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Asteroid ring top
This space-themed LEGO ring top stays up 48 sec when twirled by hand and withstands speeds in excess of 3,300 RPM.
About this creation
Please feel free to look over the images and skip the verbiage.

The recipe for endless fascination calls for predictability and surprise in just the right proportions. Good music and flowing water have this quality. So do spinning tops.

The spinning top featured on this page is the first of many LEGOŽ tops to come. Below is the updated version.



The original version in this video stayed up over 22 sec when "twirled" -- i.e., when spun up with a twirl of the fingers -- and over 45 sec when spun up by motor.



The matte-finished porcelain tile in the video isn't an ideal support surface. Both versions of this top stay up ~20% longer on glass and up to 30% longer on polished fine-grained granite. All spin times reported hereafter were obtained on granite.

The next 2 photos show the original Asteroid and its update together for comparison. The older one appears first.





The update differs from the original only in having a lighter and lower "suspension system" (hub, spokes, tip, and stem combined), but that was enough double the original's spin time.

Some of the suspension changes are evident below. Again, the original is shown first.





The 1st reason for the update's dramatic gain in spin time is a 25% gain in twirled release speed due to its lower axial moment of inertia (AMI). All other things being equal, spin time increases with release speed, and the latter varies inversely with AMI, though not in a simple way.

The next reason is the update's much lower "critical speed" -- the speed at which the top becomes unstable and falls over. Because the update's mass reduction came entirely from the suspension system, Asteroid's "specific AMI" (AMI per unit mass) rose even as its absolute AMI fell. The new suspension also reduced the top's center of mass (CM) height (measured with the spin axis vertical). A simultaneous increase in specific AMI and decrease in CM height leads to a substantial lowering of critical speed.

Finally, the update's finer tip and lower overall mass (98 vs. 110 g) reduced tip friction, and that reduced the top's deceleration rate after release. When you combine a higher release speed and much lower critical speed with a lower deceleration rate, the result can only be a substantial gain in spin time.

A subsequent section elaborates on these concepts.

On this page


Overview

I think of Asteroid as a "ring top", as most of its mass resides in a heavy ring of four 10x10 LU sloped round corners well removed from the spin axis. These photos are of the update.





This favorable mass distribution gives Asteroid a high SAMI.

Of the two factors, mass and radial distance from the spin axis, the latter is much more important in determining a particular component's contribution to the top's total AMI. Since the ring constitutes most of the top's mass and is on average farther from the axis than any other component, it's responsible for most of Asteroid's AMI. The update's lighter suspension system contributes less AMI than the original's.

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Suspension system design

The ring's suspension system had to be strong enough to hold the inherently weak 4-segment ring together at speeds in excess of 3,000 RPM. Were the ring to disintegrate at such speeds, the 10x10 ring segments would become very dangerous shrapnel indeed.

I overbuilt the original suspension with this risk in mind.







However, the suspension gradually evolved to the lightweight version below, losing both mass and AMI in the process.





In both cases, the 2-spoke, 4-point ring suspension system uses studless hub-ring connections to maximize tensile strength under centrifugal force. Since posting this page, I've made over a dozen ring tops based on 10x10 round corners using variants of this suspension.



At the core of the original hub is one my favorite new elements of 2015, here seen in LBG.



Though listed as a Hero Factory Weapon Barrel with 2 Pin Holes and 3 Axle Holes (98585), this very sturdy 3x3x1 LU disk is a bona fide Technic connector, and a most welcome one at that. It's one of the very few with 3 axle holes in a row and one of the best hubs available for rotors with 2 or 4 spokes, struts or blades.

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Stem design and twirling technique

During twirls, the fingers are in contact with the stem of the top for only a fraction of a second. On high-AMI tops like Asteroid, I've learned to make the most of that time by starting my twirls at a wider part of the stem and ending them at the narrowest part.



All of my high-AMI tops use a smooth axle joiner mounted narrow end up to widen the stem at the most efficacious point. The resulting finger grip functions as a 3-speed transmission, with the widest part of the axle joiner corresponding to first gear and the bare axle above it to 3rd gear.

Starting the twirl at the widest part of the stem (1st gear) maximizes the torque my fingers can apply to get the resting top spinning. (Torque is what ultimately overcomes AMI.) This trick is essential to long spin times in high-AMI tops.



Here my fingers are shifting upward from 2nd to 3rd gear as I twirl.

Finishing the twirl with the fingers at the narrowest part of the stem maximizes release speed. Spending ~70% of the twirl at this final station seems to work best. In my hands, an unstopped 3L axle works best for 3rd gear.

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Tips on tips for tops

Why worry about tip design? Simple. It's your one and only chance to influence how your top interacts with its supporting surface prior to falling.

Tiny tip details can have a huge impact on spin time and top behavior -- including the tendency to sleep, precess, wobble, or walk.1 In general, the broader the tip, the greater the tendency to walk, and the more irregular the tip, the greater the tendency for the top to wobble.

The tip assembly on the update is my best yet for all but the smallest tops.



The tip itself -- the small black part protruding from apex of the copper 2x2 dome -- is just the last 4 mm of a round-tipped 4L antenna (3957, not 3957b). Such tips can also be harvested from lances (3849) and ski poles (2714).



The dome -- preferably the variant with a blocked open stud and bottom axle holder (553b) -- secures the tip to the top's central axle. A top can be wobble-free only to the extent that its tip assembly is wiggle-free.

Clearly, this tip design isn't for purists, but I find its performance too good to pass up. Its smooth surface and small radius of curvature promote sleeping, suppress wobbling, and eliminate walking on level surfaces. All of these effects prolong spin time indirectly, but the small contact patch also does so directly by reducing tip friction.

The broader tip used by the original Asteroid came from the rare 6L bar with cap (61680) shown in the foreground. It allows more wobbling and walking than the update's tip and slightly shortens spin time as well.



Since you can't twirl a sleeper every time, and some perfectly good tops rarely sleep, tip design has to take into account the entire range of inclinations the spin axis can adopt. The practical upper limit here is set by the top's "grounding angle" -- i.e., the inclination at which the rotor first strikes the ground. Grounding angle varies directly with rotor height and inversely with rotor diameter.



Hence, the tip has the potential to interact with its supporting surface, not just at its apex, but over an "active tip surface" encompassing all possible contact patches allowed by the grounding angle. Ideally, contact patch size should be as small and as constant as possible over the entire active surface.

In practice, that means a smooth, hemispherical active surface with a radius of curvature just large enough to keep the tip from digging into the supporting surface. The update's tip approximates that ideal quite well on most surfaces.

The best purist tip, the sharp pointy hub of a train wheel, needs a hard surface, doesn't mount very securely, and often limits grounding angle even more than the top's rotor does. Hence, it's suitable for only the smallest of tops.



The purist alternatives suitable for bigger tops all suffer from excessive tip curvatures and active surface irregularities like flats and dimples. Bare cross-axles and flat-tipped antennas and bars are generally suitable only for battle tops, where walking is essential and wobbling can be a virtue.



Technic balls and towballs, the purist's best bets, have active surfaces of the correct overall shape, but their large curvatures encourage walking, and their slightly dull finishes add friction.





Worse yet, both ball types end in recessed flats that all but guarantee wobbling at small inclinations.

Bottom line: If a non-spinning top can be balanced on its tip, however gingerly, the tip is too flat.

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Axial moment of inertia (AMI), spin time, and twirlability

In this context, the term "axial moment of inertia " (AMI) refers to a top's moment of inertia about its spin axis. "Specific AMI" (SAMI) is the ratio of AMI to total top mass. AMI and SAMI greatly influence top performance and behavior, but they do so in different ways.

Asteroid's large AMI is both a blessing and a curse. The blessing is a fairly long spin time despite the high CM and lousy aerodynamics. The curse is that it takes a lot of muscle and skill to twirl well. This trade-off is one of the most fundamental in top design. (See my primer on AMI for more info.)

"Twirlability" is an important but largely subjective quality that varies inversely with the muscle and skill needed to twirl a top well. The most twirlable tops generally have low AMIs and a lot of rotor ground clearance, but it's not quite that simple.

"Spin time" is just the time it takes for a top to spin down from release speed (ω0) to critical speed (ωcrit) under the influence of aerodynamic drag and tip friction. For our purposes, "release speed" is the angular speed attainable with the user's finger strength and twirling skill, and "critical speed" is the angular speed at which the top finally loses stability and falls over.

Release speed can vary dramatically from user to user and even from one try to the next by the same user. This variability is especially pronounced in high-AMI, low-clearance tops like Asteroid.

Critical speed, however, is an intrinsic property of the top that varies inversely with the square of SAMI and directly with at least the cube of "CM height" (h) -- i.e., the height of the CM above the tip when the top's vertical. The importance of CM height can't be overemphasized.

NB: The 2 most common measures of rotational speed are "rotational frequency" (n, usually expressed in RPM) and "angular speed" (ω, in radians per second, abbreviated sec-1). The fixed proportionality between them is given by ω = π n / 30. Though perhaps less familiar, angular speed is much more convenient when things turn quantitative.

Increasing SAMI improves spin time by lowering ωcrit, but this strategy can only be taken so far, as it also reduces twirlability and hence ω0. Spin time starts to suffer when the reduction in ω0 finally exceeds that in ωcrit.

When twirlability is an important design goal, as it was here, it takes a lot of fiddling and testing to play this crucial trade-off against spin time to maximum advantage.

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Gravity always wins

Asteroid's large dome and ring ornaments make for a visually interesting top, but they cut into spin time in several interrelated ways. For starters, they increase tip friction by adding weight and aerodynamic drag by increasing both surface area and roughness. These dissipative effects shorten the time it takes to spin down from release speed to critical speed -- especially the drag.

But it gets worse. The ornamentation also increases critical speed by reducing the top's SAMI while raising its CM height.

The gravitational torque that ultimately brings down any top is proportional to both top mass M and CM height h. Greater gravitional torque translates directly into higher critical speed, though not in a linear way.



Minimizing the product M h is therefore an important part of maximizing spin time. However, M generally trades off against spin time, structural strength and stiffness, and cosmetics (including frills like Asteroid's ring ornaments), while h trades off against both spin time and twirlability.

The trade-off between h and twirlability stems from the fact that reductions in h generally cut into ground clearance beneath the rotor. The lower the clearance, the more skill required to twirl the top to a given release speed without grounding its rotor in the process.

Low clearance effectively reduces spin time by lowering the release speed a given user can attain in a reasonable number of tries, but the trade-off between h and spin time is also an indirect consequence of the fact that "inclination" (the angle between the spin axis and the vertical) increases steadily as a top approaches critical speed. If clearance is low enough, the rotor of an inclining top may well strike the ground before the top ever reaches critical speed.

I think of tops with this last problem as being "clearance-limited" rather than "stability-limited". Asteroid is probably the latter but comes close to the former.



The frill is gone

Removing the frills (the hemispherical cap and ring ornaments) improves twirled spin times by ~20% by (i) lowering the top's CM, (ii) reducing its overall mass by 0.028 kg, (iii) reducing air resistance, and (iv) slightly reducing axial moment of inertia.



Effect (iv) makes the top easier to twirl but ultimately penalizes spin time when the motorized spinner is used. The remaining effects are beneficial in both cases: Effects (ii) and (iii) reduce kinetic energy loss to friction and air resistance, while effects (i) and (ii) reduce the gravitational torque.




Motorized top spinners

With a properly designed motorized spinner, a reasonably aerodynamic top can be pushed to release speeds many times faster than those attainable by hand. Whether the top's structure can withstand the resulting centrifugal stresses is a separate matter with serious safety implications.

NB: Cautious tests repeated many times with good eye protection are the only way to know if a particular top is safe for motorized spin-up!

When a top fails at high speed, the shrapnel is likely to fly much farther than you would ever have imagined and can pose a great risk to onlookers, pets, and any other valuables nearby. Be sure to reseat any studded connections present before each and every spin-up by motor.



I've made tops that wouldn't stay up at all when twirled but would sleep for well over a minute when spun up by motor. Nearly all were high-AMI, low-drag tops with exceptionally large CM heights like the one above.

Motorized spinners also shine with tops that need to be kept nearly vertical during spin-up. Ff the supporting surface is slick enough, that can apply to almost any top. High-AMI tops with small grounding angles due to broad, low-slung rotors like Asteroid can also benefit, especially in unpracticed hands.

Finally, a motorized spinner may be the only way to get some tops to sleep. For example, X-pod here will sleep only when released at speeds well beyond the twirling range with the spin axis very close to the vertical.



The simple direct-drive motorized spinner below uses the fastest 9V LEGOŽ motor ever made -- the original 9V Technic motor (2838). The one used here is over 30 years old and has a no-load speed ~4,400 RPM.



Spinning up tops is an ideal application for this low-torque motor, as speed is of the essence, and little torque is needed to overcome tip friction and air resistance in most cases. (If rapid spin-up were an important design goal, a motor with a lot more torque would be in order.)



An axle joiner on the motor's built-in axle couples the motorized spinner to the top. Splitting the business end of the joiner and splaying it open just so yields clean releases ~95% of the time.



The dual black competition cannons incorporated into the motorized spinner's handle were supposed to serve as release aids. The idea was to jar a balky top loose with the impact of the yellow competition arrows (here sans rubber tips) against the handle frame. It rarely works.

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Specifications (update only)

Overall dimensions:160x112 mm (DxH, including stem)
Mass:98 g
Release speed:~500 RPM by hand; >3,300 RPM by motor
Spin-up time:15-20 sec by motor
Spin time:48 sec by hand; >60 sec by motor
Grounding angle:~15°
Spin-up motor:Original Technic 9V (2838)
Electrical power supply:Old 9V train transformer
Modified LEGOŽ parts:(i)Tip cut from 4L antenna; (ii) motor-top coupler (split axle joiner)
Non-LEGOŽ parts:None
Credits:Original MOC

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Footnotes

1 Common top behaviors include sleeping, precessing, wobbling, and walking.

A "sleeping" top spins quietly in place with its spin axis exactly vertical. If it's perfectly smooth and uniform in color, it may look like it's standing still in defiance of gravity.



In "precession", an inclined spin axis sweeps out a cone about the vertical. "Steady precession" occurs when the "inclination angle" between the spin axis and the vertical remains constant.



When the inclination angle oscillates as the top precesses, the top is said to "wobble", "nod", or "nutate".

Finally, a top "walks" or "travels" when its tip moves across the supporting surface. That's essential in a battle top, but it reduces spin time by diverting rotational kinetic energy into translational motion. Walking can coexist with eith steady precession or wobbling.

Walking is inevitable on sloping surfaces and hard to suppress on high-friction level surfaces. Whether it occurs under other circumstances depends primarily on the nature of the tip itself.

By definition, a sleeping top neither precesses nor wobbles nor walks. All other things being equal, sleeping tops spin longer. A poorly chosen tip greatly reduces the odds of twirling a sleeper.

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Comments

 I made it 
  January 30, 2015
Quoting Lego Jim .... That is one realy cool looking top. I like the motorized spinner too. This is some really clever stuff. Great job.
Very kind words from MOCpages' master of illusion.
Jeremy McCreary
 I like it 
Lego Jim ....
  January 29, 2015
That is one realy cool looking top. I like the motorized spinner too. This is some really clever stuff. Great job.
 I made it 
  January 23, 2015
Rabbit and Deus, Thanks for the kind words. I gather that a lot of folks don't care for the Asteroid Belt sphere because it's "not a real planet", but it's my favorite visually, and this MOC gave me a chance to use it.
 I like it 
  January 22, 2015
Excellent MoC and write up. A motorized spinner like yours would give a Ninjago warrior an advantage in the fighting arena if your replaced the warrior's head with an axle connector. Yeah it would look a bit weird but it but a +600 rpm spin would kick some butt in the fighting arena. Come to think of it - have you considered connecting a 6x6 Ninjago spinner turntable (e.g. bricklink part bb493c02pb02) to the bottom of your top to improve your spin time? These turntable parts are balanced, weighted, and designed to spin.
 I made it 
  January 22, 2015
Quoting Gabor Pauler Nice spinner. if I were you, I would try to build some kind of a spinning UFO from this...
Gabor, Thanks for the cool idea and the like. Problem is, if my abductors ever found out that I'd blown their saucer's cover, well, I'd rather not think about the probe they'd be coming at me with next.
  January 22, 2015
Kinda fun looking asteroid belt... And it spins like the real deal.
 I like it 
  January 22, 2015
Nice spinner! Great idea to build this with Lego
 I like it 
  January 22, 2015
Nice spinner. if I were you, I would try to build some kind of a spinning UFO from this...
 I made it 
  January 22, 2015
Quoting Walter Lee Your mechanical axle spinner is a well executed design. You might consider using a Ninjago Spinner/turntable as the base (bottom part) of your top. Ninjago turntables are weighted, balanced and designed to spin. Your top could be modified to be a Super-Ninjago spinner platform. Another balanced platform for a top that I have seen are large Technic axle wheel (wheel rims, e.g. bricklink part 88517 ) - adding rubber tire to the wheels increase the weight around the rim and improves their spin time, too. fyi - the ripcord for the Legend of Chima is not compatible with the Technic 8 tooth spur gear and the Legend of chima riding cycle flywheel does not have an axle connector so it cannot be used power anything.
Walter, many thanks for the like and the info. Hmmm, I'll have to give the Ninjago turntable a try as a replacement for the inverted 2x2 dome I used as a tip. I should emphasize for those new to tops that I'd still need a very strong studless hub above the turntable to keep the massive but intrinsically weak 20x20 LU 4-part ring in one piece at over 3,000 RPM. If the ring were to disintegrate at such speeds, the shrapnel would be very dangerous.
  January 22, 2015
Your mechanical axle spinner is a well executed design. You might consider using a Ninjago Spinner/turntable as the base (bottom part) of your top. Ninjago turntables are weighted, balanced and designed to spin. Your top could be modified to be a Super-Ninjago spinner platform. Another balanced platform for a top that I have seen are large Technic axle wheel (wheel rims, e.g. bricklink part 88517 ) - adding rubber tire to the wheels increase the weight around the rim and improves their spin time, too. fyi - the ripcord for the Legend of Chima is not compatible with the Technic 8 tooth spur gear and the Legend of chima riding cycle flywheel does not have an axle connector so it cannot be used power anything.
 
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