Aerodynamics acording to Myth Busters!

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."

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

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

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."

"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

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 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

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

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