Aerodynamics acording to Myth Busters!

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.

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

"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

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

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."

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

In article . com,
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."

On Jul 28, 7:21 pm, Alan Baker wrote:
In article . com,
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

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

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