No strobe needed for "Wagon Wheel" Reverse Rotation effect

[Bill Beaty]

I just saw something very strange while driving in to work. This was
in bright daytime sunlight on interstate 90. The nuts on the wheels
of an 18-wheel vehicle were making a shadow pattern in the blurry lug
nuts... which rotated slowly backwards.

A-HA!!!!!!! It`s soooo obvious once you've seen it. No strobe
illumination is required. The cause is: wise ass garage mechanics.

Suppose that we tighten the ten hexagonal lug nuts on a truck wheel,
then turn them a bit more so that they all point one hex facet towards
the hub. In that case they will make flashing patterns of reflected
light which remains static as the wheel rotates. (As each metal facet
rotates to the same angle, it sends a flash of sunlight to observers'
eyes.)

However, what if the mechanic has adjusted the nuts to sequentially
different angles? For ten hex nuts, if we turn the first one by 1/10/6
= 1/60th turn, the next one by 2/60, etc., then the tenth nut would be
10/60ths or 1/6th turn... which puts the hex nut facets back to the
beginning. If this wheel now rotates in the sun, the light patterns
will move at 1/10th the rate of wheel rotation. (The pattern might go
backwards or forwards depending on which side of the vehicle we were
on, and whether we adjusted the nuts by tightning CW or by loosening
CCW.)

Pretty cool, eh? A strobe effect with no strobe light. Should drive
your physics teacher crazy.

((((((((((((((((((((((( ( ( (o) ) ) )))))))))))))))))))))))
William J. Beaty http://staff.washington.edu/wbeaty/
Research Engineer UW Chem Dept, Bagley Hall RM74
[email protected] Box 351700, Seattle, WA 98195-1700
ph:206-543-6195 fax:206-685-8665

 

[Sam Wormley]

I just saw something very strange while driving in to work. This was
in bright daytime sunlight on interstate 90. The nuts on the wheels
of an 18-wheel vehicle were making a shadow pattern in the blurry lug
nuts... which rotated slowly backwards.

A-HA!!!!!!! It`s soooo obvious once you've seen it. No strobe
illumination is required. The cause is: wise ass garage mechanics.

Suppose that we tighten the ten hexagonal lug nuts on a truck wheel,
then turn them a bit more so that they all point one hex facet towards
the hub. In that case they will make flashing patterns of reflected
light which remains static as the wheel rotates. (As each metal facet
rotates to the same angle, it sends a flash of sunlight to observers'
eyes.)

However, what if the mechanic has adjusted the nuts to sequentially
different angles? For ten hex nuts, if we turn the first one by 1/10/6
= 1/60th turn, the next one by 2/60, etc., then the tenth nut would be
10/60ths or 1/6th turn... which puts the hex nut facets back to the
beginning. If this wheel now rotates in the sun, the light patterns
will move at 1/10th the rate of wheel rotation. (The pattern might go
backwards or forwards depending on which side of the vehicle we were
on, and whether we adjusted the nuts by tightning CW or by loosening
CCW.)

Pretty cool, eh? A strobe effect with no strobe light. Should drive
your physics teacher crazy.

((((((((((((((((((((((( ( ( (o) ) ) )))))))))))))))))))))))
William J. Beaty http://staff.washington.edu/wbeaty/
Research Engineer UW Chem Dept, Bagley Hall RM74
[email protected] Box 351700, Seattle, WA 98195-1700
ph:206-543-6195 fax:206-685-8665

I've seen the same effect....

Strobe effects might be caused by observer vibration.

 

[Si Ballenger]

I just saw something very strange while driving in to work. This was
in bright daytime sunlight on interstate 90. The nuts on the wheels
of an 18-wheel vehicle were making a shadow pattern in the blurry lug
nuts... which rotated slowly backwards.

A-HA!!!!!!! It`s soooo obvious once you've seen it. No strobe
illumination is required. The cause is: wise ass garage mechanics.

Suppose that we tighten the ten hexagonal lug nuts on a truck wheel,
then turn them a bit more so that they all point one hex facet towards
the hub. In that case they will make flashing patterns of reflected
light which remains static as the wheel rotates. (As each metal facet
rotates to the same angle, it sends a flash of sunlight to observers'
eyes.)

However, what if the mechanic has adjusted the nuts to sequentially
different angles? For ten hex nuts, if we turn the first one by 1/10/6
= 1/60th turn, the next one by 2/60, etc., then the tenth nut would be
10/60ths or 1/6th turn... which puts the hex nut facets back to the
beginning. If this wheel now rotates in the sun, the light patterns
will move at 1/10th the rate of wheel rotation. (The pattern might go
backwards or forwards depending on which side of the vehicle we were
on, and whether we adjusted the nuts by tightning CW or by loosening
CCW.)

Pretty cool, eh? A strobe effect with no strobe light. Should drive
your physics teacher crazy.

Probably has to do with the small movements of your eyes that you
don't notice. The eyes are always jittering to allow the
chemicals in the vision receptiors to rejuvinate.

 

[Bill Beaty]

Probably has to do with the small movements of your eyes that you
don't notice. The eyes are always jittering to allow the
chemicals in the vision receptiors to rejuvinate.

No, that doesn't work. Visual saccades are aperiodic. They can cause
random flickering effects, but they don't create the type of smooth
continuous virtual motion seen in the truck wheels.

And besides, we don't need wild speculations about the cause when such
a straighforwarded explanation exists. If you don't believe it, just
get a handful of large hex nuts and position them on an old record
player turntable. I can see the diagram of rotating lug-nuts in my
head, and describe it in words, but I guess nobody else will see the
whole phenomenon unless I create an animated GIF.

(((((((((((((((((( ( ( ( ( (O) ) ) ) ) )))))))))))))))))))
William J. Beaty http://staff.washington.edu/wbeaty/
[email protected] Research Engineer
[email protected] UW Chem Dept, Bagley Hall RM74
206-543-6195 Box 351700, Seattle, WA 98195-1700

 

[Bill Beaty]

I've seen the same effect....

Strobe effects might be caused by observer vibration.

They might be... but they're not! Once you've seen the
lugnuts-wagonwheel effect, the cause is totally obvoious. Well,
obvious if you know some simple optics. I strongly suggest that you
give a serious try at reading the message to which you responded.



Hey. HEY! Another brainstorm!

I'd been wondering how the garage mechanics could stumble across this
weird optics effect. Perhaps they hire out-of-work physics PhDs who
get bored and start pulling Feynman-esque tricks with spatial beat
patterns? It only takes one person to discover the trick, and from
then on it spreads from garage to garage. But even so, the pattern of
adjustment would be a bit complicated. I doubt the trick would spread
far. But maybe it's been spreading for many decades?

Here's the brainstorm: what happens if we adjust the lug nuts so one
flat facet of each nut is parallel to the ground? That's gotta be it!
In this case the slowly drifting "stroboscopic" pattern will still be
there, but it will move two notches ahead per revoltion of the wheel:
twice as fast as the optimized pattern I described in the first
message.

The hub-facing facet of each successive hex nut would then be off by an
angle of 360*(1/12 - 1/10) degrees, like so:

Lug nut position Angle deviation
(numbered CW) (degrees)
0 0
1 - 6
2 - 12
3 - 18
4 - 24
5 0
6 - 6
7 - 12
8 - 18
9 - 24
10 0

So the bright pattern of periodic "highlights" will drift in *reverse*
direction relative to wheel rotation.

So... communicating the trick really doesn't take any effort. Just
tell other mechanics to tighten the lug nuts so they're parallel to the
road, and everyone else will start arguing about impossible strobe
light effects; effects which occur in direct sunlight.


(((((((((((((((((( ( ( ( ( (O) ) ) ) ) )))))))))))))))))))
William J. Beaty http://staff.washington.edu/wbeaty/
[email protected] Research Engineer
[email protected] UW Chem Dept, Bagley Hall RM74
206-543-6195 Box 351700, Seattle, WA 98195-1700

 

[Rich Grise]

No, that doesn't work. Visual saccades are aperiodic. They can cause
random flickering effects, but they don't create the type of smooth
continuous virtual motion seen in the truck wheels.

And besides, we don't need wild speculations about the cause when such
a straighforwarded explanation exists. If you don't believe it, just
get a handful of large hex nuts and position them on an old record
player turntable. I can see the diagram of rotating lug-nuts in my
head, and describe it in words, but I guess nobody else will see the
whole phenomenon unless I create an animated GIF.

I guess I'm unusual (surprise, surprise!) but I have no trouble at all
visualizing that from your description, even the ones with the nuts
at incremental angles, which, it turns out, does make a pretty cool
visual (at least in my headbone. ;-) ).

Cheers!
Rich

 

[Mark L. Fergerson]

No, that doesn't work. Visual saccades are aperiodic. They can cause
random flickering effects, but they don't create the type of smooth
continuous virtual motion seen in the truck wheels.

And besides, we don't need wild speculations about the cause when such
a straighforwarded explanation exists. If you don't believe it, just
get a handful of large hex nuts and position them on an old record
player turntable. I can see the diagram of rotating lug-nuts in my
head, and describe it in words, but I guess nobody else will see the
whole phenomenon unless I create an animated GIF.

I can see it without, er, seeing it. I guess it's a spatial
perception/imagination thing.

And yeah, it's pretty cool!


Mark L. Fergerson

 

[Ben Newsam]

I just saw something very strange while driving in to work. This was
in bright daytime sunlight on interstate 90. The nuts on the wheels
of an 18-wheel vehicle were making a shadow pattern in the blurry lug
nuts... which rotated slowly backwards.

Sounds a bit dangerous, in that the nuts are clearly not tightened to
the proper torque. If they had used some kind of decorative nut cover,
perhaps...

 

[Sam Wormley]

They might be... but they're not! Once you've seen the
lugnuts-wagonwheel effect, the cause is totally obvoious. Well,
obvious if you know some simple optics. I strongly suggest that you
give a serious try at reading the message to which you responded.

I've seen the effect with wheels having recessed lug nuts, so I
have concluded that a strobed light source was not a factor for
my observation.

 

[Bill Beaty]

Sounds a bit dangerous, in that the nuts are clearly not tightened to
the proper torque. If they had used some kind of decorative nut cover,
perhaps...

Or maybe they were over-tightened? To set up the parallel-facet
pattern, you can either turn them tighter by between 0 turns and 1/6th
turn... or tighten/loosen them by plus or minus 1/12th turn. On the
other hand, if truck wheel structure has more "give" than car wheels
(so the force changes less per turn,) then the fancy pattern on truck
lug nuts would be less dangerous than a similar pattern on a car wheel.
Also note that many (all?) large truck wheels have ten lugs.


((((((((((((((((((((((( ( ( (o) ) ) )))))))))))))))))))))))
William J. Beaty Research Engineer
[email protected] UW Chem Dept, Bagley Hall RM74
[email protected] Box 351700, Seattle, WA 98195-1700
ph425-222-5066 http//staff.washington.edu/wbeaty/

 

[Ben Newsam]

Or maybe they were over-tightened?

I didn't say they were looser than they should be, I just said not
correct.

 

[Doc]

I've always found this interesting as well. I've read things relating
this effect to the framerates of our eyes but I wasn't impressed. The
lugnuts here ARE the strobe with a frequency of however many times
they're aligning just right with the sun to relfect at your eye. Lots
of mag type wheels will produce this effect as well because of the huge
difference in reflection between the mettallic spokes and the dark
voids in between them. As far as the backwards effect, I'm guessing
that is caused by differences between the rate of reflection and the
angular velocity of the wheel, almost like a beat frequency. If that
doesn't make much sense, think of a wheel spinning at 120rpm with one
mirror on it compared to a wheel spinning at 120rpm with two mirrors on
it. If the speed of the wheel changes just a tiny bit, that changes
everything. Any takers?
-Doc

 

[Mark]

I would guess the mechanics are not doing this on purpose but rather is
a reuslt of the torque wrench or some other aspect of the way the nuts
are tightnened.

Intreesting...thanks

Mark

 

[Bill Beaty]

I've always found this interesting as well. I've read things relating
this effect to the framerates of our eyes but I wasn't impressed. The
lugnuts here ARE the strobe with a frequency of however many times
they're aligning just right with the sun to relfect at your eye.

Exactly. That provides the first frequency.

Lots
of mag type wheels will produce this effect as well because of the huge
difference in reflection between the mettallic spokes and the dark
voids in between them. As far as the backwards effect, I'm guessing
that is caused by differences between the rate of reflection and the
angular velocity of the wheel, almost like a beat frequency.

Yes and no. If all the similar reflectors are aligned to the same
angle relative to a radial line passing through the hub... then they
will always flash at the same wheel angle, and the flashing spot of
light will seem to remain still as the wheel rotates. (In other
words, a small flat mirror mounted on a spinning wheel will sweep out a
reflection and act somewhat like an unmoving cylindrical mirror.)

If that
doesn't make much sense, think of a wheel spinning at 120rpm with one
mirror on it compared to a wheel spinning at 120rpm with two mirrors on
it.

But then the glowing spot will just flash twice as fast. But it won't
move along.

To make it move, we just have to progressively adjust each mirror angle
in sequence. If there are many mirrors equally spaced around a circle
on the wheel, and if each one is progressively tilted, then each flash
position will advance by that extra angle, and the flashing spot will
move along. Unfortunately it will move with sawtooth motion: moving
slowly forwards, then suddenly jumping back to the initial position.
BUT... but if instead we use faceted nuts as mirrors, in that case
there's a magic number for the progressive extra angle we give to each
nut, an angle where all the progressive tilting will add up so it
tightens the last nut by one entire facet.

Or said another way: the facets of whirling hex nuts produce not one,
but *several* bright and rapidly flashing spots, like a dotted line on
the wheel circumference, but if we tighten each nut a bit more than the
previous one, we can make this dotted line start drifting either
forward or back. And if we turn each nut to a particular angle, the
row of flashing spots will advance by one entire spot per revolution of
the wheel (and so there will be no "sawtooth" motion.) As I said
before, for ten hex nuts, one magic angle is quantized 1/60th turn
relative to a radially-adjusted pattern. So first adjust the ten nuts
to point at the hub, then turn nut #1 by 1/60th turn (6deg,) turn the
second nut by 2/60ths (12deg), etc. The last one would turn 10/60ths
or 60deg or one facet-space, so don't bother turning it at all.

For ten nuts, another magic angle is 1/10 turn or 36deg. It's doubly
magic, since it cause two parallel facets of all nuts be aligned
parallel to one line (such as the surface of the earth.) That makes
the alignment instructions very easy to remember.

HEY! I wonder if garage mechanics *know* that this happens? Maybe
they just align the nuts in a decorative pattern for the hell of it,
never realizing that it produces a pattern of backwards-rotating
lights?

PS I just saw the moving pattern in some really grubby truck lug
nuts. Apparantly we don't require a specular reflector, but just a
flat nut-facet surface which changes its apparent brightness as it
rotates.

((((((((((((((((((((((( ( ( (o) ) ) )))))))))))))))))))))))
William J. Beaty http://staff.washington.edu/wbeaty/
Research Engineer UW Chem Dept, Bagley Hall RM74
[email protected] Box 351700, Seattle, WA 98195-1700
ph:206-543-6195 fax:206-685-8665 http://amasci.com/

 

[Bill Beaty]

I guess I'm unusual (surprise, surprise!) but I have no trouble at all
visualizing that from your description, even the ones with the nuts
at incremental angles, which, it turns out, does make a pretty cool
visual (at least in my headbone. ;-) ).

Whew! I was afraid that this was yet another instance where my
explanation seems perfectly simple to me, yet nobody here has any idea
what I'm talking about.



Maybe I can only explain it to other visual thinkers?


Here's another interesting bit: with nuts adjusted with one facet
parallel to the ground, the hex nuts must rotate 60deg before they all
flash again. But since they're spaced ten nuts per 360deg, or 36deg
apart, then when a nut flashes the next time, it has moved amost to the
position of a previous flash (which was located at 2x36 or 72deg.) But
since the wheel rotated by 60deg between flashes, rather than by 72deg,
our brains will perceive the flash as having jumped along by 60deg -
72deg = -12deg.

The wheel physically rotated by 72deg each time the flash jumps
backwards by 12deg. So the pattern of ten spots of light is rotating
backwards at 1/6th the RPM of the wheel! No wonder it's so easily
seen. 6x slower.

Now we just have to harness the effect for the benefit of humankind.
Like making cheap plastic hubcaps which seem to rotate backwards or
forwards (or both,) at quantized speeds of 1/6th, 1/3rd, etc.

((((((((((((((((((((((( ( ( (o) ) ) )))))))))))))))))))))))
William J. Beaty http://staff.washington.edu/wbeaty/
Research Engineer UW Chem Dept, Bagley Hall RM74
[email protected] Box 351700, Seattle, WA 98195-1700
ph:206-543-6195 fax:206-685-8665 http://amasci.com/

 

[Ben Newsam]

Whew! I was afraid that this was yet another instance where my
explanation seems perfectly simple to me, yet nobody here has any idea
what I'm talking about.

I think several of us can visualise it quite easily. All the same, it
would be really cool is if you could manage to get a video of it
actually happening on a truck!

 

[Ben Newsam]

HEY! I wonder if garage mechanics *know* that this happens? Maybe
they just align the nuts in a decorative pattern for the hell of it,
never realizing that it produces a pattern of backwards-rotating
lights?

Maybe this thread should be crossposted to a truckers newsgroup. Hmmm,
then again maybe not.

 

[Shadowland]

All very interesting but I have a hard time believing there's a truck
mechanic
on the planet that would take the time to arrange the nuts in such a
pattern.
One that did would be fired for wasteing time.


And the odds of the nuts just working out that way upon being
tightened would be
millions to one.

Anyhow the angles probably aren't that critical since there will be
some reflection
from highly polished nut regardless.

 

[Bill Beaty]

All the same, it
would be really cool is if you could manage to get a video of it
actually happening on a truck!

How about happening instead on my workbench?

A video will convince no one, since the camera aliasing creates all
sorts of drifting patterns. However, when I spin my lugnuts-disk quite
fast, I see in the camera viewfinder the same drifting pattern which I
see with my unaided eyes.


Apparent reverse motion in truck wheel lug nuts

(Note that I stopped the shutter way down for this video, so the bright
pattern would be more easily seen.)

PS

videos on other topics (such as FLIR infrared kitten footage.)

billb videos
http://www.youtube.com/view_play_list?p=6E21BC6820E062A6

((((((((((((((((((((((( ( ( (o) ) ) )))))))))))))))))))))))
William J. Beaty http://staff.washington.edu/wbeaty/
Research Engineer UW Chem Dept, Bagley Hall RM74
[email protected] Box 351700, Seattle, WA 98195-1700
ph:206-543-6195 fax:206-685-8665 http://amasci.com/

 

[Bill Beaty]

All very interesting but I have a hard time believing there's a truck
mechanic
on the planet that would take the time to arrange the nuts in such a
pattern.

In the last few days I've already seen two trucks with a very clear
"drifting dots" pattern in their spinning lug nuts.

It's perfectly possible that garage mechanics know nothing about this
stuff, and the pattern is accidental.

Suppose you use a tire iron rather than an air-driven wrench. Suppose
you tighten each nut so the tire iron is parallel to the ground. That
creates exactly the arrangement needed to make pseudo-strobelight
motion patterns.

Anyhow the angles probably aren't that critical since there will be
some reflection
from highly polished nut regardless.

Yes, and angle error can be crudely determined: if the hex nut angles
are randomized by more than +-30 degrees, there will never be any
coherent patterns in the reflections. And for the parallel-facets
arrangement, if nuts are randomized by +-18 degrees, their reflections
will wiggle around but never overlap with the apparent locations of
neighbors' reflections.

So when you're stamping on the tire iron, if you always leave it at the
same angle within ~10 degrees, your truck's lug nuts will probably make
an easily seen "drifting dots" pattern when on the highway.

((((((((((((((((((((((( ( ( (o) ) ) )))))))))))))))))))))))
William J. Beaty http://staff.washington.edu/wbeaty/
Research Engineer UW Chem Dept, Bagley Hall RM74
[email protected] Box 351700, Seattle, WA 98195-1700
ph:206-543-6195 fax:206-685-8665 http://amasci.com/