Apparent reverse motion in truck wheel lug nuts

The flat facets of chromed lug nuts on spinning truck wheels create patterns of light. These patterns appear to spin slowly backwards. The patterns can be produced by several orientations of lug nuts. Simplest is to adjust all the nuts to have facets parallel to the same line. Also see an improved pattern with much less flashing: http://www.youtube.com/watch?v=6LRnJIPoF9w

Category:

Science & Technology

Tags:

• physics
• science
• optics
• driving
• cartalk
• puzzler
• optical
• illusion
• strobe
• amasci

[BaronMorte]

CAN SOMEONE PLEASE TALK TO ME

i cant watch this video GL

[wbeaty]

> i cant watch this video

Try again. Works fine for me. It's a fairly old vid.

[jarikcbol]

is this non similar to a rotoscope film, where you look through the slots, and see the image on the far side of the drum, and for it to move correctly, you have to spin the drum in apparently the opposite direction of the movements on the film strip?

[wbeaty]

> image on the far side of the drum

Yes, but in this case there are small mirrors instead of slots, and they reflect parts of the outside world.

Unlike slots, the mirrors can be tilted. If the nuts are aligned in a starburst, then the pattern remains unmoving when the wheel rotates.

But if they are aligned to a single straight line, then the pattern moves slowly backwards when the wheel rotates rapidly forward.

[jmalmsten]

After reading up a bit on other causes you could be looking for than camera-syncing I only find text stating a 20-40hz needed for a light to seize to blink and start to be stable. But that wouldn't explain the lugs, Because then, in steady sunlight sans camera it would cause a ring, not a set of dots. So all I can summize is that the effect sought are reflections causing a strobe-effect-themselves while rotating, creating a blinking result that fuses in your eyes to steady or slowly-moving dots.

[jmalmsten]

Yes, that's basically what I said. But my point was that the shutter-speed is probably too slow to produce distinguishable shapes. There's simply too much motion-blur. The latter-test you did with the reflections of the lugs worked better since the reflections where visible for shorter periods than the shutter-speed of the camera, in a sense that reduces the time of exposure and they appear more as shapes than the blur around them. Like using multiple flashes in a single still-photo.

[wbeaty]

> don't exactly sync up with the camera/eye

True strobe effects are beat frequencies, and the motion freezes at a particular RPM (and, the pattern would drift forward or backward if the speed is slightly different from the camera frame rate.)

This lug-nuts effect never goes forward.  Also, it drifts in direct proportion to RPM, which is completely unlike true strobe effects. Also, the geometry of the progressive flashes is trivially simple (but only if you build one and watch it work.)

[jmalmsten]

... But if the rotors (or truck wheels) don't exactly sync up with the camera/eye then for each frame the blades or bolts happen to be either slightly before or slightly after the position it was in the last frame (depending on the sync). And the result is the false-strobing effect of the wheels going slower or even reverse that it really is.

[jmalmsten]

I'm quite certain that it'll show up if you shorten the shutter-time on the camera. For example, there's a video on YT where a russian helicopter seems to be floating mid-air with it's rotors standing perfectly still. The short shutter-time made the blades distinguishable instead of just motion-blur (as is the problem in this demonstration) and the framerate of the camera happened to sync up with the rotors so that every time the camera took a pic, the rotor happened to be in the same place...

[sambroughton]

Works for me too.