No strobe needed for "Wagon Wheel" Reverse Rotation effect

DOH!

I should have included sci.optics from the start.

To sci.optics: take a look at these youtube videos below, then go and make a spinning-disk device yourself. After all, we cannot trust that videos lack enormous aliasing artifacts. Plus the possibility of 120Hz fluorescent lighting strobe. (Well, actually the patterns in the video look identical to the patterns observed by eye in sunlight, so the aliasing effects aren't very significant.)

This one uses lug-nuts where the facets are all aligned parallel to a single line:

Apparent reverse motion in truck wheel lug nuts

The improved version uses my original brainstorm: radially aligned lug nuts, but with each nut in sequence having excess twist of -6deg,

-12deg, -18deg, etc.

Apparent reverse motion: original 6deg pattern

Also see the entire thread online:

google groups: sci.electronics.design

((((((((((((((((((((((( ( ( (o) ) ) ))))))))))))))))))))))) William J. Beaty Research Engineer snipped-for-privacy@chem.washington.edu UW Chem Dept, Bagley Hall RM74 snipped-for-privacy@eskimo.com Box 351700, Seattle, WA 98195-1700 ph425-222-5066 http//amasci.com

I was wrong about this. The pattern is actually made of six bright dots spaced equally around the circle, with wide dark spaces between. So it's much more obvious than the other type.

((((((((((((((((((((((( ( ( (o) ) ) ))))))))))))))))))))))) William J. Beaty Research Engineer snipped-for-privacy@chem.washington.edu UW Chem Dept, Bagley Hall RM74 snipped-for-privacy@eskimo.com Box 351700, Seattle, WA 98195-1700 ph425-222-5066 http//amasci.com/

I have noticed, from time to time, vibrations such as might be present in a moving vehicle can produce a stroboscopic effect. IIRC the phenomenon seemed to be emphasized if my teeth were rattling at the time.

Bill

-- Ferme le Bush

So ask the question. Was my car significantly vibrating? Answer: no. This was on I-90, east of Seattle. Now if I was on I-5 between Seattle and Tacoma, then vibration might be an issue. The road surface there is terrible! :)

If the truck-wheel phenomenon was caused by vibrating eyes, we'd also see the same effect in many other places. It wouldn't be limited to

*only* the ten lug nuts of large truck wheels. And it wouldn't be limited to *only* the facets of those lug nuts. (Perhaps this last point was missed. When you see an example on the highway, it's very obvious that the backwards-moving pattern is located in the whirling facets on the sides of the lug nuts.)

But yes, I agree that strong vibrations commonly produce strobe effects, particularly when a scene is viewed through vibrating rear-view mirrors on a slightly rough road.

((((((((((((((((((((((( ( ( (o) ) ) ))))))))))))))))))))))) William J. Beaty Research Engineer snipped-for-privacy@chem.washington.edu UW Chem Dept, Bagley Hall RM74 snipped-for-privacy@eskimo.com Box 351700, Seattle, WA 98195-1700 ph425-222-5066 http//amasci.com

What you're seeing is the response time of the eye to the light it is getting in. Remember that the eye isn't a continous sender of info to the brain but rather sends it in blocks and when you get near the cycle time, the strobe effects kick in because the eye is seeing the moving objects at differing places and when the objects are a little bit advanced or retarded, the object doesn't look like it is moving correctly. Strobes will allow for more motion (several steps of the rotation) than the eye will but the effect is the same. The reason for this is that the strobe is a very short imaging time while the eye takes a lot longer to image the object.

-- Yeppie, Bush is such an idiot that He usually outwits everybody else. How dumb!

For a really good demonstration of that, try watching a computer monitor, or better still a TV, with a teaspoon clamped between the teeth by the handle, and "twang" it gently.

The whole point, though, of this "lug nut effect", is that ther *is* no strobe"!

I"ve seen wheel covers that have bearings so the covers are balanced with respect to gravity. They' really do turn a slow rate (forward or backwards with minor adjustment) from the airflow across them. It startled me the first time I saw them as the fake lug bolts were clear and destinct, unlike with strobe effects. They can also be set up to spin with the wheels when the car is moving but continue to spin for some time after the car stops.

Or say, "R-r-r-r-r-r-r-r-r-ruffles have r-r-r-r-r-r-r-r-r-r-ridges."

;-)

Cheers! Rich

Hi Bob,

I'm having some trouble buying in to your explanation. If the eye sends info in discrete blocks, does it transmit them with such regularity that a strobe effect is even possible? And, would you know at what rep rate an average person's eyes has?

I would have expected to have heard of this effect before, which is the main reason I am doubting it. Of course, I could still be wrong :-) Bill Beaty's explanation of the different alignments of the lug nuts seemed pretty plausible to me.

Regards,

Mark

Lol!

No, the eye has no frame rate. If it did, we'd be seeing "strobe light" effects in smoothly spinning wheels.

Have you watched the videos of my whirling lug nuts disk? Those slowly drifting patterns are not a camera artifact: they move proportionally to disk rotation rate, NOT to rotation rate minus some sampling frequency. The camera records them, but if I view the spinning disk out in the sun, the moving patterns are still there.

If the eye/brain had such a "frame rate," then ANY spinning wheel would produce strong aliasing effects; the same effects we see when wagon wheels spin backwards in TV shows and movies.

But if you watch a fast spinning wheel by eye in sunlight, the details of the wheel are blurred into smooth circles, and there are no visible slow-moving patterns. Don't believe ME. Go make a striped cardboard wheel and try it. I suggest using a hand-operated twist drill so you can easily control the RPMs.

Wikipedia mentions additional effects.

But I don't see them myself. The article mentions that they are strongest at 8Hz-12Hz. Yet the effects I observe in the whirling-lug nuts are independant of frequency, and I can easily observe them at 30Hz and faster (and at any rate below 30Hz too.)

Also, "strobe light" effects are easily detected because there is a particular RPM where the pattern slows to a stop. The pattern reverses direction if the wheel should spin faster. But with the lug-nuts patterns, there is no magic rate. Instead the patterns always move in the same direction, and they always rotate at exactly one-sixth of the wheel's rate of rotation. Strobelight effects involve a difference frequency relative to the wheel rotation frequency. Lug-nuts patterns are just dividing the wheel rotation frequency by six.

Heh! OK, whatever works for you!--

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Even R-r-r-r.......ich Gr-r-r-r-r.......ise.

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OOO - Own Opinions Only. Try www.jmwa.demon.co.uk and www.isce.org.uk
2006 is YMMVI- Your mileage may vary immensely.

John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK

Wow! I've been punned by none other than the esteemed John Woodgate!

Sir, you honor me. :-) :-) :-)

Thanks! Rich

Try this one:

Instructions:

We did this when I was a kid, using an old vacuum cleaner motor and a train transformer (like a 0-19V variac).

Cheers! Rich

In message , dated Sat, 26 Aug 2006, Rich Grise writes

My pleasure.

-- OOO - Own Opinions Only. Try

and

2006 is YMMVI- Your mileage may vary immensely.

John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK

I won't even bother to view the video. The camera has already strobed the turning of the wheel and that is the strobe source that is causing the effect.

-- Yeppie, Bush is such an idiot that He usually outwits everybody else. How dumb!

I'm just trying to explain why the strobe effect happens with just the naked eyeball out in the bright sunlight. You can see the effect also with airplane propellers. High contrast objects are needed for the effect to happen and once it gets past a certain point, it goes away. I figure that it only shows for most people at the first order, when the object of interest moves to the next spot. If the object moves to the second next spot, the show may happen but for most, the blur is too large to see the effect. Go play with it and see what happens to you and ask your kids to view the effort also. Kids generally have much more acute eyesight and will often see things that us adults won't.

-- Yeppie, Bush is such an idiot that He usually outwits everybody else. How dumb!