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[Jeremy McCreary]

Nice experiment, Jeremy.
From your pictures, your top wouldn't seem to me one with a so low CG to stay leaned always towards the heavy side.

Thanks, Iacopo! I think that for any given top with static unbalance, the phase angle in effect at the time of observation depends on a combination of top properties and speed. The top properties involved include CM height, AMI, TMI, and the magnitude of the unbalance. The fact that topple speed also depends on these properties (especially AMI and CM height) only adds to the confusion.

The LEGO tops I've tested so far show just one phase angle (180°) throughout their operating speed ranges (~6,400 RPM to topple speed). In practice, however, it's impossible to know what phase angle might do below topple speed, and that "blind spot" only grows with CM height. The top shown in my post has a minimum topple speed of ~480 RPM. There could easily be another whirling mode with 0° phase angle below 480 RPM, but I'd never know it.

I suspect that the tip of your top was slipping on the spinning surface, because of high speed and slippery surface, (if you used the glass pane in the picture);
if the tip slips on the base, the direction of leaning becomes unreliable.

That may well be true of some tops, but I haven't observed any variation in phase angle due to tip and surface properties alone. For any given top, I get the same phase angles on the glass table shown in the photos, on concave glass lenses of varying curvature, on finished wood, on lids from plastic containers, on coarse- and fine-grained polished granite, and even on polished granite covered with 1.8 mil plastic film, which, due to tip sinkage, tends hold the contact point in place.

[Jeremy McCreary]

Good work Jeremy.

Thanks, Alan!

I just want to verify, that when you held the top, viewing the bottom with the dull stripe at 9 o'clock, the azimuth of the gap on the ceiling indicated the heavy side, which is opposite of my experiments.

Yes, I followed your instructions to the letter, triple-checked everything, and still got gaps pointing to the heavy side in all cases. As I noted in my last reply to Iacopo, I think the difference in observed phase angle boils down to differences in top properties and test speeds. These differences were substantial in our cases.

Were any of your observed gap azimuths either near the top (~12 o'clock, and nearest you), or near the bottom (~6 o'clock and farthest from you)?   This as opposed to being only on the right or left sides).

No, never saw any phase angle other than 180° with this particular batch of 6 medium-AMI, medium-CM height tops, but I have about ~600 more with AMIs, TMIs, and CM heights all over the map. I know from past work with the paintbrush method that some of the low-slung high-AMI tops among them show 0° phase angles at low speeds, and I'll test the laser method on them next. (Bear in mind that high AMI for me is generally low AMI for you and Iacopo.) Unfortunately, our inability to test for phase angle below topple speed leaves a lot of uncharted territory.

Incidentally, recall that I posted a slow motion video of a top with a penny taped to it.  At low speed the video showed that the azimuth of the penny was lowest and that azimuth scraped first, just as we would intuit.  I have since removed that video from dropbox to make room for other items.  If you need to see it again, I can re-post it.

That video nicely documented a top with 0° phase angle at very low speed. It would be great to have it back on the forum for future reference. Have you considered posting it on YouTube to save your dropbox space?

Since then, I've had another top which behaved the same way.  I balanced it by first taping a small square of copper foil to the opposite side.  Then experimentally adjusted the size of that piece of copper to achieve balance.  Next, I calculated the volume of the copper piece and milled a shallow hole of equal volume on the heavy side (0.25" dia and .008" deep).  Balance was achieved.  The volume of the material removed calculated to a weight of .058 grams.

Interesting method. Your fabrication skills and output are quite impressive.

[Aerobie]

Were any of your observed gap azimuths either near the top (~12 o'clock, and nearest you), or near the bottom (~6 o'clock and farthest from you)?   This as opposed to being only on the right or left sides).

No, never saw any phase angle other than 180° with this particular batch of 6 medium-AMI, medium-CM height tops, but I have about ~600 more with AMIs, TMIs, and CM heights all over the map.

Would you try placing your unbalance weights 90 degrees from the gap in your reflective disc?

Thank you,

Alan

[Jeremy McCreary]

Would you try placing your unbalance weights 90 degrees from the gap in your reflective disc?

Happy to, but it'll take a few days, as we're holed up in a motel while the hardwood floors get refinished.

[Jeremy McCreary]

Would you try placing your unbalance weights 90 degrees from the gap in your reflective disc?

Grabbed my test top project box when they let me back in the house briefly this afternoon. The gap on the ceiling still points toward the heavy side of the top when the non-reflective strip is 90 degrees from the unbalance vector in either direction. Ditto for 60 and 120 degree offsets.