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[Aerobie]

My latest twirler is 2.25" OD, bronze sleeve, 158g, spinning on a 0.5" steel ball.  I decided to try this big ball to achieve lower topple speed.  But I expected the price of the big ball would be higher decay.  But decay is quite low. 

Topple speed is about 160 RPM.

Yesterday, I twirled it to a new personal best of 24:27.
Today I broke yesterday's record with a twirl of 24:50.

I've now made several tops which have the same rotational inertia, but different diameters (2.25", 2.5" and 3.0").  In order to have constant rotational inertia, the smaller tops are heavier.

The smallest one has the least decay, despite being the heaviest and thus having the most tip friction.  I hadn't expected this.  I thought tip friction would be the most important factor at twirling speeds.

I'll probably try 2.0" diameter next.

Alan

[Aerobie]

What is the optimum shape for the wheel edge of a twirler?

Focusing on twirlers has been very interesting to me.  My most recent finding (above) is that for a given rotational inertia, the diameter/height ratio should not be too large. 

Now, what about the edge of the wheel?

Iacopo favors rounded edges.  Iacopo, have you experimentally determined that the round edge is superior to my cylinder?

Some thoughts:

The rounded edge has lower surface area for a given rotational inertia and diameter.

Radial (centrifugal) airflow may break free of the cylinder at the corners, thus reducing the effective surface area.  This is why autos now have a sharp corner at the upper rear end.  (They were rounded decades ago).

I'm searching for published data, but haven't found it yet.  My next top will probably have a rounded edge to compare to my 2.25" cylindrical top.

Best,

Alan

[Iacopo]

Iacopo, have you experimentally determined that the round edge is superior to my cylinder?

Tested in the vacuum chamber, the best aerodynamics seems to me the rounded shape; all my best tops have that shape.  I have only one top with the shape of a cylinder and it seemed inferior to me; but more shapes, weights, proportions should be tested to be sure.
What I see with clarity is that the best shapes are very simple and clean, they have no ridges, angles, holes...  One of my older tops has many holes under the flywheel, (I made them for balancing it), and this top clearly performs poorly at high speed.

[Aerobie]

I made a second 2.25" top with rounded edges.  In order to match rotational inertia, the rounded top is slightly thicker and heavier (173g vs 158g for square edged top).

To date the rounded top has slightly lower decay above 1,000 RPM (string launched) and they are identical below 1,000 RPM, which is the twirling range.  But the rounded top is not as well balanced.  I will mount the 0.5" ball in a cup with three set screws to adjust its centering and repeat decay measurements after it is balanced.

Alan

[the Earl of Whirl]

By the way, I love the video.  Thanks for posting it.  I can't wait to watch it again tomorrow, which is my day off!

[Iacopo]

This is my new top.  It has replaceable tips, (PTFE balls).
I adjusted its balance inserting a little weight in the wood below the top; this has been made for a definitive balance of the top and balance shouldn't (hopefully) tuned again in the future.  But it wouldn't be bad to have anyway a system (which there is not in this top) for adjusting the tip position; with spiked tips filing one side of it was enough, but with ball tips this can't be done.  I will invent something for my next top, (maybe without screws).

[Aerobie]

Another beauty indeed.

Can you tell us more about Teflon balls?
How does its decay compare to carbide on glass with a very thin film of oil?
How many 40 minute spins before noticeable flattening?
Is oil of any benefit with Teflon?  I think it should further reduce decay

I noted the deep concavity of your base.  My experience is that deep concavity has much greater decay.

Best,
Alan

[Jeremy McCreary]

This is my new top.

The video is almost as gorgeous as the top, Iacopo!

[Iacopo]

Can you tell us more about Teflon balls?
How does its decay compare to carbide on glass with a very thin film of oil?
How many 40 minute spins before noticeable flattening?
Is oil of any benefit with Teflon?  I think it should further reduce decay
I noted the deep concavity of your base.  My experience is that deep concavity has much greater decay.

Teflon is generally a very good material for the tips, as for spin decay, compared to other materials. 
I made my first records with it.  It is particular because it is a very slippery material and you really have this slippery sensation when you spin the top; a teflon tip doesn't help the top to stay in place when spinning it.  It tend to slip sideways (as you see in the video).  So the top it is easier that exits from a little base, while spinning it, which it happened not rarely with my first tops.  Now I make my bases with a raised edge to avoid this problem.
It is not a durable material so certainly it can't be used as a fixed tip. Also it dents easily.
Teflon works better with oil.
For the shape of my base I am constrained by the recessed tip; the spinning surface must be little enough to enter in the base of the top and stay in contact with the tip never touching the flywheel at its sides.  In such a little base, if I don't give to it enough concavity, the top with a ball tip (with spiked tips is different) tends to spin in circles around the circumference of the base; so I have to compromise.
 

[Aerobie]

For the shape of my base I am constrained by the recessed tip; the spinning surface must be little enough to enter in the base of the top and stay in contact with the tip never touching the flywheel at its sides.  In such a little base, if I don't give to it enough concavity, the top with a ball tip (with spiked tips is different) tends to spin in circles around the circumference of the base; so I have to compromise.

Yes.  I appreciate the problem.  But perhaps the base could be shaped with slight concavity on the inner half, and steeper concavity for the outer half.  If you are buying the glass (concave lens), you could buy a shallow lens and surround it with a steeper border, which you machine from any rigid material (metal or plastic).  The top would circle when twirled, but soon settle in the central glass.

Best,
Alan

PS  I'm almost finished making the cup I mentioned with three set-screws to position the ball for balancing.  I'm excited about how I can also use this to test different ball sizes to measure their effect on decay and topple speed.  I can also try different ball materials.  All of this will keep me busy for a week.

[Iacopo]

perhaps the base could be shaped with slight concavity on the inner half, and steeper concavity for the outer half.  If you are buying the glass (concave lens), you could buy a shallow lens and surround it with a steeper border, which you machine from any rigid material (metal or plastic).  The top would circle when twirled, but soon settle in the central glass.

When it begins circling around the circumference, with the tip leaned on the external ridge surrounding the concavity, it continues in that way for a long time.  The circumference is short, and the top travels it in very little time, so this circling motion is rapid, and the resulting centrifugal force does not permit to the top to settle at the center until the top spins slow enough, towards the end of the spin.
This doesn't happen if there is enough concavity in the base, for this reason I make it so.
I should give to only the central part of the base a shallow curvature, but I have to think how to do it; with my technique for making bases ,(it is polished steel, not glass), I tend to generate a uniform curvature in the spinning surfaces.   

With high Cg, slight lean moves the Cg in the direction of lean and the side it leans towards becomes the heavy side, even if it was the light side before the lean. 

I think you are right.  Leaning amplifies, (low CG), or even changes, (high CG over a certain angle of leaning), the effect of weight of the heavy side on the direction of leaning.  Still it is not clear what triggers the change of direction towards the 90 degrees position.
 

[Aerobie]

Update on two 2.25" tops:

I've already written about my 2.25" cylindrical and rounded tops.  The rounded version had inferior balance so I tried Iacopo's method of moving the pivot.  (In a prior post I moved weights in the rim of a 2.5" top).

As Iacopo has warned us, this requires very slight movement of the pivot.  I secured the pivot (0.5" steel ball) with three 2-56 screws.  After a long learning process I achieved excellent balance as evidenced by smooth sleeping and an abrupt transition from sleeping to topple with no intervening wobble.

But the odd thing is that prior to balancing, the rounded top toppled at 173 RPM after a period of ugly wobble.  When balanced it topples suddenly at 210 RPM.  My prior balancing work had resulting in both eliminating the wobble and lowering the topple RPM.

The balance mechanism elevated the Cg from about 0.57" to 0.64", but that should not be sufficient to account for the substantial increase in topple RPM.  However I'll sink the balance assembly to restore the prior Cg height and repeat the tests.

Another bit of data is that the decay rate in the range of 300-500 RPM increased about 20%, presumably due to the windage of the screws.  The decay rates were closer to each other at 250 RPM, where windage is lower.  I'll make a cover to hide the screws from windage.

Best,

Alan

[Atomic]

Update on two 2.25" tops:

As Iacopo has warned us, this requires very slight movement of the pivot.  I secured the pivot (0.5" steel ball) with three 2-56 screws.  After a long learning process I achieved excellent balance as evidenced by smooth sleeping and an abrupt transition from sleeping to topple with no intervening wobble.

Wow, those are really nice tops and love the way you have the set screws to adjust the pivot point. Must be a nightmare to get the balance right in the first place. Still, really nice bit of engineering.

**Edit. PS: Even with so many amazing ornamental tops on the market, this top with it's craftsmanship and movable ball bearing is something that far outdoes many of the tops I have seen. The only thing I would change is to use a ceramic silicone nitride ball thus reducing the weight at the center and possibly reducing the topple speed even more.

[Jeremy McCreary]

But the odd thing is that prior to balancing, the rounded top toppled at 173 RPM after a period of ugly wobble.  When balanced it topples suddenly at 210 RPM.  My prior balancing work had resulting in both eliminating the wobble and lowering the topple RPM.

The balance mechanism elevated the Cg from about 0.57" to 0.64", but that should not be sufficient to account for the substantial increase in topple RPM.  However I'll sink the balance assembly to restore the prior Cg height and repeat the tests.

Beautiful tops, interesting data, Alan! I think you're right to repeat the tests at the previous Cg height. Cg height has a dramatic effect on w_min, the minimum speed for stable sleeping or steady precession, and you've increased Cg height here by 12%. The equation for w_min² includes important terms with the Cg height cubed.

[Aerobie]

Update:

I sunk the ball cup deeper into the wheel, thus reducing Cg height and the topple speed came down as it should.  Interestingly I had one run where the top slowed to a substantially lower speed before it toppled.  I think it might have found a slight depression in my mirror or sweet spot on the very thin film of oil.

I also tried changing from a 1/2" ball to a 3/8" ball.  To my great surprise the decay rate was about the same.  But with the smaller ball, the top tends to circle more after launch and takes longer to stand up and sleep.

I also made a smooth shroud which covers my three screws.  The decay rate surprisingly increased very slightly!  I think some aero shear between the 1.27" diameter shroud and the base is contributing to decay.  The air gap from the base to the shroud is 0.11".

Alan