Marty Wallace said:
<[email hidden]> wrote in message
news:[email hidden]...
Quoted message said:jim beam said:[email hidden] wrote:
> On Mon, 13 Dec 2004 18:02:41 -0700, Mark Hickey
> <[email hidden]> wrote:
>
>
>>"Leo Lichtman" <[email hidden]> wrote:
>>
>>
>>>TIC: Golf balls are dimpled to reduce drag. This may be an
unexplored
Quoted message said:Quoted message said:>>>direction in bicycle technology.
>>
>>The trick is to get the frame spinning at a few hundred RPM to make
>>the dimples work - it's REALLY hard to stay on the bike.
>>
>>Mark Hickey
>>Habanero Cycles
>>http://www.habcycles.com
>>Home of the $695 ti frame
>
>
> Dear Mark,
>
> As I understand it, the dimpling effect would actually work
> without spinning.
>
> The trick is to present a regularly irregular surface
> (sorry, but I can't think of a better phrase) to act as a
> trip-wire and produce a thin, even layer of turbulent air
> over the ball's surface that reduces separation and drag.
>
> Here's a page with the famous trip-wire demonstration
> pictures at the bottom:
>
> http://www.princeton.edu/~asmits/Bicycle_web/blunt.html
fig 5 is a rather good illustration! thanks carl.
>
> Dimples are just the easiest way to reduce drag. They give
> increased distance, while the spin is what gives loft,
> shank, and slice--and would do so with a smooth ball, too:
>
> http://www.princeton.edu/~asmits/Bicycle_web/sportsballs.html
>
> Carl Fogel
Dear Jim,
Here's an even better copy of the drag pictures, which
eluded me at first:
http://www.efluids.com/efluids/gallery/flow_past_sphere.htm
The text is nice, too, since it explains that the pictures
are of air bubbles in flowing water, not smoke in a wind
tunnel as most people assume.
The odd parallel lines that often puzzle people, the ones
sticking out to the right of the sphere, are just the bar
that holds the sphere in place against the force of the
flowing water--in air, a sphere would have been suspended
from a wire.
Dimples would achieve roughly the same effect as the
circular trip wire.
Carl Fogel
Most frames have cables running the length of the down tube. I wonder if
they would have the same effect?
I suppose John Cobb will tell us one day.
Marty
Dear Marty,
Probably not much effect because of the difference in the
surfaces.
The rear brake cable typically runs under the horizontal
frame tube--it's already behind the short upright steering
stem tube, which would create the flaring turbulence. It's
already in the turbulence and running down the tube, not
around it, so it's too little and too late. Effectively, it
would function like the horizontal bar that you can see in
the wake in the demonstration pictures, doing next to
nothing.
The front and rear derailleur cables usually stick out on
the diagonal downtube. Again, they aren't likely to help
matters--in fact, they're more likely to increase drag.
Remember, the idea of golf-ball dimpling is to provide a
very thin layer of turbulent air on the leading surface of a
blunt object, which causes the air flowing around it to hug
the surface instead of separating into flaring turbulence.
The trailing dimples are no help and actually hurt things,
but there's no way to persuade a golf-ball to fly through
the air with a smooth half in back and its cheerful dimples
facing the wind.
First, the derailleur cables tend to be too far away from
the down-tube to function as trip wires. Close as they are
to the tube, they're considerably more than a golf-ball
dimple away.
Second, the cables and down-tube are already enjoying an
enormous amount of turbulence--the front wheel and its
welter of spokes are spinning wildly right in front of them
them.
Theoretically, we might expect some improvement if a rider
in shorts shaved only the backs of his legs, leaving his
hairy glory intact in front to whip up a thin layer of
turbulence over his shins and reduce the invisible flare and
drag behind his calves.
To show how tricky all this is, Armstrong's suit in the
opening time trial in 2004 is said to have been slick in
front and fuzzy in the rear, exactly the opposite of what I
just suggested:
" . . . panels of slick material to enhance the flow of air
at the front, fuzzy panels to break up vortexes at the rear,
and stitches that follow the direction of the airflow."
--"23 Days in July," John Wilcockson, p. 17
Frankly, the description sounds backwards to me, but I'm
willing to believe that it worked out that way in a wind
tunnel test.
And I'm also willing to believe that it would have made no
discernable difference if the skin-tight shirt had been put
on backwards by mistake--these kinds of improvements are
dwarfed by the momentary drag of the rider sticking an elbow
out, letting his knees widen slightly, or raising his head
for a few moments in a bumpy turn.
Carl Fogel
