Photo of Spoke Deflection?

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?

On Sun, 10 Jul 2005 05:05:55 GMT, 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?

Dear Moe,

Alas, it's just a photography effect, not an example of the
rims spreading out wildly as the wheel stands on its lower
spokes.

The camera is set up to capture bicycles and riders moving
leftward at 30 mph.

Notice how clear they are.

At the hub, the spokes are also moving around 30 mph
leftward.

But at the rim, a spoke at 12 o'clock is doing 60 mph
leftward, and a spoke at 6 o'clock is doing 0 mph leftward.

So we have a camera seeing things moving leftward at 30 mph
nice and clear.

The rim-end of a straight-up spoke is moving "through" the
camera's view at 30 mph--more upper spokes will appear in
the picture.

Look at the lower rider's upper spokes. If they really were
packed together that closely, he'd have 40+ spoke wheel.
(Look even more closely and you can see three spokes
sticking straight up and parallel to each other.)

The rim-end of a straight-down spoke is do 0 mph relative to
the ground and trailing the camera by 30 mph--fewer lower
spokes will appear in the picture.

Look at the lower spokes on both front wheels. If they
really were spaced that far apart on the rim, the riders
would be using 5-spoke front wheels.

The odd curving reflects the different velocities of each
point of the spoke--rim-ends can almost double or halve the
forward speed, while hub-ends have very little effect.

A further complication is the very high camera angle--we're
looking down at about a 45-degree angle, but the wheels are
just about straight up.

The real rim deflections require a micrometer to
measure--the rim flattens out very slightly above the
contact patch, but even a tiny flattening reduces the
tension on the lowermost spokes significantly.

Carl Fogel

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 though t 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 be nt because wheel speed is constant
around the wheel.

Is this spoke bending real?

How could it be? How could the rim or hub bend in such a way as to
distort the spokes in that pattern? Or do you think the spokes were
bent that way by the wind? How do you account for the fact that some of
the spokes seem to have disappeared?

"Wheel speed" may be constant, either as a whole with respect to the
ground or by angular velocity, but linear spoke speed is certainly not
constant with respect to the shutter or film. The top spokes are
rotating forward while the bottom ones are rotating backward, so to
speak, and the velocity of the spokes near the hub is much lower than
that near the rim.

The pattern of apparent spoke distortion might seem consistent, apart
from some details, with a horizontally running, left-right travel focal
plane shutter. However, the rest of the image is clearly not. Since the
picture depicts a photo-finish, it is no great leap to guess that the
image was taken with a photo-finish camera. Traditionally such cameras
have a fixed slit with a moving strip of film that goes in the
direction of the racers (in this case, right to left) and at
approximately the same speed. This speed of travel is much lower than
that of a typical focal plane shutter, hence the relatively large
effect on the spokes, the near lack of distortion in the rest of the
bicycles or riders, and the streaks in the background. This is a
classic photo-finish camera image.

The effect can be used to produce curious images of many kinds. See for
example
http://tinyurl.com/9uqudd

In article ,
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?

Oh, this has got to be a troll.

On the chance that you are since

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

The bendy spokes are an artifact of a composite, time-lapse photo
consisting of a series of 1-pixel-wide photos of the finish line. Those
bent spokes thus represent an even more interesting phenomenon:
sequential photos of the same spoke at equally spaced times, leading to
these notable curves which would tell me something insightful if I could
just remember what it was.

-RjC.

--
Ryan Cousineau http://www.wiredcola.com/
"I don't want kids who are thinking about going into mathematics
to think that they have to take drugs to succeed." -Paul Erdos

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.

That's cos the top spokes are moving faster than the bike (relative to
the fixed camera) and the bottom spokes aren't moving at all.

41 wrote:

Since the
picture depicts a photo-finish, it is no great leap to guess that the
image was taken with a photo-finish camera. Traditionally such cameras
have a fixed slit with a moving strip of film that goes in the
direction of the racers (in this case, right to left) and at
approximately the same speed.
....
The effect can be used to produce curious images of many kinds. See for
example
http://tinyurl.com/9uqudd

http://tinyurl.com/9uqud works much better. What a neat photo!

Chalo Colina

Chalo wrote:
41 wrote:
...
The effect can be used to produce curious images of many kinds. See for
example
http://tinyurl.com/9uqudd

http://tinyurl.com/9u qud works much better. What a neat photo!

Sorry about the typo- thanks, and sharp work for cleaning it up.e

On Sun, 10 Jul 2005 05:05:55 GMT, 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.

It is. It's what a mathematician might call a mapping effect. The
plane in space at which the image is captured is fixed, and the image
itself is a composite taken as the object traverses it. 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
data to the finished product must have been done; in order for the
image to show the bikes with this little linear stretch or
compression, the sampling rate had to pretty closely match the transit
speed of the objects crossing the capture point.


--
Typoes are a feature, not a bug.
Some gardening required to reply via email.
Words processed in a facility that contains nuts.

On Sun, 10 Jul 2005 15:27:05 GMT, Werehatrack
wrote:

On Sun, 10 Jul 2005 05:05:55 GMT, 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.

It is. It's what a mathematician might call a mapping effect. The
plane in space at which the image is captured is fixed, and the image
itself is a composite taken as the object traverses it. 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
data to the finished product must have been done; in order for the
image to show the bikes with this little linear stretch or
compression, the sampling rate had to pretty closely match the transit
speed of the objects crossing the capture point.

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.
--
Typoes are a feature, not a bug.
Some gardening required to reply via email.
Words processed in a facility that contains nuts.

On Sun, 10 Jul 2005 00:13:18 -0700, Ryan Cousineau
wrote:

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

Dear Ryan,

Nice link--thanks.

If you look at the animations of the horses and runners, you
can see the same distortion that curves the spokes.

The camera is set up to track the body speed in order to
show the body accurately.

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.

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 end to the
other except when dead horizontal, different parts of the
same jointed leg are moving forward at speeds, so the same
leg will show increasing distortion.

This means that both spokes and legs will look more normal
the closer that they get to their pivot point (the hub and
hip).

Carl Fogel