Photo of Spoke Deflection?

Werehatrack wrote:
On Sun, 10 Jul 2005 05:05:55 GMT, Moe Kit wrote:

At first I thought it was a shutter/moving object effect with the camera.

It is.

It's actually
remarkable that the image of these two riders is not linearly
distorted. I have to think that some image manipulation from the raw

Nope. The shutter, or rather slit, is stationary, the film (or digital
scanning equivalent technology) moves at approximately the speed of the
racers. This explains their relative lack of distortion.

By the way: do you really think there is no linear distortion in either
of the riders? Look closely at the upper one. It's impossible to say
with precision, but that's an interesting left arm he has.

wrote:

So the part of a leg or spoke that's going forward faster
than the body is widened, while the part of a leg or spoke
that's going slower than the body is narrowed.

Going fishing? It's the reverse. Proved by the fact that stationary
objects appear as continuous streaks the length of the image.



The closer the leg or spoke is travelling to the body speed,
the less the camera will distort it.

Like a spoke, whose forward speed varies from one e nd to the
other except when dead horizontal

You seem to believe that only horizontal speed matters in the
distortion. Vertical speed matters too. Think about it, or try a
little experiment yourself. Or look at the picture again.i

On 10 Jul 2005 15:23:54 -0700, "41"
wrote:



wrote:

So the part of a leg or spoke that's going forward faster
than the body is widened, while the part of a leg or spoke
that's going slower than the body is narrowed.

Going fishing? It's the reverse. Proved by the fact that stationary
objects appear as continuous streaks the length of the image.



The closer the leg or spoke is travelling to the body speed,
the less the camera will distort it.

Like a spoke, whose forward speed varies from one e nd to the
other except when dead horizontal

You seem to believe that only horizontal speed matters in the
distortion. Vertical speed matters too. Think about it, or try a
little experiment yourself. Or look at the picture again.i

Dear 41,

Aaargh! Backwards confusion.

As my previous posts explicitly stated, there's also the
vertical motion.

rim end of spoke, 30 mph forward + 30 mph forward at top
|r 30 mph forward + 30 mph diagonally
| \ at 45 degrees to vertical
| \ 30 mph forward + 30 mph downward
| \ at 9 o'clock
| h
slit hub -30 mph forward

Run the frame and hub at a steady 30 mph past the slit and
the thin spoke will appear to topple forward past the slit
and sag downward like a chain, forming the curve that we see
in the upper left quadrant of the front wheels in this
pictu

http://www.velonews.com/images/details/8436.11841.f.jpg

For a relatively thicker leg with little downward component,
moving forward relative to the main body distorts by
thinning the leg (not widening it--I got my phrases
topsy-turvy).

Unlike a spoke-end that rotates at a constant speed, a leg
trails behind with the foot planted on the ground and is
then whipped forward at roughly twice body speed to get in
front of the body again, only to decelerate wildly again
when the foot is planted.

Legs are therefore harder to figure out that spokes because
a leg at the same angle can be moving either way relative to
the body (zipping forward or trailing backward).

But these two animations from Ryan's nice link show the same
kind of distortion:

http://www.finishlynx.com/products/f...rview/body.htm

Objects moving more slowly past the slit than the body
widen, while objects moving faster past the slit than the
body narrow. (Click on the minimize button to freeze the
endless animations.)

The lower leg of the lowest runner is vertical and is just
finishing its rapid forward motion--you can see that the
front foot is in mid-air, well above its own shadow.

Since that vertical lower leg is moving forward much faster
than the body speed for which the camera is adjusted, it's
absurdly narrowed.

If you look at the front foot of each runner, you can see
that they were all zipping forward much faster than the
bodies--they're absurdly shortened.

The leading thighs of each runner, on the other hand, are
moving forward at roughly the same speed as the bodies or
even starting to trail behind, so they're roughly normal or
even wider.

Compare them to the trailing thighs, which are narrowing
because (although they trail behind), they're being swept
forward at a greater speed than the body--they're quite
noticeably narrower than real thighs.

The same things apply to the horses. When the legs and
hooves narrow absurdly, they're moving faster forward than
the bodies. When they widen (streak), they're moving forward
more slowly than the bodies--the fifth horse leg from the
left catches a forward-sung rear leg that was almost
motionless relative to the ground and widens it into a
weird-looking thing like an elephant's trunk.

Carl Fogel

Most distortions like this one are caused by a camera with a focal plane
shutter. Most modern 35mm's are of this type. The spokes are for all
intents and putposes, straight. The shutter causes this distortion.

Sometimes you see pictures of moving bicycles where the wheels are
obviously oval (not even round), this is also caused by the focal plane
shutter effect.

HTH,
Ernie

Moe Kit wrote:
If you look carefully at this photo

http://www.velonews.com/images/details/8436.11841.f.jpg

the spokes appear to be bending in some sort of complex curve, like a
higher-order polynomial or a catenary.

At first I thought it was a shutter/moving object effect with the camera.
But if you look at the rear wheel of the foreground rider the top spokes
appear linear while the bottom spokes are 'bent.' If it was a camera/speed
artifact then all spokes should appear bent because wheel speed is constant
around the wheel.

Is this spoke bending real?

If it's real, then why don't the spokes bend under load while the bike is
stopped?

If it's real, and spokes bend and unbend to this degree with each
revolution, why aren't spokes failing under fatigue loading more
frequently?

"E Willson" wrote: (clip) Sometimes you see pictures of moving bicycles
where the wheels are obviously oval(clip)
^^^^^^^^^^^^^^
If you come across old racing car pictures, there are three things that are
always the leather headgear, with chin strap, wrap-around close-fitting
goggles, oval wheels, leaning forward. (This is because the slit in those
old cameras moved from top to bottom. The image is up-side-down on the
film, so the top of the wheel is exposed last.

Are there distortions that effect the determination of who won? Also,
how is the finish line superimposed on the image and what guarantees
that it is absolutely perpendicular to the track? It seems to me that
even the slightest canting of the line would give someone an advantage.
-- Jay Beattie.

In article ,
Werehatrack wrote:
Looking closely at the image, I see that some manual manipulation was
also applied to make the wheels look round. Had this not been done,
they would appear very decidedly egg-shaped; flatter across the
bottom.

There's no reason for the bottom to appear flattened. Imagine
attaching to the fork and rear triangle, non-rotating rings that mask
the wheel's brake tracks (picture something like a fender that wraps
around the entire wheel). As these rings pass by the camera, there's
no difference between what happens at the top and bottom of the ring.

The rims' rotation behind these rings only affects which spot
on the ring each spot on the rim maps to in the image (tire labels get
stretched on the bottom and compressed at the top, as seen with the
spoke spacing), but not the shape.

I don't believe there's any manipulation involved aside from
adjusting the horizontal scale for the riders' speed.

Aside: what we see happening to the image of the wheel is
actually analagous to the relativistic dilation a moving observer
sees of charges moving around in a loop of current. The changes in
perceived charge distribution creates an electric field for the
observer. This field and its dependence on the observer velocity is
more commonly referred to as a magnetic field.

-Luns

"Luns Tee" wrote: (clip) As these rings pass by the camera, there's no
difference between what happens at the top and bottom of the ring. (clip)
^^^^^^^^^^^^^^^^^
Your idea of masking the spinning wheel to eliminate rotational effects is
clever, but it is not on point. The "ovalization" is not caused by the
rotation of the wheel, but by its translation, so the ring masks would also
appear as ovals.

Lets do this thought experiment: Take a picture of a vertical line moving
past the camera. If you need a physical example, paint a vertical white
stripe on the side of a semi-trailer. Set up your camera, with a
focal-plane shutter, on the sidewalk, and make the exposure as the truck
passes. If it takes 1/50 second for the slit to move from the top to the
bottom of the film, and the truck is moving 60 MPH (88 ft/sec) the line will
advance 88/50 = 1.76 ft during the exposure, producing a distinctly sloping
line on the negative.

If, instead of a vertical line you have a bicycle wheel, it will be as
though you painted the wheel on the side of a tall stack of cards, and then
slid a tilt into the stack.

The reason the wheels are not oval in the OP's photo is that the film is
moving instead of the slit, so that the image and film are moving together.
So the film records the wheels in their true shape. Since the spokes are
not synchronized with the film, they are distorted.

On 10 Jul 2005 19:05:51 -0700, "Jay Beattie"
wrote:

Are there distortions that effect the determination of who won? Also,
how is the finish line superimposed on the image and what guarantees
that it is absolutely perpendicular to the track? It seems to me that
even the slightest canting of the line would give someone an advantage.
-- Jay Beattie.

Dear Jay,

If the camera is slightly angled toward the racers as below,
then the racer farthest from it will indeed have a slight
advantage (and vice-versa):

R1-\
R2--\
\

But the finish-line camera is set as perpendicular to the
track as possible--and just as an actual metal bar once
defined the size of a meter before wavelengths, so does the
camera angle define the finish line. They're usually lined
up and sighted across the visible finish line.

Ryan's nice link shows horses and runners distorting and
approaching the finish line:

http://www.finishlynx.com/products/f...rview/body.htm

To one side is a simultaneous narrow digital camera view
from which things are created by the computer. You can
freeze the animation by clicking on your browser's minimize
button.

***

Here's an old and famous finish photo--notice the leg
distortions:

http://www.greyhound1.homestead.com/Winning_Treble.html

"The photo-finish camera cannot give a false result. If
three dogs hit the winning line with their noses touching it
together, they will be filmed at the same moment a triple
dead heat. If one Greyhound's nose hits the line
fractionally before the others, the camera's film will have
shifted to show the gap. Once the photographic principle is
understood it becomes clear that the camera (in this
instance) cannot lie. The Greyhound is only captured on film
as he/she crosses the line, and not before."

***

Here's a description of how the celluloid version works:

http://www.rit.edu/~andpph/text-strip-photofinish.html

"The order of arrival of each car at the finish line can be
read off the film by simply "reading" the image from right
to left. The car farthest to the right always beats any car
behind it. This is because the images arrive sequentially at
the slit in the camera and are similarly recorded onto the
passing film."

***

And here are some drawings showing the runner's chest as the
mark:

http://www.stuffo.com/olympic-timing1.htm

Again, the principle is to pick some reasonably stable,
visible, and obvious leading portion of the racer, such as a
nose or chest, something will be going at about the expected
rate of the main body, unlike paws, hooves, and feet.

A perverse set of rules would allow us to pick the tip of
the greyhound's tail, a the saddlehorn, the back of a
runner's waistband, or even the back of the heel of the left
shoe--the first left heel to reach the finish line always
shows up ahead of the other left heels.

The reason for picking a leading feature like a nose or
chest is to make judging things easier--the winning animal's
nose is usually plainly visible from the side, just as some
part of a runner's chest is usually visible, no matter what
the arms are doing.

If a runner's hand were used, the odd situation would arise
in which one runner clearly behind another happened to be
swinging his hand forward at just the right moment to be "in
front" of the leading runner's hands which were going past
his hips at that moment in opposite directions.

Animals can do the same thing, putting their forepaws and
hooves ahead of their noses. (Well, maybe not basset
hounds.)

Runner's noses aren't used for the same reason--a head-bob
can put one person's nose ahead of another's in a very close
finish, even though the head-bobber's chest is plainly
behind his rival's chest.

With bicycles, the leading edge of the front tire makes a
wonderful mark--the edge is very clear, and it's almost
impossible to get anything else that far forward.

Carl Fogel

In article ,
Leo Lichtman wrote:

"Luns Tee" wrote: (clip) As these rings pass by the camera, there's no
difference between what happens at the top and bottom of the ring. (clip)
^^^^^^^^^^^^^^^^^
Your idea of masking the spinning wheel to eliminate rotational effects is
clever, but it is not on point. The "ovalization" is not caused by the
rotation of the wheel, but by its translation, so the ring masks would also
appear as ovals.

An oval is not an egg shape - Werehatrack seemed to be
describing the bottom of the wheel appearing flatter [than the top].
My point is that the shape seen for the top of the wheel will be no
different from what's seen of the bottom of the wheel.

-Luns