'Zuk at work
- Thread starter Bspeed
- Start date
- May 23, 2000
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The first thing that catches my eye is the front wheel. How come the bottom half of the front wheel has the spokes frozen in motion, but in the top half of the wheel there are no spokes to be found?
I've been meaning to ask also, what camera and lens are you using for these shots?
I've been meaning to ask also, what camera and lens are you using for these shots?
- Thread starter
- #4
Kawidude said:
Wow, now thats some wierd science! Maybe Rich can figure that one out, LOL.
It may well be from resizing or sharpening? this image is about 150k give or take, versus the original.
I am glad you noticed that, I will look at the original tonight.
camera is a Nikon D200, lens is a Sigma 50-150 f2.8
SpeedyManiac
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- Aug 8, 2000
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The reason you can't see the spokes is because they're moving fast. From our frame of reference (the ground), the bottom of the wheel is not moving relative to the ground (perfect traction) while the top of the wheel is moving quite quickly.
Nice shot, by the way.
Nice shot, by the way.
my guess is totally different than SM's. i'd think it'd be the background that the spokes are displayed on. the top spokes are rotating just as fast as the bottom, but they are displayed with the background being much more distant in relation to the bottom.
the lighting may also play a factor in that. i'd imagine the shadow cast by the brake/brake protector would make the spokes more visible in high light conditions.
the lighting may also play a factor in that. i'd imagine the shadow cast by the brake/brake protector would make the spokes more visible in high light conditions.
SpeedyManiac
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- Aug 8, 2000
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I'll explain it more. This is the correct answer (not to sound like an ass, but I've taken way too many courses on this stuff).
Consider a wheel rolling without slipping on the ground. It does not pivot about its center. Instead, it pivots about the point on the ground where the wheel is contacting the ground. Now, using the simple relation that the velocity of a point is equal to perpendicular distance from center of rotation multiplied by rotational (angular) velocity (v = r·w), it is easy to determine the velocity of various points. As you can see from my diagram, the top of the wheel is moving at twice the speed of the center of the wheel.
In Bspeed's picture, he just happened to have the shutter speed fast enough to freeze the slower moving spokes on the bottom, but not fast enough to catch the upper spokes. This is emphasized in the rear wheel. The rear wheel has a lot of slip, so it's not rolling without slipping, instead it is essentially spinning about the axle (not quite, but close enough) This means all the spokes have the same velocity so they are all blurred.
Consider a wheel rolling without slipping on the ground. It does not pivot about its center. Instead, it pivots about the point on the ground where the wheel is contacting the ground. Now, using the simple relation that the velocity of a point is equal to perpendicular distance from center of rotation multiplied by rotational (angular) velocity (v = r·w), it is easy to determine the velocity of various points. As you can see from my diagram, the top of the wheel is moving at twice the speed of the center of the wheel.
In Bspeed's picture, he just happened to have the shutter speed fast enough to freeze the slower moving spokes on the bottom, but not fast enough to catch the upper spokes. This is emphasized in the rear wheel. The rear wheel has a lot of slip, so it's not rolling without slipping, instead it is essentially spinning about the axle (not quite, but close enough) This means all the spokes have the same velocity so they are all blurred.
SpeedyManiac
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- Aug 8, 2000
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No problem. If you want to see it for yourself, take a circular object (a plate, for example) and roll it along the floor, making sure it doesn't slip. Mark a dot on the plate and you'll see that the plate pivots around whichever point is touching the ground.
Chili said:
Excellent explanation SM. Well done. :cool:
Doug - Think of it like the pendulum on a grandfather clock (just upside down :whoa: ) For that tiny slice of time you have a fixed pivot point at the contact patch (like the pendulum pivot) and the rest of the wheel is basically swinging off over it. Of course then the contact patch moves, you have a new momentary pivot point and the whole thing starts again, at which point you crash because you are looking down at your wheel trying to figure all of this out instead of watching the guy in front of you on the Yamaha who is brake checking you for being overly curious. :yikes:
