View Full Version : Ball dimensional variability
We just wanted to pass on a bit of news.
Last night, as we began prototyping shooting and pickup mechanisms, we realized that we had been accepting the specs and CD for the dimensions of the ball. As a double check, we actually measured our ball. We did this by placing it against a wall, with a vertical surface touching the other side of the ball, measuring its diameter caliper style.
The first thing we noticed is that after 4 days the ball is no longer round. Rather, it is "lobed". Rotating the ball to measure the max diameter across the lobes gives a max dimension of 25.75", a good deal larger than the ~24" specified in the game manual.
We wanted to get this out there to make teams aware that if they design mechanisms for a 24" diameter ball, the actual balls may become jammed.
Anupam Goli
08-01-2014, 09:11
Todd,
How much of wear (if any) does the ball have, and how much of use has it gone through? We'll have to check our own ball's dimension and see if it's matching the change in shape that you seem to be getting. Perhaps the colder temperature is affecting the air pressure of your ball?
wilsonmw04
08-01-2014, 09:15
Todd,
How much of wear (if any) does the ball have, and how much of use has it gone through? We'll have to check our own ball's dimension and see if it's matching the change in shape that you seem to be getting. Perhaps the colder temperature is affecting the air pressure of your ball?
If the temp were the case, it would make it smaller not larger.
My guess is that it has more to do with the cover than the ball. Variability is going to be something we are going to have to deal with. 1.5" is not that big of a concern. It's better than the basketballs in Rebound Rumble.
Very little wear, and very little use. We've tossed it around and bounced it by hand a bit. We also took it outside to test some crude shooter schemes (see-saw using a 2x4). After a few minutes in the cold (~20 deg F), it had shrunk noticeably, with the cover becoming wrinkled. Bringing it back inside restored the ball to size.
Rishabhgadi
08-01-2014, 09:35
If the temp were the case, it would make it smaller not larger.
My guess is that it has more to do with the cover than the ball. Variability is going to be something we are going to have to deal with. 1.5" is not that big of a concern. It's better than the basketballs in Rebound Rumble.
After the experiance with the Basketballs I beleive everyone is prepared for this.
MechEng83
08-01-2014, 09:40
After the experiance with the Basketballs I beleive everyone is prepared for this.
Except for all the rookies from 2013 and 2014, whether it be new teams, or new students/mentors.
Anupam Goli
08-01-2014, 09:46
If the temp were the case, it would make it smaller not larger.
Whoops, got it backwards. Sorry about that!
My guess is that it has more to do with the cover than the ball. Variability is going to be something we are going to have to deal with. 1.5" is not that big of a concern. It's better than the basketballs in Rebound Rumble.
Very little wear, and very little use. We've tossed it around and bounced it by hand a bit. We also took it outside to test some crude shooter schemes (see-saw using a 2x4). After a few minutes in the cold (~20 deg F), it had shrunk noticeably, with the cover becoming wrinkled. Bringing it back inside restored the ball to size.
I think wilsonmw04 is correct in suspecting that the change in ball shape may be due to the cover of the ball and the shrinking/expanding it went through. As the ball's volume changes, the cover gets more worn and stretched.
RaxusPrime
08-01-2014, 11:51
Do you think it might be a major factor during competition then?
wilsonmw04
08-01-2014, 13:03
I don't think so, unless you are designing something that needs to be snug to the ball. Since is gives a bit when force is applied, I don't see it being an issue.
Do we happen to have a mass on this ball? Can't be much more than a pound and a half, right?
M.O'Reilly
08-01-2014, 18:28
I am surprised that your ball is deformed, and I surmise it was manufactured that way. The only way I could imagine that the ball would get deformed is from someone sitting on it. Again, normal exercise balls are meant to bear weight, so my guess is it was manufactured this way, and we can expect to have to deal with heterogeneity at competitions.
Henrique Schmit
08-01-2014, 18:48
Do we happen to have a mass on this ball? Can't be much more than a pound and a half, right?
I've heard that one of our mentors calculated the ball's mass as more than 1kg, i think it was 1,3kg.
I've heard that one of our mentors calculated the ball's mass as more than 1kg, i think it was 1,3kg.
So that works out to 2.86 pounds. Thanks!
TheOtherGuy
08-01-2014, 19:01
I've heard that one of our mentors calculated the ball's mass as more than 1kg, i think it was 1,3kg.
Does this include the mass of the air when filled?
JamesCH95
08-01-2014, 19:45
Our ball measured 25in +/- 3 in as measured over several different diameters.
The fact that yours is 25.75in OD is somewhat disconcerting...
mman1506
08-01-2014, 20:20
The bladders are actually supposed to be 25 inches in diameter. That could explain why they tend to be on the large side. http://www.sportogo.com/2565cmbladder.aspx
gotta think that inflation pressure is going to have some effect here, too.
how uniform will *that* be at comps?
gotta think that inflation pressure is going to have some effect here, too.
how uniform will *that* be at comps?
At comp, they might do the same as what they did in 2011 and have an inflation box that they use that is roughly the volume the ball is supposed to be.
wilsonmw04
08-01-2014, 21:38
At comp, they might do the same as what they did in 2011 and have an inflation box that they use that is roughly the volume the ball is supposed to be.
psst...
http://www.youtube.com/watch?v=Azc8AKzgEwE
JamesCH95
09-01-2014, 06:58
At comp, they might do the same as what they did in 2011 and have an inflation box that they use that is roughly the volume the ball is supposed to be.
If you noticed during 2011 some (many?) field workers just pumped up the tubes as hard as they could, shoved the sizing gauge over the tube and went "hey, it fits!" and put them out on the field grossly over-sized. In 2007 the exact opposite was true, tubes were generally grossly under-inflated.
No matter what the regulation is we better count on someone who does not appreciate the implications of their seemingly trivial volunteer job at an event. Balls could be very over-inflated, under-inflated, or both. Plan accordingly.
The bladders are actually supposed to be 25 inches in diameter. That could explain why they tend to be on the large side. http://www.sportogo.com/2565cmbladder.aspx
We called Sportogo and confirmed that this is the piece used in the official FIRST balls. Because we already popped ours and needed a replacement!
We followed this inflation guide:
http://www.usfirst.org/sites/default/files/uploadedImages/Robotics_Programs/FRC/Game_and_Season__Info/2014/2014_BALL_Inflation_and_Maintenance_Guide.pdf
We measured the circumference. Independent measurements were made by three students. The average diameter of eight measurements was 24.7 inches.
We got burned by larger tournament tubes than we had designed and practiced with in 2011 so we'll make sure that we take ball diameter variance into account this year.
AdamHeard
09-01-2014, 16:27
We followed this inflation guide:
http://www.usfirst.org/sites/default/files/uploadedImages/Robotics_Programs/FRC/Game_and_Season__Info/2014/2014_BALL_Inflation_and_Maintenance_Guide.pdf
We measured the circumference. Independent measurements were made by three students. The average diameter of eight measurements was 24.7 inches.
We got burned by larger tournament tubes than we had designed and practiced with in 2011 so we'll make sure that we take ball diameter variance into account this year.
That guide is junk, it essentially is a spec based on feelings and opinion and not numbers.
Jared Russell
09-01-2014, 16:29
That guide is junk, it essentially is a spec based on feelings and opinion and not numbers.
Essentially, "Inflate the balls until they are properly inflated."
Alan Anderson
09-01-2014, 16:49
Essentially, "Inflate the balls until they are properly inflated."
It's not quite as tautological as you make it sound. Using the zipper as a tightness gauge makes the subjective interpretation of "properly inflated" a narrower range of possibilities than it might otherwise be.
Joe Ross
09-01-2014, 17:04
That guide is junk, it essentially is a spec based on feelings and opinion and not numbers.
I thought it was fine. It says to design for several inches of variability.
waialua359
09-01-2014, 17:14
I would assume that ball size variability would affect ball intake designs the most.
Take it out of the equation.:)
waialua359
09-01-2014, 17:15
Essentially, "Inflate the balls until they are properly inflated."
Did I just see your username correctly? DaisyPoof? Team 254? haha...
AdamHeard
09-01-2014, 18:09
Did I just see your username correctly? DaisyPoof? Team 254? haha...
DaisyPoof sounds like what EJ would name the robot if the rest of the team would let him.
DampRobot
09-01-2014, 21:40
DaisyPoof sounds like what EJ would name the robot if the rest of the team would let him.
Who started the Decepticon thing then?
G Fawkes
13-01-2014, 15:03
Can more teams please post their measured ball diameter?
We measured ours with what we felt was robust fill and got 25". We also noticed it shrinks ~0.25" over a few hours.
mblake31
15-01-2014, 06:28
We measured our ball shortly after inflating to what we thought was a "normal" inflation, and got 24".
It's since been left overinflated for about a week, and after filling it until the zipper just closes, we measured 25.75". Unlike you guys in the States, though, we've had a week of 90 degree days, so possibly that's a factor.
Jared Russell
15-01-2014, 10:25
FWIW: We cannot zip our ball shut if blown up above 26" diameter. It seems underinflated below 25" diameter.
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