Coefficient of Drag

Recently I've been wondering why truck manufacturers, including Toyota,
don't do a better job making pickup trucks aerodynamic. Nowadays
fuel economy at speeds > 55 mph is determined mostly by air resistance,
not vehicle weight or engine size.

This article says my Prius has a .29 coefficient of drag (cD), while
the Honda Insight has a .25 cD. Would Prius cD improve by installing
rear-wheel fenders? They're ugly on the Insight but what else could
explain the huge difference between .25 and .29 cD?

[url]http://www.edmunds.com/advice/fueleconomy/articles/45188/article.html[/url]

I remember .29 is what our 1994 Volvo 850 wagon had, but looking up
this information now on the web, answers vary from .32 to .33 cD.
I'm fairly certain the wagon had better cD than the sedan, so maybe
the wagon was .32 and the sedan was .33.

The teardrop is the most aerodynamic shape, so wagons have an advantage
in that they start out closer to the best shape, although glass in the
rear windows is heavier than a trunk lid. Teardrop also might be why
pickup truck owners report improved fuel economy with a camper shell.

The Tundrasolutions.com website (owned by Toyota?) reports the Tundra
has a cD ranging from .37 - .38 depending on model.

The 2007 Chevy Silverado has a cD of .43, reportedly "best in class"
according to Wards Auto, so something is amiss here.

Actually they are doing a better job the higher box sides, like on the F150
an the Silverado, are part of the reason. Trucks are designed to be trucks,
pretty hard to get a bunch of passengers and cargo into something shaped
like the Pruis that looks like a door stop.

Yeas ago GM and others tried designing streamlined trucks and even
locomotives and railroad cars. The problem for trucks was the streaming
and extra weight reduced load capacity that offset the economy gained

mike


"Bill Tuthill" <ccreekin@yahoo.com> wrote in message
news:45b27851@news.meer.net...[color=blue]
> Recently I've been wondering why truck manufacturers, including Toyota,
> don't do a better job making pickup trucks aerodynamic. Nowadays
> fuel economy at speeds > 55 mph is determined mostly by air resistance,
> not vehicle weight or engine size.
>
> This article says my Prius has a .29 coefficient of drag (cD), while
> the Honda Insight has a .25 cD. Would Prius cD improve by installing
> rear-wheel fenders? They're ugly on the Insight but what else could
> explain the huge difference between .25 and .29 cD?
>
> [url]http://www.edmunds.com/advice/fueleconomy/articles/45188/article.html[/url]
>
> I remember .29 is what our 1994 Volvo 850 wagon had, but looking up
> this information now on the web, answers vary from .32 to .33 cD.
> I'm fairly certain the wagon had better cD than the sedan, so maybe
> the wagon was .32 and the sedan was .33.
>
> The teardrop is the most aerodynamic shape, so wagons have an advantage
> in that they start out closer to the best shape, although glass in the
> rear windows is heavier than a trunk lid. Teardrop also might be why
> pickup truck owners report improved fuel economy with a camper shell.
>
> The Tundrasolutions.com website (owned by Toyota?) reports the Tundra
> has a cD ranging from .37 - .38 depending on model.
>
> The 2007 Chevy Silverado has a cD of .43, reportedly "best in class"
> according to Wards Auto, so something is amiss here.
>[/color]

"Bill Tuthill" <ccreekin@yahoo.com> wrote in message
news:45b27851@news.meer.net...[color=blue]
> Recently I've been wondering why truck manufacturers, including Toyota,
> don't do a better job making pickup trucks aerodynamic. Nowadays
> fuel economy at speeds > 55 mph is determined mostly by air resistance,
> not vehicle weight or engine size.
>
> This article says my Prius has a .29 coefficient of drag (cD), while
> the Honda Insight has a .25 cD. Would Prius cD improve by installing
> rear-wheel fenders? They're ugly on the Insight but what else could
> explain the huge difference between .25 and .29 cD?
>
> [url]http://www.edmunds.com/advice/fueleconomy/articles/45188/article.html[/url]
>
> I remember .29 is what our 1994 Volvo 850 wagon had, but looking up
> this information now on the web, answers vary from .32 to .33 cD.
> I'm fairly certain the wagon had better cD than the sedan, so maybe
> the wagon was .32 and the sedan was .33.
>
> The teardrop is the most aerodynamic shape, so wagons have an advantage
> in that they start out closer to the best shape, although glass in the
> rear windows is heavier than a trunk lid. Teardrop also might be why
> pickup truck owners report improved fuel economy with a camper shell.[/color]

I've read that the most aerodynamic shape looks more like a banana , with
the ends of the banana pointing downwards. A truncated back end will create
a low pressure area immediately behind the vehicle and increase drag.

The other area that people often forget about is the undercarriage. A
shield under the engine compartment will help reduce drag.
[color=blue]
>
> The Tundrasolutions.com website (owned by Toyota?) reports the Tundra
> has a cD ranging from .37 - .38 depending on model.
>
> The 2007 Chevy Silverado has a cD of .43, reportedly "best in class"
> according to Wards Auto, so something is amiss here.
>[/color]

Since the 2007 Trunda is not for sale yet, the Silverado probably is "best
in class" for cD.
--

Ray O
(correct punctuation to reply)

On 20 Jan 2007 12:15:13 -0800, Bill Tuthill <ccreekin@yahoo.com>
wrote:
[color=blue]
>Recently I've been wondering why truck manufacturers, including Toyota,
>don't do a better job making pickup trucks aerodynamic. Nowadays
>fuel economy at speeds > 55 mph is determined mostly by air resistance,
>not vehicle weight or engine size.
>
>This article says my Prius has a .29 coefficient of drag (cD), while
>the Honda Insight has a .25 cD. Would Prius cD improve by installing
>rear-wheel fenders? They're ugly on the Insight but what else could
>explain the huge difference between .25 and .29 cD?
>
> [url]http://www.edmunds.com/advice/fueleconomy/articles/45188/article.html[/url]
>
>I remember .29 is what our 1994 Volvo 850 wagon had, but looking up
>this information now on the web, answers vary from .32 to .33 cD.
>I'm fairly certain the wagon had better cD than the sedan, so maybe
>the wagon was .32 and the sedan was .33.
>
>The teardrop is the most aerodynamic shape, so wagons have an advantage
>in that they start out closer to the best shape, although glass in the
>rear windows is heavier than a trunk lid. Teardrop also might be why
>pickup truck owners report improved fuel economy with a camper shell.
>
>The Tundrasolutions.com website (owned by Toyota?) reports the Tundra
>has a cD ranging from .37 - .38 depending on model.
>
>The 2007 Chevy Silverado has a cD of .43, reportedly "best in class"
>according to Wards Auto, so something is amiss here.[/color]

Bill, I agree that truck manufacturers could be doing things to
improve the aerodynamics of some trucks, but most of those things
would nat make the truck "look" more massive so they get put on the
back burner.

Two comments on your input above:

1) Granted a tear drop is the most aerodynamic shape, but it is blunt
end forward, not pointy end, and the tear drop works so well because
the fluid is constantly being swept toward the back to be replaced by
the next "layer" of fluid. It is a lot like the action of team bicycle
racers. The lead racer peals off to rest in the back while the rest of
the pack forges on.

2) I have always wondered why automotive manufacturers advertize CD
(Coeficient of Drag) rather than D (Drag). In order to compute D for a
vehicle, you multiply CD * Frontal area. It is Drag that your engine
has to push through the wind, not CD. CD is the measure of resistance
per unit area.

That is one of the reasons the exact same car with narrow wide tires
will get better mileage than one with wide tires. The wide tires add
to the frontal area.

Take two cars with the same CD that weigh the same. Drive them at the
same speed over the same course. the one with the smaller frontal area
will get better gas mileage.

In article <45b27851@news.meer.net> [email]ccreekin@yahoo.com[/email] "Bill
Tuthill" writes:
[color=blue]
> This article says my Prius has a .29 coefficient of drag (cD),
> [...][/color]

Then the article disagrees with commonly available authoritative
data sheets, that (no doubt while lying through their collective
teeth) claim 0.26 -- not a big error but whattheheck. <g>
[color=blue]
> Would Prius cD improve by installing rear-wheel fenders?[/color]

If so, I suspect Toyota would have installed them. Overall fuel
efficiency is a selling point on that model. T would be keen to
improve the Cd wherever possible.
--
Andrew Stephenson

Ray O <rokigawaATtristarassociatesDOTcom> wrote:[color=blue]
>
> I've read that the most aerodynamic shape looks more like a banana, with
> the ends of the banana pointing downwards. A truncated back end will create
> a low pressure area immediately behind the vehicle and increase drag.[/color]

Maybe this has to do with whether aerodynamicity refers to a falling raindrop
or a vehicle moving along the ground?

The teardrop is the reverse of my expectations: the blunt end is in front,
with the tapered end at the back.
[color=blue]
> The other area that people often forget about is the undercarriage. A
> shield under the engine compartment will help reduce drag.[/color]

Aha, another use for skid plate! ;-)
[color=blue]
> Since the 2007 Trunda is not for sale yet, the Silverado probably is
> "best in class" for cD.[/color]

That explains it, thanks.

A friend got 23 MPG from his 1999(?) Silverado small V8 on the highway,
with camper shell and nothing on the roof. The EPA for that vehicle
was 21 MPG highway. Pretty good for a pickup truck, eh? His mileage
went down when he replaced the crappy factory tires. (They were crappy
off-road anyway, but obviously had low rolling resistance.)

"Bill Tuthill" <ccreekin@yahoo.com> wrote in message
news:45b3ab60@news.meer.net...[color=blue]
> Ray O <rokigawaATtristarassociatesDOTcom> wrote:[color=green]
>>
>> I've read that the most aerodynamic shape looks more like a banana, with
>> the ends of the banana pointing downwards. A truncated back end will
>> create
>> a low pressure area immediately behind the vehicle and increase drag.[/color]
>
> Maybe this has to do with whether aerodynamicity refers to a falling
> raindrop
> or a vehicle moving along the ground?
>
> The teardrop is the reverse of my expectations: the blunt end is in front,
> with the tapered end at the back.
>[/color]

There are many things that contribute to drag and make the propulsion system
work harder. Anything that contributes to turbulence will increase drag.
The spoilers and wings on an Indy car are smaller for high speed races than
for road races, where the additional downforce to aid cornering is more
important than reducing drag.

Having the pointy end at the back of the car reduces the turbulence and drag
behind the car. A blunt rear end creates a low pressure area, which sucks
the car backwards, but a vehicle following in the low pressure area doesn't
have to work as hard to move through the low pressure area, the principle
behind drafting in races.
[color=blue][color=green]
>> The other area that people often forget about is the undercarriage. A
>> shield under the engine compartment will help reduce drag.[/color]
>
> Aha, another use for skid plate! ;-)[/color]

could be![color=blue]
>[color=green]
>> Since the 2007 Trunda is not for sale yet, the Silverado probably is
>> "best in class" for cD.[/color]
>
> That explains it, thanks.
>
> A friend got 23 MPG from his 1999(?) Silverado small V8 on the highway,
> with camper shell and nothing on the roof. The EPA for that vehicle
> was 21 MPG highway. Pretty good for a pickup truck, eh? His mileage
> went down when he replaced the crappy factory tires. (They were crappy
> off-road anyway, but obviously had low rolling resistance.)
>[/color]

The tailgate on a pickup can act as a large wind brake. Shells and tonneau
covers keep air away from the tailgate and reduce drag, as does lowering or
removing the tailgate or replacing it with a net or mesh.
--

Ray O
(correct punctuation to reply)

"Bill Tuthill" <ccreekin@yahoo.com> wrote in message
news:45b3ada6@news.meer.net...[color=blue]
> Andrew Stephenson <ames@deltrak.demon.co.uk> wrote:[color=green][color=darkred]
>>> [Edumunds.com] says Prius has a .29 coefficient of drag (cD).[/color]
>>
>> Then the article disagrees with commonly available authoritative
>> data sheets, that (no doubt while lying through their collective
>> teeth) claim 0.26 -- not a big error but whattheheck. <g>[/color]
>
> Aha! So Toyota managed to nearly equal the Insight without specifying
> rear-wheel fenders, which make tire changes more difficult.
>
> The Honda Insight is also very noisy inside at highway speeds.
> I'd rather pay more for gasoline than lose my hearing.
>[/color]

There are lots of tricks to reduce drag and improve fuel mileage besides
fender skirts. Reducing interior sound insulation saves weight. I expect to
see exterior rearview mirrors go away, replaced by cameras that doesn't
stick out into the airstream.

The Prius has some subtle design tricks that maintain decent interior room
without major contributions to drag. Look at the roofline from behind, and
notice that the centerline of the roof is lower than the left and right
side.
--

Ray O
(correct punctuation to reply)

"Bill Tuthill" <ccreekin@yahoo.com> wrote in message
news:45b3ae84@news.meer.net...[color=blue]
> [email]username@mailserver.mail[/email] wrote:[color=green]
>>
>> 2) I have always wondered why automotive manufacturers advertize CD
>> (Coeficient of Drag) rather than D (Drag). In order to compute D for a
>> vehicle, you multiply CD * Frontal area. It is Drag that your engine
>> has to push through the wind, not CD. CD is the measure of resistance
>> per unit area.[/color]
>
> Holy smokes! This is a huge revelation to me. Thanks.
>[color=green]
>> That is one of the reasons the exact same car with narrow wide tires
>> will get better mileage than one with wide tires. The wide tires add
>> to the frontal area. Take two cars with the same CD that weigh the same.
>> Drive them at the same speed over the same course. The one with the
>> smaller frontal area will get better gas mileage.[/color]
>
> Pickup trucks should have an advantage over SUVs because they are
> generally lower profile.[/color]

Actually, some pickups have the same or a higher profile than comparable
SUV's. The tailgage on pickups are a pretty big source of drag. The
biggest advantage pickups have over an SUV bases on the same chassis is
weight. Without the interior trim, carpets, seats, sheet metal, AC and
heating capacity, etc., the pickup will weigh less.
[color=blue]
>
> The reason I'm thinking about this is that a(nother) friend has a
> relatively new Silverado and reports that fuel economy is worse now
> than the one he bought 10 years ago, especially with a full load.
>
> Just looking at the windshield wipers and underside, it looks like
> the manufacturers could make 10-20% improvements in fuel economy
> with only minor engineering changes.
>[/color]

There are many reasons your friend's new Silverado gets worse mileage. The
new one is probably larger and weighs more and has more amenities that add
weight. It may have a larger engine or be heavier-duty than the old one.
--

Ray O
(correct punctuation to reply)

Both the 2007 Tundra and Silverado were compared by MT for the truck of the
year award, The Tundra lost out in that comparison. ;)


mike


"Ray O" <rokigawaATtristarassociatesDOTcom> wrote in message
news:c18c4$45b2a202$47c2b532$19522@msgid.meganewsservers.com...[color=blue]
>
> "Bill Tuthill" <ccreekin@yahoo.com> wrote in message
> news:45b27851@news.meer.net...[color=green]
>> Recently I've been wondering why truck manufacturers, including Toyota,
>> don't do a better job making pickup trucks aerodynamic. Nowadays
>> fuel economy at speeds > 55 mph is determined mostly by air resistance,
>> not vehicle weight or engine size.
>>
>> This article says my Prius has a .29 coefficient of drag (cD), while
>> the Honda Insight has a .25 cD. Would Prius cD improve by installing
>> rear-wheel fenders? They're ugly on the Insight but what else could
>> explain the huge difference between .25 and .29 cD?
>>
>> [url]http://www.edmunds.com/advice/fueleconomy/articles/45188/article.html[/url]
>>
>> I remember .29 is what our 1994 Volvo 850 wagon had, but looking up
>> this information now on the web, answers vary from .32 to .33 cD.
>> I'm fairly certain the wagon had better cD than the sedan, so maybe
>> the wagon was .32 and the sedan was .33.
>>
>> The teardrop is the most aerodynamic shape, so wagons have an advantage
>> in that they start out closer to the best shape, although glass in the
>> rear windows is heavier than a trunk lid. Teardrop also might be why
>> pickup truck owners report improved fuel economy with a camper shell.[/color]
>
> I've read that the most aerodynamic shape looks more like a banana , with
> the ends of the banana pointing downwards. A truncated back end will
> create a low pressure area immediately behind the vehicle and increase
> drag.
>
> The other area that people often forget about is the undercarriage. A
> shield under the engine compartment will help reduce drag.
>[color=green]
>>
>> The Tundrasolutions.com website (owned by Toyota?) reports the Tundra
>> has a cD ranging from .37 - .38 depending on model.
>>
>> The 2007 Chevy Silverado has a cD of .43, reportedly "best in class"
>> according to Wards Auto, so something is amiss here.
>>[/color]
>
> Since the 2007 Trunda is not for sale yet, the Silverado probably is "best
> in class" for cD.
> --
>
> Ray O
> (correct punctuation to reply)
>[/color]

In article <1f547$45b3bc56$47c2b532$10307@msgid.meganewsservers.com>
rokigawaATtristarassociatesDOTcom "Ray O" writes:
[color=blue]
> [...] A blunt rear end creates a low pressure area, which sucks
> the car backwards, [...][/color]

OTOH, a while back I posted this [trimmed]...
[color=blue]
> Subject: Re: $30,000 for a PRIUS?
> Message-ID: <1150129866snz@deltrak.demon.co.uk>
> Date: Mon, 12 Jun 2006 16:31:06 GMT
>
> [...]
>
> Second, streamlining. The best shape for a car, I read way back
> (when the idea was considered laughable), was a banana set up as
> an arch running on wheels recessed into its tips. Look around:
> note how cars are now approximating to this. And a chopped-off
> rear is good for streamlining (discovered by accident, according
> to one story, during WW2 when a Royal Navy ship damaged in action
> had her stern patched with a flat plate, to get her home for full
> repairs -- she did markedly better mpg on that journey). These
> give the Prius its overall "arched" ("hunchbacked") form.
>
> [...][/color]

I'd be interested to hear whether the WW2 ship story is true. My
source was an engineering-oriented schoolmaster, in my teens (ie,
early 60s).
--
Andrew Stephenson

"Mike Hunter" <mikehunt2@mailcity.com> wrote in message
news:2LOdndSGxpWeTy7YUSdV9g@ptd.net...[color=blue]
> Both the 2007 Tundra and Silverado were compared by MT for the truck of
> the year award, The Tundra lost out in that comparison. ;)
>
>
> mike
>[/color]

From what I have seen and read of the 2007 Tundra, it is a much better truck
than the previous generation Tundra and getting closer to the F series Ford
and Silverado in utility but have not passed them yet. I predict that a few
more people will purchase it as a work truck than the previous Tundra but it
will still appeal primarily to urban cowboys.
--

Ray O
(correct punctuation to reply)

"Tomes" <askme@here.net> wrote in message
news:k9Rsh.13809$yx6.11883@newsread2.news.pas.earthlink.net...[color=blue]
> "Ray O" <rokigawaATtristarassociatesDOTcom> wrote in message
> news:1f547$45b3bc56$47c2b532$10307@msgid.meganewsservers.com...[color=green]
>> Having the pointy end at the back of the car reduces the turbulence and
>> drag behind the car. A blunt rear end creates a low pressure area, which
>> sucks the car backwards, but a vehicle following in the low pressure area
>> doesn't have to work as hard to move through the low pressure area, the
>> principle behind drafting in races.[/color]
>
> This gets me to thinking about drafting behind trucks on the highway to
> save fuel. Just how close does one need to be to make this effective
> (notwithstanding safety issues of being that close). How about for behind
> a pickup or a 'regular' car?
> Tomes[/color]
Notwithstanding safety issues, depending on speed and the shape of the
leading vehicle, you probably need to be within 1 to 3 feet.
--

Ray O
(correct punctuation to reply)

Ray O wrote:
[color=blue]
> I've read that the most aerodynamic shape looks more like a banana , with
> the ends of the banana pointing downwards.[/color]

Is that why the Oscar Meyer wienermobile, with the ends of the hot dog
pointing upward, has never broken any speed records at the Bonneville
Salt Flats?

"larry moe 'n curly" <larrymoencurly@my-deja.com> wrote in message
news:1169426453.651362.45820@s34g2000cwa.googlegroups.com...[color=blue]
>
> Ray O wrote:
>[color=green]
>> I've read that the most aerodynamic shape looks more like a banana , with
>> the ends of the banana pointing downwards.[/color]
>
> Is that why the Oscar Meyer wienermobile, with the ends of the hot dog
> pointing upward, has never broken any speed records at the Bonneville
> Salt Flats?
>[/color]

Yup, they should have flipped the dog over!
--

Ray O
(correct punctuation to reply)