So it seems I have found a nice LEGO tip for not so low CM (I guess Jeremy will know the exact radius of that curvature off the top of his head.)
My own lingo, just to keep things straight: "Pivot" = the entire surface subject to touching the ground at tilts short of the scrape angle. "Contact" = the instantaneous physical interface between the pivot and the ground.
We use "tip" pretty loosely around here. The meaning's usually clear from context, but not always. As a modular topmaker, I tend to think of the "tip" as an assembly including the part providing the pivot and all the other parts needed to secure the pivot to the rest of the top.
Of the 5 LEGO pivots above, the 4 on the left see the most use by far. They also go by other names. Each has its pros and cons. The pivots on the Mixel joint ball and Technic ball have tiny flats at bottom dead center, but the effect on behavior is generally negligible. All 5 are otherwise spherical.
The Technic ball (red) is very handy, as it fits securely on the end of a Technic cross-axle (black) with no other mount needed. Many different parts can be used to mount the minifig mic and Mixel joint ball securely — provided you're willing to shorten the small rods attached to them.
We've discussed the profound effect pivot radius of curvature has on top behavior and contact resistance many times. When I want to maximize spin time, favor sleep, and minimize travel, I cut myself a 1.6 mm pivot off the end of a round-tipped 4L antenna. (This pivot is found on several other parts as well.)
I use the boat skid (far right) as a pivot only when I want lots of travel or certain behaviors from a high-CM top. Seems to have more resistance than all the others -- especially when translucent, and especially on concave surfaces. (The polycarbonate LEGO uses to make translucent parts has significantly higher coefficients of static and sliding friction than the ABS used for opaque parts.)
Often, my choice of pivot is dictated by how securely I can mount it on a given top and how desperate I am to lower the top's CM. Pivot wiggle is bad.
- Ah, it is not enough for a thing to be a top to ask that CM is always above contact point! The radius of the curvature of the contact point tip needs to be smaller than the distance from contact point to CM !! Otherwise is can't fall.
Great point! Hadn't quite put it together that way. So in order for a spintoy to fall, its CM must be above
both the contact
and the pivot's center of curvature when the spin axis is vertical.
Two tops made from weighted Spinjitzu turntables below. The purple one with the Mixel joint ball pivot falls right over at rest. The red one with the boat skid pivot stays upright. Vertical CM heights are about the same.
You must have discussed those things before around here... Did you agree on rules what can be considered a spinning top for this forum at least?
Many discussions, but still no unanimous agreement. For a spintoy to qualify as a true top, I
personally require that it have a non-zero critical speed, below which it has no stability against gravity and falls at the slightest provocation. Many others share that view, though not necessarily in those words.
(the head slap emoji is still missing)
Yes, and I'm really going to need it for myself in my next post. So from now on, I'm using
as the dope-slap emoji.