Poof Ball Dynamics

[Stephen Kowski]

Originally Posted by John Neun

Our contact point is very short, so the ball has to be accelerated very quickly.

would the gains that you would recieve by elongating the contact point be worth the effort of setting it up so that you would have a overall greater velocity? (i.e. some sort of pulleys/belt system)

Or is the small contact point more than enough to accelerate the ball to an ideal velocity?

[Andy A.]

95 measured the dimensions of 48 balls. While I do not have the raw data, there was one interesting note.

On average, the balls are slightly oblong. In one axis, their diameter is about a 1/16th smaller. I believe this corresponds to the injection spot. The other two axis where on average within a 64th or two of each other.

There where a few outliers, perhaps 4% to 6% that had at least one dimension up to an 8th off from the other two. They where just visibly oblong. We noted one ball that was much stiffer and harder to compress then the others. Many balls where very soft, which likely had much to do with the outer skin being torn on arrival. We noted at on a scale of 1 to 5, with 1 being near perfect and 5 being trashed, the 48 balls we have averaged around a 2. Most defects occurred around the balls equator, and consisted of torn skin. A few voids or dent's were noted.

We had one ball that consistently preformed poorly in our shooting tests. We had already numbered the balls and #32 was widely unpredictable. Of roughly 1 dozen shots fired where over %80 of other balls scored, it failed to score once. It either would fire much faster or much slower then the other balls. All the other balls where quite consistent. I examined #32, but couldn't come up with any explanation. With out dissecting it, it seemed identical to every other ball in our collection.

I experimented on one ball quickly by stabbing it perhaps 2 dozen times randomly around it's surface with my knife. I found that it quickly became softer, as the air in the ball escaped much faster. Competition balls will probably follow a similar route as the skin is ripped off.

So, overall the balls are pretty consistent out of the box. They are on average a little oblong and will soften up over time. If our collection is Representative, then it's possible that a very small percentage (.5%) of balls are demonicly possessed and simply will not go in the goal.

I'll see about getting the .xls of the measurement data and methods posted. Right now it is stuck on a Windows ME machine that doesn't recognize USB hard drives.
-Andy A.

[Bill Moore]

Originally Posted by Andy A.

95 measured the dimensions of 48 balls. While I do not have the raw data, there was one interesting note.

On average, the balls are slightly oblong. In one axis, their diameter is about a 1/16th smaller. I believe this corresponds to the injection spot. The other two axis where on average within a 64th or two of each other.

There where a few outliers, perhaps 4% to 6% that had at least one dimension up to an 8th off from the other two. They where just visibly oblong. We noted one ball that was much stiffer and harder to compress then the others. Many balls where very soft, which likely had much to do with the outer skin being torn on arrival. We noted at on a scale of 1 to 5, with 1 being near perfect and 5 being trashed, the 48 balls we have averaged around a 2. Most defects occurred around the balls equator, and consisted of torn skin. A few voids or dent's were noted.

We had one ball that consistently preformed poorly in our shooting tests. We had already numbered the balls and #32 was widely unpredictable. Of roughly 1 dozen shots fired where over %80 of other balls scored, it failed to score once. It either would fire much faster or much slower then the other balls. All the other balls where quite consistent. I examined #32, but couldn't come up with any explanation. With out dissecting it, it seemed identical to every other ball in our collection.

I experimented on one ball quickly by stabbing it perhaps 2 dozen times randomly around it's surface with my knife. I found that it quickly became softer, as the air in the ball escaped much faster. Competition balls will probably follow a similar route as the skin is ripped off.

So, overall the balls are pretty consistent out of the box. They are on average a little oblong and will soften up over time. If our collection is Representative, then it's possible that a very small percentage (.5%) of balls are demonicly possessed and simply will not go in the goal.

I'll see about getting the .xls of the measurement data and methods posted. Right now it is stuck on a Windows ME machine that doesn't recognize USB hard drives.
-Andy A.

Great work Team 95! Thanks for sharing it!

Andy, any possibility that the center of gravity for Ball #32 is significantly offset (moreso than the other balls)? Kind of like one of those "trick" baseballs that you can't throw straight?

[MikeDubreuil]

Our team conducted an environmental test on the ball. We measured the diameter of the ball at ambient temperature and then at 40 degrees Celsius, 95% relative humidity. Testing showed that at the high temp/high humidity condition, the weight of the ball increased from 183.5g to 188.8g and the circumference increased from 21 5/8" to 22".

[RogerR]

Originally Posted by Andy A.

95 measured the dimensions of 48 balls. While I do not have the raw data, there was one interesting note.

On average, the balls are slightly oblong. In one axis, their diameter is about a 1/16th smaller. I believe this corresponds to the injection spot. The other two axis where on average within a 64th or two of each other.

There where a few outliers, perhaps 4% to 6% that had at least one dimension up to an 8th off from the other two. They where just visibly oblong. We noted one ball that was much stiffer and harder to compress then the others. Many balls where very soft, which likely had much to do with the outer skin being torn on arrival. We noted at on a scale of 1 to 5, with 1 being near perfect and 5 being trashed, the 48 balls we have averaged around a 2. Most defects occurred around the balls equator, and consisted of torn skin. A few voids or dent's were noted.

We had one ball that consistently preformed poorly in our shooting tests. We had already numbered the balls and #32 was widely unpredictable. Of roughly 1 dozen shots fired where over %80 of other balls scored, it failed to score once. It either would fire much faster or much slower then the other balls. All the other balls where quite consistent. I examined #32, but couldn't come up with any explanation. With out dissecting it, it seemed identical to every other ball in our collection.

I experimented on one ball quickly by stabbing it perhaps 2 dozen times randomly around it's surface with my knife. I found that it quickly became softer, as the air in the ball escaped much faster. Competition balls will probably follow a similar route as the skin is ripped off.

So, overall the balls are pretty consistent out of the box. They are on average a little oblong and will soften up over time. If our collection is Representative, then it's possible that a very small percentage (.5%) of balls are demonicly possessed and simply will not go in the goal.

I'll see about getting the .xls of the measurement data and methods posted. Right now it is stuck on a Windows ME machine that doesn't recognize USB hard drives.
-Andy A.

could the air trapped in the ball explain the the behavior of the poof-balls under impulse loads? and if so, do you think that the difference between a new and 'compromised' (i.e., missing chunks of foam and skin) poof ball might be enough that their behavior under impulse loads might change (i.e., compromised poofs won't regain as much energy as was invested in the compression by the rollers, and therefore won't shoot as far or as fast?)?

[Andy A.]

I can only hazard guesses at all the physics behind the poofs behavior when varying how compressible they are in a roller system and in flight. I can say that the ball I had stabbed and rendered squishy didn't seem to behave much differently. admittedly, our testing was using our '02 bot (you may remember it, the popcorn popper). With some modification, it was found that the roller system used to suck up soccer balls happened to shoot the poofs pretty well to. The poofs were not being compressed a great deal, but it seems they don't have to be.

From what I saw, no, squishy poofs fly about as fast as hard poofs. This may be highly dependent on how they are fired- the method used involved a track of rollers about 8 inches long, so the poofs had plenty of time to accelerate. As you said, impulse loads and sudden accelerations that a single 'tennis' ball shooter might impart could affect the poofs very differently. I've played with a single wheel shooter, but only with a single ball, so I have no good reference for that type.

I guess the only way to know is to put one of your poofs to the knife and see what happens. My gut feeling is that the longer the acceleration is, the more consistent the balls will behave. Short of mocking up all sorts of shooting mechanisms, my gut is about as far as I can go with this. John has done it, and figured out it doesn't matter. Go figure. Like I said, this is complicated!

I think FIRST may be inspiring the most accurate and largest knowledge base on the proper handling and hurling of squishy Nerf type balls. Heck, we've already got kickballs down pat.

-Andy A.

Edit- As to #32's misbehavior- I hadn't thought about the CG being a factor, but it makes some sense. I suppose it's possible that there is a dense spot in the foam. My best guess had been that the skin was somehow not homogenus and perhaps one color was grippier then the other, or perhaps the dense spot near the injection nipple is larger or harder. I really don't know. I don't have access to the balls for a while, but another member of 95 may be able to take a closer look.

[Rick TYler]

Originally Posted by Andy A.

Edit- As to #32's misbehavior- I hadn't thought about the CG being a factor, but it makes some sense.

It's obvious, a disgruntled worker at the Poof Slinky(r) factory tossed a rat into the plastic molding machine. I bet if you cut Old 32 apart, you would find the modern equivalent of a Dutch sabot in the works.

I am interested if anyone is considering a multi stage acceleration, either by using multiple wheels or some kind of catapult launcher? Also is anyone thinking about adding a spin to the ball by either a rifled barrel or having paired wheels at a slight skew?

[BrianR]

[quote=Dr.BotAlso is anyone thinking about adding a spin to the ball by either a rifled barrel or having paired wheels at a slight skew?[/QUOTE]

We are considering putting spin of the ball, but it would not be due to skew, as it can be done in multiple ways. We are thinking that we could run two wheels, one above the other, with the top going slower, providing backspin, or simply having a plate above one wheel, which also provides backspin. Having the wheels aligned on the same axis allows them to be controlled off of the same motor, which saves weight and complexity.

[Eric Scheuing]

I'm not sure if you said this or not, and if you did, I probably missed it somehow, but do the balls stay slightly compressed permanently?

[John Neun]

Originally Posted by Stephen Kowski

would the gains that you would recieve by elongating the contact point be worth the effort of setting it up so that you would have a overall greater velocity? (i.e. some sort of pulleys/belt system)

Or is the small contact point more than enough to accelerate the ball to an ideal velocity?

Hi Steve,

I don't think a long contact point is a good idea, unless you have to move the balls. We have found that the launch seems to be most efficient with a "point" loading of the balls. Again, we are using a single wheel to accelerate the ball, and a plate to "aim" it. I believe that the longer contact point will just waste energy. Our testing seems to corroborate this, as regards the ultimate range of the shooter. The whole compression, viscoelasticity thing kind of came up just for this reason. I might have thought that a long, gradual acceleration with minimal ball compression (because less normal force is required for friction) would be more efficient than a short, highly compressed shooting zone. Not so, surprisingly to me.