Date:         Fri, 26 Sep 2008 23:18:30 -0600
Reply-To:     Andrew Grebneff <goose1047@GMAIL.COM>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Andrew Grebneff <goose1047@GMAIL.COM>
Subject:      Re: Aerodynamics and the Westie luggage rack, etc..
In-Reply-To:  <000801c92059$dddd0f00$4001a8c0@gateway.2wire.net>
Content-Type: text/plain; charset=ISO-8859-1

On Fri, Sep 26, 2008 at 10:31 PM, Don Hanson <dhanson@gorge.net> wrote:
> I did some low budget research about aerodynamics, read up on all the
> sources I could find, etc.  So I know a little, but I certainly can't spout
> data or numbers, just general principals.


Aerodynamics really only come into play at above 110kmh (~70mph).The
vehicle may not feel as though it's run into a wall above that speed,
but your fuel bill will suggest otherwise, almost no matter what
vehicle (figure that phrase out if you can).

That's probably why the top speed of a 25hp (the so-called "36hp"
engine) Split will top out at 110kmh, empty ot with a tonne of cargo.

Modern vans still have external gutters, but that's because they have
a large roof area and concealed gutters couldn't cope with decent
rain... and of course it would be expensive (anda  rust trap) to put
concealed full-length gutters into one.

The rear of a vehicle is important in drag calculations. A vehicle
with a sloping back roof and tail (eg a real Beetle) will have a poor
drag coefficient, because there is no separation of airflow over the
vehicle from the air behind it, causing a relarive low-pressure area
behind the entire rear surface of the vehicle, from roof high-point to
bumper. That's why the more aerodynamic cars have a sharp lip on the
back of the trunk edge, and probably why T3s have a very good CD (and
a T2 has a better CD than an E-Type Jaguar)...

Of course, for a given shape, the larger the frontal area, the more
overall drag there will be, as you are trying to push a larger surface
through the air (which is a fluid). Take a T3 and double its size in
all dmensions and you'd be increasing the drag 4 times. So take this
into account if you're widening your bus by 20cm and want to retain
what we laughingly call its fuel-economy.

... the rear lip causes separation of the airflow over the car from
the turbulent air trail behind, reducing overall drag. It probably
also creates a little downforce at the rear, but it wouldn't be
significant.

Putting an airdam on the front, IF the dam is windtunnel-tested... and
believe me, most are NOT), can decrease drag by reducing the amount of
air going under the car (which also, by being compressed as it paasses
under the car, which is acting as one side of a venturi, produces
lift, which is most noticable at the front). However the airdam itself
adds frontal area, which increases drag... so a properly-designed
airdam is a compromise.

Rear spoilers are supposed to work by forcing air upward as it passes
over the tail, which results in downpressure on the car's tail. That
is the ideal... again, as with airdams, most "spoilers" are just
eyecandy and have only one effect... increased drag.

Sideskirts are only functional on cars utilizing radical
"ground-effect" for downforce eg the banned Formula 1 cars.
On a road car there is nothing to suck air out from under the car, so
skirts have nothing but esthetic function... and are one more thing to
have to repair (as well as being rust-traps, as are most plastic
addons eg spoilers).

Just as well that parentheses don't cause rust, or this mess

                age

                      would be fall in g  t   o

                                                         pi


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--
Andrew Grebneff
Dunedin, New Zealand
Fossil preparator
Mollusc, Toyota & VW van nut
Temporarily in Calgary, AB, Canada
<goose1047@gmail.com>