Aerodynamics acording to Myth Busters!

"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...

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.

"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

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

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

"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

"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

"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

"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

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