fresh air flowing down from the top, and keeps the bad air down lower than
> > > the open window.
>
> > > They have been doing that since the 60's, on old station wagons, before
> > > they cared about mileage and aerodynamics.
> > > --
> > > Jim in NC
>
> --
> Alan Baker
> Vancouver, British Columbia
> "If you raise the ceiling four feet, move the fireplace from that wall
> to that wall, you'll still only get the full stereophonic effect if you
> sit in the bottom of that cupboard."- Hide quoted text -
>
> - Show quoted text -

I think it will cost a bit more than that. From my experiences in the
B.C. lakes region and the Coq the cops are a bit anal about the posted
limits. It has been a few years but the Coq was posted 110 (about
62mph). Then there is that stretch from Merritt until you pick up the
Eastern extension of the freeway that is posted 90. Then from
Peachland to Kelowna is again max of 90 with stretches down to 80
(IIRC). You might be parked alongside the road a bit. :)

I got a ticket in there "Exceeding speed limit while passing".

Harry K

[Alan Baker]

In article om>,
Harry K > wrote:

> On Jul 27, 7:49 pm, Alan Baker > wrote:
> > In article >,
> > "Bill Daniels" <bildan@comcast-dot-net> wrote:
> >
> > > You guys haven't mentioned the biggie - roof racks. Removing the cross
> > > bars
> > > from my Grand Cherokee added 2MPG. Glider pilots look at roof racks and
> > > see
> > > open air brakes. I can't believe the number of vehicles I see in Denver
> > > with ski racks in July.
> >
> > > Then, of course, you can park the truck. I understand that trucks have
> > > very
> > > low drag when parked.
> >
> > > If the vehicle has a MPG computer, you can see what speed does to MPG.
> > > At
> > > 65, my Jeep gets 19MPG. At 55, it gets 24MPG. At 45, it gets 36MPG. If
> > > you do the numbers, saving two minutes by driving fast costs big time.
> > > Those are REALLY expensive minutes - more than the rental on some
> > > airplanes.
> >
> > Real world case; let's use your figures for simplicity's sake:
> >
> > I want to drive from Vancouver to Kelowna. A little check with Google
> > Maps tells us that that's a distance of...
> >
> > ...393 kilometers, or about 245 miles.
> >
> > At 65, that takes me about 3 hours, 45 minutes and I use about 13
> > gallons of gas.
> >
> > At 55, it takes 4 hours, 30 minutes and I use about 10 gallons of gas.
> >
> > Even at Canadian gas prices, those 10 litres of fuel cost me only about
> > $11.50. Is that really too much to pay to save 45 minutes of my precious
> > time? 25 cents a minute seem pretty cheap to me.
> >
> >
> >
> >
> >
> >
> >
> > > Bill Daniels
> >
> > > "Morgans" > wrote in message
> > ...
> >
> > > > "Ernest Christley" > wrote
> >
> > > >> I've seen SUV's with what looks like reverse scoops that could have to
> > > >> catch the air passing over the top of the vehicle and blow it down the
> > > >> back. Counterintuitive until you realize that attached flow offers
> > > >> less
> > > >> drag.
> >
> > > > That scoop helps drag, but more importantly, it prevents carbon
> > > > monoxide
> > > > from entering in the back window when it is open and driving at highway
> > > > speeds.
> >
> > > > When a chopped off SUV has the window open, that low pressure area in
> > > > the
> > > > back gathers up the exhaust, and the swirling motion of the vortex
> > > > allows
> > > > some to enter in though the open window. Not good. The scoop supplies
> > > > fresh air flowing down from the top, and keeps the bad air down lower
> > > > than
> > > > the open window.
> >
> > > > They have been doing that since the 60's, on old station wagons, before
> > > > they cared about mileage and aerodynamics.
> > > > --
> > > > Jim in NC
> >
> > --
> > Alan Baker
> > Vancouver, British Columbia
> > "If you raise the ceiling four feet, move the fireplace from that wall
> > to that wall, you'll still only get the full stereophonic effect if you
> > sit in the bottom of that cupboard."- Hide quoted text -
> >
> > - Show quoted text -
>
> I think it will cost a bit more than that. From my experiences in the
> B.C. lakes region and the Coq the cops are a bit anal about the posted
> limits. It has been a few years but the Coq was posted 110 (about
> 62mph). Then there is that stretch from Merritt until you pick up the
> Eastern extension of the freeway that is posted 90. Then from
> Peachland to Kelowna is again max of 90 with stretches down to 80
> (IIRC). You might be parked alongside the road a bit. :)
>
> I got a ticket in there "Exceeding speed limit while passing".
>
> Harry K

Sorry, Harry, but I can't agree. Stay below about 20 klicks over the
limit and you won't ever get stopped. If you're truly nervous, limit
yourself to 15 over.

--
Alan Baker
Vancouver, British Columbia
"If you raise the ceiling four feet, move the fireplace from that wall
to that wall, you'll still only get the full stereophonic effect if you
sit in the bottom of that cupboard."

[Peter Dohm]

"Bill Daniels" <bildan@comcast-dot-net> wrote in message
. ..
> You guys haven't mentioned the biggie - roof racks. Removing the cross
bars
> from my Grand Cherokee added 2MPG. Glider pilots look at roof racks and
see
> open air brakes. I can't believe the number of vehicles I see in Denver
> with ski racks in July.
>
> Then, of course, you can park the truck. I understand that trucks have
very
> low drag when parked.
>
> If the vehicle has a MPG computer, you can see what speed does to MPG. At
> 65, my Jeep gets 19MPG. At 55, it gets 24MPG. At 45, it gets 36MPG. If
> you do the numbers, saving two minutes by driving fast costs big time.
> Those are REALLY expensive minutes - more than the rental on some
airplanes.
>
> Bill Daniels
>
>
Those numbers really bother me. The numbers are all fairly low; but the
curve shape fits aerodynamic drag, with no other influence such as tires or
"pumping losses." In effect, the numbers fit a much larger, but
aerodynamically atrocious and very lightly loaded vehicle. Therefore, I
really doubt that your mileage computer is telling you the whole truth.

Have you 1) verified you odometer against at least 10 miles of highway mile
markers, 2) verified your speedometer reading at 60MPH as a result of the
same test, and 3) checked the cumulative fuel mileage, as shown on the
computer, against your fuel mileage calculated in the usual way?

Peter

[Harry K]

On Jul 28, 7:21 pm, Alan Baker > wrote:
> In article om>,
> Harry K > wrote:
>
>
>
>
>
> > On Jul 27, 7:49 pm, Alan Baker > wrote:
> > > In article >,
> > > "Bill Daniels" <bildan@comcast-dot-net> wrote:
>
> > > > You guys haven't mentioned the biggie - roof racks. Removing the cross
> > > > bars
> > > > from my Grand Cherokee added 2MPG. Glider pilots look at roof racks and
> > > > see
> > > > open air brakes. I can't believe the number of vehicles I see in Denver
> > > > with ski racks in July.
>
> > > > Then, of course, you can park the truck. I understand that trucks have
> > > > very
> > > > low drag when parked.
>
> > > > If the vehicle has a MPG computer, you can see what speed does to MPG.
> > > > At
> > > > 65, my Jeep gets 19MPG. At 55, it gets 24MPG. At 45, it gets 36MPG. If
> > > > you do the numbers, saving two minutes by driving fast costs big time.
> > > > Those are REALLY expensive minutes - more than the rental on some
> > > > airplanes.
>
> > > Real world case; let's use your figures for simplicity's sake:
>
> > > I want to drive from Vancouver to Kelowna. A little check with Google
> > > Maps tells us that that's a distance of...
>
> > > ...393 kilometers, or about 245 miles.
>
> > > At 65, that takes me about 3 hours, 45 minutes and I use about 13
> > > gallons of gas.
>
> > > At 55, it takes 4 hours, 30 minutes and I use about 10 gallons of gas.
>
> > > Even at Canadian gas prices, those 10 litres of fuel cost me only about
> > > $11.50. Is that really too much to pay to save 45 minutes of my precious
> > > time? 25 cents a minute seem pretty cheap to me.
>
> > > > Bill Daniels
>
> > > > "Morgans" > wrote in message
> > > ...
>
> > > > > "Ernest Christley" > wrote
>
> > > > >> I've seen SUV's with what looks like reverse scoops that could have to
> > > > >> catch the air passing over the top of the vehicle and blow it down the
> > > > >> back. Counterintuitive until you realize that attached flow offers
> > > > >> less
> > > > >> drag.
>
> > > > > That scoop helps drag, but more importantly, it prevents carbon
> > > > > monoxide
> > > > > from entering in the back window when it is open and driving at highway
> > > > > speeds.
>
> > > > > When a chopped off SUV has the window open, that low pressure area in
> > > > > the
> > > > > back gathers up the exhaust, and the swirling motion of the vortex
> > > > > allows
> > > > > some to enter in though the open window. Not good. The scoop supplies
> > > > > fresh air flowing down from the top, and keeps the bad air down lower
> > > > > than
> > > > > the open window.
>
> > > > > They have been doing that since the 60's, on old station wagons, before
> > > > > they cared about mileage and aerodynamics.
> > > > > --
> > > > > Jim in NC
>
> > > --
> > > Alan Baker
> > > Vancouver, British Columbia
> > > "If you raise the ceiling four feet, move the fireplace from that wall
> > > to that wall, you'll still only get the full stereophonic effect if you
> > > sit in the bottom of that cupboard."- Hide quoted text -
>
> > > - Show quoted text -
>
> > I think it will cost a bit more than that. From my experiences in the
> > B.C. lakes region and the Coq the cops are a bit anal about the posted
> > limits. It has been a few years but the Coq was posted 110 (about
> > 62mph). Then there is that stretch from Merritt until you pick up the
> > Eastern extension of the freeway that is posted 90. Then from
> > Peachland to Kelowna is again max of 90 with stretches down to 80
> > (IIRC). You might be parked alongside the road a bit. :)
>
> > I got a ticket in there "Exceeding speed limit while passing".
>
> > Harry K
>
> Sorry, Harry, but I can't agree. Stay below about 20 klicks over the
> limit and you won't ever get stopped. If you're truly nervous, limit
> yourself to 15 over.
>
> --
> Alan Baker
> Vancouver, British Columbia
> "If you raise the ceiling four feet, move the fireplace from that wall
> to that wall, you'll still only get the full stereophonic effect if you
> sit in the bottom of that cupboard."- Hide quoted text -
>
> - Show quoted text -

I keep hearing that on various forums. My experience (1 trip yearly
over the Peachland-Merritt-Kamloops) is that if I punch in a bit under
the posted (don't like to flog my car on those grades), I will be
passed by just about everybody but only very rarely by someone really
cooking. Eyeball says the "flow" is only slightly over the posted
both on and off the Coq.

Harry K

[Scott[_5_]]

On Sun, 29 Jul 2007 10:04:39 -0400, in rec.aviation.homebuilt, "Peter Dohm"
> wrote:

>Have you 1) verified you odometer against at least 10 miles of highway mile
>markers, 2) verified your speedometer reading at 60MPH as a result of the
>same test, and 3) checked the cumulative fuel mileage, as shown on the
>computer, against your fuel mileage calculated in the usual way?

Seconded. I've never seen an automotive trip computer that was worth more
than the recyclable metals in it.

My Grand Prix has a trip computer, and what it tells me is scarcely better
than a wild guess. It reports better fuel mileage than I really get, and
underreports the amount of fuel I've used. When the tank's full it tells me
I have a range of 430 miles (best I've ever gotten was ~350), and it raises
a fuel alarm when I've still got 80 miles in the tank. Now and then, it
will raise a low fuel alarm when the tank is completely full.

It probably kicks puppies and steals candy from babies, too.

-Scott

[Harry K]

On Jul 29, 6:33 pm, (Scott) wrote:
> On Sun, 29 Jul 2007 10:04:39 -0400, in rec.aviation.homebuilt, "Peter Dohm"
>
> > wrote:
> >Have you 1) verified you odometer against at least 10 miles of highway mile
> >markers, 2) verified your speedometer reading at 60MPH as a result of the
> >same test, and 3) checked the cumulative fuel mileage, as shown on the
> >computer, against your fuel mileage calculated in the usual way?
>
> Seconded. I've never seen an automotive trip computer that was worth more
> than the recyclable metals in it.
>
> My Grand Prix has a trip computer, and what it tells me is scarcely better
> than a wild guess. It reports better fuel mileage than I really get, and
> underreports the amount of fuel I've used. When the tank's full it tells me
> I have a range of 430 miles (best I've ever gotten was ~350), and it raises
> a fuel alarm when I've still got 80 miles in the tank. Now and then, it
> will raise a low fuel alarm when the tank is completely full.
>
> It probably kicks puppies and steals candy from babies, too.
>
> -Scott

Probably quite true but...

It is a useable instrument to detect what mode of driving is most
efficient even it it is not accurate. That is if it reports, for
example, 19 mpg over a stretch of road with tailgate down and then you
repeat the run in the same direction, same speed, same conditions,
etc. the reported mpg is then a useable bit of data. Yes, it will be
inaccurate but the comparison is useable and meaningful.

Harry K

[[email protected]]

On Jul 27, 8:49 pm, Alan Baker > wrote:
> In article >,
> "Bill Daniels" <bildan@comcast-dot-net> wrote:

> > If the vehicle has a MPG computer, you can see what speed does to MPG. At
> > 65, my Jeep gets 19MPG. At 55, it gets 24MPG. At 45, it gets 36MPG. If
> > you do the numbers, saving two minutes by driving fast costs big time.
> > Those are REALLY expensive minutes - more than the rental on some airplanes.

At about $4.25 per US gallon around here (Alberta, Canada)
65 MPH and 19 MPG is $14.54 an hour for fuel. At 45 MPH and 36 MPG it
comes to $5.31 per hour. The savings come to $9.23 per hour. I'd like
to know what airplane--even an ultralight--that would go for under $10
per hour? Even my little old homebuilt with its 65 hp engine costs me
about $20 per hour for fuel. Our Cessna 172s rent for $127 per hour,
which is fairly typical.


Dan

[Peter Dohm]

"Harry K" > wrote in message
ups.com...
> On Jul 29, 6:33 pm, (Scott) wrote:
> > On Sun, 29 Jul 2007 10:04:39 -0400, in rec.aviation.homebuilt, "Peter
Dohm"
> >
> > > wrote:
> > >Have you 1) verified you odometer against at least 10 miles of highway
mile
> > >markers, 2) verified your speedometer reading at 60MPH as a result of
the
> > >same test, and 3) checked the cumulative fuel mileage, as shown on the
> > >computer, against your fuel mileage calculated in the usual way?
> >
> > Seconded. I've never seen an automotive trip computer that was worth
more
> > than the recyclable metals in it.
> >
> > My Grand Prix has a trip computer, and what it tells me is scarcely
better
> > than a wild guess. It reports better fuel mileage than I really get,
and
> > underreports the amount of fuel I've used. When the tank's full it
tells me
> > I have a range of 430 miles (best I've ever gotten was ~350), and it
raises
> > a fuel alarm when I've still got 80 miles in the tank. Now and then, it
> > will raise a low fuel alarm when the tank is completely full.
> >
> > It probably kicks puppies and steals candy from babies, too.
> >
> > -Scott
>
> Probably quite true but...
>
> It is a useable instrument to detect what mode of driving is most
> efficient even it it is not accurate. That is if it reports, for
> example, 19 mpg over a stretch of road with tailgate down and then you
> repeat the run in the same direction, same speed, same conditions,
> etc. the reported mpg is then a useable bit of data. Yes, it will be
> inaccurate but the comparison is useable and meaningful.
>
> Harry K
>
Probably true. However, the coastdown test mentioned elsewhere in this
tread is probably the most consistently reliable method--when properly
controlled. The most obvious requirements are: state the coast down from
the same place and speed each time, record the remaining speed at the
seconde marker, continue testing in the same direction, warm up the tires
before the first test, and choose a day (or days) with constant temperature
and wind conditions. There are certainly more, but those are enough to give
a far more reliable result than any test in traffic that I can think of--and
even then, since we are discussing the drag of detached flow, the test may
only be valid for the speed(s) at which the test was run.

Peter

[Scott[_5_]]

"Harry K" > wrote in message
ups.com...

> It is a useable instrument to detect what mode of driving is most
> efficient even it it is not accurate. That is if it reports, for
> example, 19 mpg over a stretch of road with tailgate down and then you
> repeat the run in the same direction, same speed, same conditions,
> etc. the reported mpg is then a useable bit of data. Yes, it will be
> inaccurate but the comparison is useable and meaningful.

I would not assume that a faulty instrument is going to give me readings
that are inaccurate in a consistent way across a given range.

-Scott

[Peter Dohm]

"Scott" > wrote in message
news:46ae8308.426924174@localhost...
> "Harry K" > wrote in message
> ups.com...
>
> > It is a useable instrument to detect what mode of driving is most
> > efficient even it it is not accurate. That is if it reports, for
> > example, 19 mpg over a stretch of road with tailgate down and then you
> > repeat the run in the same direction, same speed, same conditions,
> > etc. the reported mpg is then a useable bit of data. Yes, it will be
> > inaccurate but the comparison is useable and meaningful.
>
> I would not assume that a faulty instrument is going to give me readings
> that are inaccurate in a consistent way across a given range.
>
> -Scott

Wait a minute Scott,

You got a little too ambitious with the clipping and the part you left in
was from a previous contributor. I only agreed that the errors in the fuel
computer would probably be consistent--electronics usually are--and then
recommended a proceedure to conduct coast down testing.

My contribution was the following:

"Probably true. However, the coastdown test mentioned elsewhere in this
tread is probably the most consistently reliable method--when properly
controlled. The most obvious requirements are: state the coast down from
the same place and speed each time, record the remaining speed at the
seconde marker, continue testing in the same direction, warm up the tires
before the first test, and choose a day (or days) with constant temperature
and wind conditions. There are certainly more, but those are enough to give
a far more reliable result than any test in traffic that I can think of--and
even then, since we are discussing the drag of detached flow, the test may
only be valid for the speed(s) at which the test was run.

Peter"

Peter

[Scott[_5_]]

On Tue, 31 Jul 2007 08:23:18 -0400, in rec.aviation.homebuilt, "Peter Dohm"
> wrote:

>"Scott" > wrote in message
>news:46ae8308.426924174@localhost...
>> "Harry K" > wrote in message
>> ups.com...
>>
>> > It is a useable instrument to detect what mode of driving is most
>> > efficient even it it is not accurate. That is if it reports, for
>> > example, 19 mpg over a stretch of road with tailgate down and then you
>> > repeat the run in the same direction, same speed, same conditions,
>> > etc. the reported mpg is then a useable bit of data. Yes, it will be
>> > inaccurate but the comparison is useable and meaningful.
>>
>> I would not assume that a faulty instrument is going to give me readings
>> that are inaccurate in a consistent way across a given range.
>>
>> -Scott
>
>Wait a minute Scott,
>
>You got a little too ambitious with the clipping and the part you left in
>was from a previous contributor. I only agreed that the errors in the fuel
>computer would probably be consistent--electronics usually are--and then
>recommended a proceedure to conduct coast down testing.

Sorry about the piggyback post, but I was replying to HarryK, as the
(edited) attribution shows. I responded via your post because his was
already expired, and I was not ambitious enough to download it again.

I agree with you about the coastdown test. I disagree that a faulty fuel
computer will necessarily be reliable in any useful way. In my GP I have an
example of a computer that not only produces incorrect results, but produces
results that are wildly inconsistent when compared to results from the usual
method of measuring fuel mileage. Perhaps others have more reliable errors,
but you can't know without testing them first.

(To be fair, my GP's computer is probably very accurate based on the data it
gets. The problem is most likely with the analog data sources for fuel flow
rate and/or fuel level.)

-Scott

[Mark Hickey]

(Scott) wrote:

>On Tue, 31 Jul 2007 08:23:18 -0400, in rec.aviation.homebuilt, "Peter Dohm"
> wrote:
>
>>You got a little too ambitious with the clipping and the part you left in
>>was from a previous contributor. I only agreed that the errors in the fuel
>>computer would probably be consistent--electronics usually are--and then
>>recommended a proceedure to conduct coast down testing.
>
>Sorry about the piggyback post, but I was replying to HarryK, as the
>(edited) attribution shows. I responded via your post because his was
>already expired, and I was not ambitious enough to download it again.
>
>I agree with you about the coastdown test. I disagree that a faulty fuel
>computer will necessarily be reliable in any useful way. In my GP I have an
>example of a computer that not only produces incorrect results, but produces
>results that are wildly inconsistent when compared to results from the usual
>method of measuring fuel mileage. Perhaps others have more reliable errors,
>but you can't know without testing them first.
>
>(To be fair, my GP's computer is probably very accurate based on the data it
>gets. The problem is most likely with the analog data sources for fuel flow
>rate and/or fuel level.)

I think Peter's point is still valid - while the mileage computers
might not be entirely accurate, the inaccuracies should be relatively
consistent. That is, if you're doing a test where all other variables
are the same, and the computer tells you you're getting an extra 1mpg,
you can't be sure that the delta is precisely 1mpg, but you can bet
that there IS a difference, and that it's probably not TOO far from
that value.

For example, my Jeep Grand Cherokee has a computer, and I drive a lot
on fhe Arizona roads that are absolutely flat and have very little
traffic. I can get the cruise control set, let everything stabilize,
and see a 1mpg difference from clicking the overdrive on and off
(which is about what I'd expect the actual delta to be). Can I say
with any certainty that the difference is very close to 1mpg? Nope.
But can I surmise there IS an advantage to running with the overdrive
engaged, based on the computer? Absolutely.

Mark "now if I could only get it to go up 10mpg..." Hickey

[Capt. Geoffrey Thorpe]

"cavelamb himself" > wrote in message
link.net...
<...>
> But those webbed tailgate thingies have GOT to increase drag.
>
> Don't they???
>
> Richard

OK, Following up on on old thread.

I spent the day working in an automotive wind tunnel today, and I thought to
ask the operator if he had tried tailgate up / down in the tunnel.

He said they did a bunch of tests on an F150 and found that a tonnau cover
was best, a cap was nearly as good, tailgate up was better than down (I
forget if there was a difference between down and removed) and the web
things were the worst.

With the tailgate up, when they used smoke, the smoke would go over the roof
and then come down and touch the top of the tailgate.

--
Geoff
The Sea Hawk at Wow Way d0t Com
remove spaces and make the obvious substitutions to reply by mail
When immigration is outlawed, only outlaws will immigrate.

[Denny]

On Oct 31, 6:17 pm, "Capt. Geoffrey Thorpe" <The Sea Hawk at wow way
d0t com> wrote:
> "cavelamb himself" > wrote in message
>
> link.net...
> <...>
>
> > But those webbed tailgate thingies have GOT to increase drag.
>
> > Don't they???
>
> > Richard
>
> OK, Following up on on old thread.
>
> I spent the day working in an automotive wind tunnel today, and I thought to
> ask the operator if he had tried tailgate up / down in the tunnel.
>
> He said they did a bunch of tests on an F150 and found that a tonnau cover
> was best, a cap was nearly as good, tailgate up was better than down (I
> forget if there was a difference between down and removed) and the web
> things were the worst.
>
> With the tailgate up, when they used smoke, the smoke would go over the roof
> and then come down and touch the top of the tailgate.
>
> --
> Geoff
> The Sea Hawk at Wow Way d0t Com
> remove spaces and make the obvious substitutions to reply by mail
> When immigration is outlawed, only outlaws will immigrate.

Jeez, there goes my excuse for not fixing the broken latch on the
tailgate... I hate it when that happens...

denny

[Morgans[_2_]]

"Denny" > wrote
>
> Jeez, there goes my excuse for not fixing the broken latch on the
> tailgate... I hate it when that happens...

Ahh, don't sweat it too much.

A couple bungee chords will do just fine. <g>
--
Jim in NC

[Lou]

I admit, I didn't follow the original thread, but I've driven
a pickup for the past 30 years and I can tell you that each
time I've installed a cap I lost 2mpg.
Lou

[clare at snyder.on.ca]

On Thu, 01 Nov 2007 13:46:34 -0700, Lou > wrote:

>I admit, I didn't follow the original thread, but I've driven
>a pickup for the past 30 years and I can tell you that each
>time I've installed a cap I lost 2mpg.
> Lou
And on my 1957 Fargo, putting the gate down gave me an extra couple of
MPH and MPG. Mind you that whole truck had no aerodynamics at all.
Stepside with sidemount.

--
Posted via a free Usenet account from http://www.teranews.com

[Harry K]

On Nov 1, 1:10 pm, "Morgans" > wrote:
> "Denny" > wrote
>
>
>
> > Jeez, there goes my excuse for not fixing the broken latch on the
> > tailgate... I hate it when that happens...
>
> Ahh, don't sweat it too much.
>
> A couple bungee chords will do just fine. <g>
> --
> Jim in NC

I hate modern technology. They never work as well as the old stuff.
Just today I saw the broken end of a bungee on the side of the
highway. That never happened in the days of baling wire.

Harry K

[J.Kahn]

Capt. Geoffrey Thorpe wrote:
> "cavelamb himself" > wrote in message
> link.net...
> <...>
>> But those webbed tailgate thingies have GOT to increase drag.
>>
>> Don't they???
>>
>> Richard
>
> OK, Following up on on old thread.
>
> I spent the day working in an automotive wind tunnel today, and I thought to
> ask the operator if he had tried tailgate up / down in the tunnel.
>
> He said they did a bunch of tests on an F150 and found that a tonnau cover
> was best, a cap was nearly as good, tailgate up was better than down (I
> forget if there was a difference between down and removed) and the web
> things were the worst.
>
> With the tailgate up, when they used smoke, the smoke would go over the roof
> and then come down and touch the top of the tailgate.
>
> --
> Geoff
> The Sea Hawk at Wow Way d0t Com
> remove spaces and make the obvious substitutions to reply by mail
> When immigration is outlawed, only outlaws will immigrate.
>
>
I work on the RJ program and it was once explained to me, in a
discussion of devices to slow airflow in ducts, that a wire mesh
structure has far more drag and will create a much larger and more
permanent (longer lasting effect downstream) drop in velocity than a
single obstruction, even if the single obstruction is much larger. In
our case, a nozzle type restrictor used on the cockpit air duct had a
fraction of the effect on velocity downstream, that the mesh screen at
the outlet had, and was basically useless. So it's easy to see how
those tailgate screens are the worst thing to do for drag.

John