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Author Topic: Weight barycentre  (Read 5942 times)

MrYo

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Weight barycentre
« on: August 31, 2009, 03:50:17 PM »

Where is better the weight barycentre for a top? My first one is near the middle of the height of the top, but i fear this will slow down the changes of plane, is better to move it more up?

Maurizio
« Last Edit: October 23, 2009, 10:52:39 PM by ta0 »
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ta0

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Re: Weight barycentre
« Reply #1 on: August 31, 2009, 11:13:46 PM »

All the commercial tops have the center of mass considerably higher (farther away from the tip) than the geometrical center (when the tip is included).

The Throwback and the Gates are very stable tops so I was curious to find their centers of mass. I did it by hanging them from a thin (sewing) string taped to the body (the tape to one side to let the string pivot freely.) By trial an error one I found the position where the axis remains horizontal.


For the Throwback (bearing tip) I got the center of mass at a height of 42 mm (where the yoyofactory lettering is), while its total height is 66 mm. For the Gates the values are 47 mm and 73 mm.
« Last Edit: August 31, 2009, 11:15:37 PM by ta0 »
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MrYo

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Re: Weight barycentre
« Reply #2 on: September 01, 2009, 06:39:50 AM »

Tnx, i will try to find a similar center of mass, this will be near simple due to the closing system I want to try, but will be easy moving a bit the center of mass using an o-ring or a metal ring in the lower part of the body or the closing cap. Now i must modify the original cad project with this objective, finding a base weight that can be rised in order to change the center of mass.

Maurizio
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poptop

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COM
« Reply #3 on: September 01, 2009, 02:48:18 PM »

The center of mass has a little something to do with inertia, true? How about momment of inertia?  It seems like we were discussing this a while back when we were measuring various tops. 
I need to update my file as I've collected a few more tops since then, but I got a bunch of measurements in excel.  I was looking for measurements, ratios, etc. that "good tops" had in common.  I think we've got a pretty good data set to mess with.  I'll try to spend some time updating.
« Last Edit: October 28, 2009, 12:39:33 AM by poptop »
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Erratic Wobbler

MrYo

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Re: Weight barycentre
« Reply #4 on: September 02, 2009, 06:35:23 AM »

ta0, i made the same test with my top and, with a little surprise i discovered that the center of mass is very similar to the throwback, between 60-65% of the total height of the top.

poptop, your work is very interesting for me, I'm not a good player (for now i hope) but i like a lot to make yoyo, tops, modding ecc so any help is welcome for me :-)

Maurizio
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ta0

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Re: Weight barycentre
« Reply #5 on: September 02, 2009, 09:53:30 AM »

Oh surprise . . . I found that the height of the center of mass of the Acrobat is at 64% (46/72), almost the same as Throwback (62%). But more surprisingly, for a BK I got a lower one: 60% (47/78)!

I guess this means that the height of the center of mass with respect to the geometrical center is not a good indication of its stability. Considering that the shape of the top changes with height and also its material distribution (weight rings, etc.) it is not that surprising in hindsight. It probably has to do more with the ratio between the moments of inertia around the spintop axis and a perpendicular axis. Crabtree anybody?
« Last Edit: September 02, 2009, 10:07:38 AM by ta0 »
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SpinQueen

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Re: Weight barycentre
« Reply #6 on: September 02, 2009, 11:08:00 AM »

Ta0, now please do all that same kind of figuring for a double tipped top and then re design it to play like a tippy top!   Then I can boomerang it...watch it flip over in my hand, and then wire walk to my other hand ;)
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poptop

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Re: Weight barycentre
« Reply #7 on: September 04, 2009, 03:37:33 PM »

OK, so why is the Acrobat easier to lean over compared to the Throwback?  More weight concentrated at the shoulder?  How does this translate to moment of inertia speak? I'm a little dim, sorry...
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killswitch

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Re: Weight barycentre
« Reply #8 on: September 06, 2009, 03:07:36 PM »

just got throwback and acrobat. and wow i cant really throw throwback right horizontaly. ugh
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ta0

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Re: Weight barycentre
« Reply #9 on: September 06, 2009, 03:51:04 PM »

Have you shorten the string? Both tops come with strings way too long, specially the Throwback. You need to cut like 30 cm (1 ft) at least. I use 145 cm (47") on my Throwback.
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Pulpowsky

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Re: Weight barycentre
« Reply #10 on: September 06, 2009, 04:39:20 PM »

Before to cut a high quality string, do some knots to short the string.

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killswitch

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Re: Weight barycentre
« Reply #11 on: September 08, 2009, 07:10:25 AM »

yeah i did that first the knots then i used the short string from ripcord :D and even that has to be shorten a bit. but even with that long it was possible but the throwback has to be hold very awkward to rotate to right position :)

i was wondering if it is ok to have that short string.
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Pulpowsky

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Re: Weight barycentre
« Reply #12 on: July 06, 2010, 01:14:19 AM »

All the commercial tops have the center of mass considerably higher (farther away from the tip) than the geometrical center (when the tip is included).

The Throwback and the Gates are very stable tops so I was curious to find their centers of mass. I did it by hanging them from a thin (sewing) string taped to the body (the tape to one side to let the string pivot freely.) By trial an error one I found the position where the axis remains horizontal.


For the Throwback (bearing tip) I got the center of mass at a height of 42 mm (where the yoyofactory lettering is), while its total height is 66 mm. For the Gates the values are 47 mm and 73 mm.

Ta0, could you explain in detail how you do that?
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ta0

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Re: Weight barycentre
« Reply #13 on: July 06, 2010, 08:30:22 AM »

Quote
Ta0, could you explain in detail how you do that?
What I did was very simple. I used a piece of sewing thread to hang the top. If the crown of the top rises, I move the string closer to the tip and if the tip rises I move it closer to the crown. When the top stays horizontal the string is attached at the height of the center of mass.

I wanted to make sure that the top could rotate freely so the string comes from under the edge of the tape that is parallel to the top axis..

(wow,  it was difficult to right that:  my head can only think about the world cup semifinal right now.  ;) )
« Last Edit: July 06, 2010, 08:34:11 AM by ta0 »
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