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hi all, thank you for the nice words and support .
the pain won't leave me forever


jim

I did a couple of experiments.
First I built a top using a metal ruler, a ball magnet as tip and a Lego stem. The aspect ratio was a huge 18:1.  Did not work. The ruler is heavy so I was hoping that the effect of the tip and stem would be negligible.
But note that with such a large aspect ratio, the ratio of moments of inertia between the two biggest principal axes is: Iz/Ix= 1 + (a/b)² = 1.003, so they are basically the same.
Even with an aspect ratio of 1:4, the ratio would be only 1.0625. So the stem axis wouldn't be very dominant.

Then I decided to 3D print one, and came up with this "penny top" (inspired by The Toycrafter):





It says 5:1 but that was a mistake. It should say 7.5:1 as it's 195 mm long and 26 mm wide (and 2 mm thick).

As you can see, it has two possible orientations for the penny: longitudinal or transversal. I suspected one might work while the other wouldn't. And it did work! I call it the Penny Riddle: can you guess which orientation is correct? (solution below)

It does have a lot of tendency to scrape but I found the perfect pedestal: a spice jar!



With the penny in the good orientation I only get about 5 seconds spin times, so this aspect ratio is pushing it. But the difference is very clear with the bad orientation, for which it refuses to spin. The solution to the riddle is that it only can spin when the penny is transversal:



For the transversal orientation, the penny adds the same amount of inertial to each of the main axes (and decreases a little the difference with the light axis), so it does not affect the order of moments. But for the longitudinal orientation, it can flip which of the main axes is the larger one. This is because the moment of inertia of a disk with respect to a diameter is half than with respect to the perpendicular axis.

Sure.  You can always call or email Dale or me.  I'll be making an Amazon shop shortly too, but if you contact us directly, there are less fees and probably less shipping charge. 
Or email scienceofspin7@gmail.com

Thanks! Is there a place besides Facebook where I can look over your tops to pick out some to email you about? (I'm mostly a finger top fancier and not familiar with throwing top brands.)

Please update the list:
Spintastics tops
scienceofspin7@gmail.com
www.scienceofspin.com (no shopping cart but will add a page of spin tops and pricing)

Done!

Correction.  Yoyoplay in Dallas does NOT supply Spintastics tops (they did have some on their website but have recently returned all of their stock).
We did sell the yoyo line to them...but NOT the spin tops.

Please update the list:
Spintastics tops
Science of Spin, Inc. Dale or Val Oliver
scienceofspin7@gmail.com
www.scienceofspin.com (no shopping cart but will add a page of spin tops and pricing)

Sure.  You can always call or email Dale or me.  I'll be making an Amazon shop shortly too, but if you contact us directly, there are less fees and probably less shipping charge. 
Or email scienceofspin7@gmail.com

I guess you could make a detachable stem like you have done before so as to be able to use a large stem. You could also have a pedestal like Iacopo to avoid the scratch problem.

Great suggestions worth looking into, but not sure they'd help with the simple test tops at hand. A pedestal would definitely allow a shorter tip assembly, but I'm already using the shortest secure LEGO tip I can think of. And unclear as to how to make a workable detaching stem here.

But the critical speed could be a more critical issue. I'm not sure if the usual equation can be applied to the case non equal transversal moments.

You're right: The usual critical speed formulas assume 2 (and only 2) equal central moments with no easy workaround.

But page 3 of the "Spin-It" paper (Bächer et al., 2014) gives a simple critical speed formula for an arbitrary triaxial rigid body based on Lewis et al., (1992). The notation in the latter is clear as mud, but the formula looks legit. Working to apply it to this problem as we speak.

You are probably right about the practical limits to elongation. Although 4:1 doesn't seem too large. 

I guess you could make a detachable stem like you have done before so as to be able to use a large stem.
You could also have a pedestal like Iacopo to avoid the scratch problem.
But the critical speed could be a more critical issue. I'm not sure if the usual equation can be applied to the case non equal transversal moments.

If you made the stem light enough with respect to the plates, it should work. Actually, just remove the stem and spin it by pinching it 

In theory, yes. But for a real finger top to have any play value in my book, it must have at minimum...
1. A decent chance of spin-up without scraping
2. At least a few seconds of spin time (and more if other redeeming qualities like a cool visual effect or spin-down behavior are lacking)
3. Tolerable amounts of wobble and hop

From a design standpoint, that means...
A. A critical speed I can exceed by hand by a useful margin
B. A stem long and thick enough for my well-practiced adult fingers to actually twirl
C. A wiggle-free tip
D. A workable scrape angle -- which in turn means a certain minimum ground clearance that grows with max radius

In both practice and theory (see coming post), the more elongated the rotor, the greater the critical speed forced by these requirements. And beyond some maximum elongation, exceeding critical speed by a useful margin by hand becomes impossible. Meaning that the top falls over almost immediately. Not much play value there.

For example, here's the 4:1 test top above on edge, now with the lightest and shortest LEGO stem and tip assemblies I could muster. Note the original stem and tip for comparison.



A stem any thinner or shorter than this just doesn't work for me. And the scrape angle's now marginal at best.

Note that the CM's much lower than before for 2 reasons: (i) The tip assembly's a lot shorter, and (ii) had to point the rotor studs downward to make this stem and tip work. That definitely reduces critical speed, but my best spin time here is still < 0.5 s.  And no luck pinching the rotor with the stem removed.

So, a 4:1 aspect ratio is a definite fail on play value in my book. Moreover, it fails at all LEGO-compatible max rotor dimensions, not just this one.

The question then becomes, would play value impose a practical elongation limit on a thin non-LEGO rotor? Well, the limit might not be 2:1 or 4:1, but I'm quite certain it would.

Is it a balancing problem, lack of weight or maybe just my technique?

Welcome to the Forum, Nitro2k01.

It is a balancing problem.  There is a problem of lack of accurate simmetry, somewhere, maybe in the tip, which could be dented or slightly bended.
Or maybe the density of the material is not homogeneous where needed.
Recently I made a little wooden top. The shape is symmetrical, but the wood is not homogeneous, the top was very unbalanced, so I had to add a brass piece in the flywheel to make it balanced. Only after having added the brass piece the top spins well.





Certainly in your case it is not lack of weight, nor the spinning technique.