Air Drag - - Why a Bed Cover is the Way To Go!
An
interesting study at the University of Michigan ( photo on left was posted on
their web site which is no longer active) showed that a full hard bed cover decreased
drag by 22%. This seems intuitively reasonable. A key benefit is the
reduced drag at the back of the Cab since there is less of it exposed.
They also found that eliminating the tailgate
increased drag
by 4%! Why? What occurs with the tailgate up is an
air bubble forms in the bed which causes air to move over the bed and reduces
the low pressure area behind the cab. When you drop the tailgate you
eliminate the bubble and your back to a greater low pressure area behind the
cab! Air flow is not always intuitive.
If your convinced you get better mileage with the tailgate down don't blame
me for the University of Michigan providing facts! Aerodynamics are
tricky, that's why NASCAR teams spend lots of time and big bucks in wind
tunnels! I suspect some trucks might be slightly different but it
would not come close to what a bed cover will provide.
Here Are Comments Copied from Click and Clack, the "Tappet Brothers"
Web Site (for what
its worth):
"I'm an aerodynamics engineer. When I was in the
U.S. Air Force a few years back, I worked with folks from the Lockheed low-speed
wind tunnel. In the 1970s, aircraft production went into a slump, and Lockheed
started looking for other customers for its wind-tunnel services. Prime
candidates were the auto makers, and Lockheed was successful in convincing Ford,
among others, that the wind tunnel would help them reduce drag and wind noise on
their vehicles. Needless to say, in the past 15-20 years, Lockheed has learned a
lot about car and truck aerodynamics. Anyway, they actually performed drag tests
on pickups with the tailgate both up and down, and found that drag was actually
LOWER with the tailgate CLOSED! This ran counter to their intuition (and yours).
The reason is that a closed tailgate sets up a large "bubble" of stagnant air
that slowly circulates around the bed of the truck (we aero types call this a
"separated bubble"). When air approaches the truck, it "sees" the bubble as part
of the truck. So to the air, the truck looks like it has a nice, flat covering
over the bed, and the air doesn't "slam" into the vertical tailgate. If the
tailgate is open, or replaced by one of those "air gate" nets, however, that
nice, separate bubble in the truck bed does not form (it "bursts"). Then the air
approaching the truck "sees" a truck with a flat bed on the back of a tall cab.
This is a very nonaerodynamic shape with a very LARGE drag. So, believe it or
not, it's best for gas mileage to keep the tailgate CLOSED. Hope this
information is helpful. Ed Fitzgerald, Research Assistant, Dept. of
Aero/Mechanical Engineering, U. of Notre Dame"
Some Additional Data For The Skeptics (found on a web
site):
"...it is generally better to
keep your pickup truck tailgate up instead of down or removed. It seems
counter intuitive, but research by the National Research Council of Canada
determined that there was generally a lower drag coefficient (Cd) with the
tailgate up than with the tailgate down or removed. The difference wasn't big
but it was measurable, with a 2002 Ford F150 Crew Cab measuring a Cd of 0.5304
with the tailgate up and 0.5425 with tailgate down. With the tailgate removed,
the Cd was 0.5596. However, placing a tonneau cover on the bed lowered the Cd
to 0.4967, probably enough to offset the weight of the tonneau cover if you do
much highway driving." [Note
(0.5425-0.5304)/0.5304 = 2.3% more drag with a lowered tail gate and
(0.5596-.5304)/0.5304 = 5.5% more drag with it removed!]
You Have to Believe
"MYTHBUSTERS!"
Adam and Jamie, hosts of
the TV show MYTHBUSTERS, drove two identical Ford F150 pickup trucks
filled with identical amounts of gas. One with the tailgate up the other
down. They drove the same road and after 500 miles the one with the
tailgate up went 30 miles further before it ran out of gas! That's 6% better (30/500!)
Spotlight Example
of Air Flow
Many years ago in a fluid dynamics class it was mentioned there is more drag
with a bullet
shaped spotlight on a car pointing with the bullet shape toward
the front than with the flat face moving forward (Back in the "Old Days" some
"Custom Car Guys" mounted spotlights on each A pillar, bullet end forward!) Why
we asked? Similar reasoning as with the truck tailgate. With the bullet shape pointing forward there
is somewhat less drag in the front but a large low pressure area in the rear.
In essence a slight partial vacuum is created pulling the spotlight backward. You
can visualize the massive turbulence and vortexes created as the spotlight moves
through the air. With the bullet shape pointed backward the air moves
smoothly to the rear. Even the front drag increase is not as great as it
might seem since the air will partially create its own path over the front.
The net result is there is the reduced rear drag more than offsets the increased
front drag with the bullet shaped spotlight having the glass front facing
forward!
In the early days of streamlining race cars very long tails were
being used. Then someone found a spoiler helped create a similar effect
and also produced some down force...but that's another story!
Final
Thoughts: As a cost reduction, for the 2009 Formula 1 Racing Series
the rules are limiting teams to use no more than one wind tunnel and state that
devices can only be used for 15 runs per eight-hour day and only five days per
week. The lead teams have their own wind tunnels and reportedly would
spend three shifts looking for the small advantage it takes to win.
How much does a wind tunnel cost? Clemson University announced they plan to
build one for NASCAR vehicles at a cost of $40 million! What does a
top Formula 1 team cost to run? Honda quit F1 for 2009 saying with
the economic downturn it needs to focus on its core business of making and
selling cars rather than spending $291 million a year to race them on Grand Prix
tracks! Ferrari and McLaren probably spent more than that in 2008 when the
Championship got very light at the end!
So when you think your
"common sense" says you'll get better mileage with the tailgate down think about
the many millions race car teams spend in a wind tunnel since they know even
their '"educated intuition" is usually wrong when it comes to air flow!
Maybe you need to get your truck in a wind tunnel!
By the way;
according to a Ford Aero Systems Engineer, flow-through, web-like fabric tailgates tend to
increase drag even more than just leaving the tailgate down! They have
measured as much as 4- to 5-percent more!
Chevy
S-10 Mods
Click for YouTube Video
Overview of Car
