View Full Version : Best Way to Shoot the Balls??
Hey everyone! I would like to wish everyone luck in the upcoming competition! I think it will prove to be an exciting and fun new challenge.
My question envolves the method of getting the ball to the tall goal. After they showed the softball thrower during the kickoff, that was my best idea! Any other ideas out there that you want to share? its seems to me that anything pneumatic or spring loaded would take way too long to shoot, considering the number of balls on the field. Thanks a bunch and good luck in the upcoming competition!
Ben
Team 281 - ex student
|20807 61|2|_
07-01-2006, 23:33
im thinking of a hot wheels sort of thing where u pick the balls up they go down into a basket or some sort of holding device and then shoot up between 2 wheels kinda the same thing but w/e
Chuck Glick
07-01-2006, 23:34
Hey everyone! I would like to wish everyone luck in the upcoming competition! I think it will prove to be an exciting and fun new challenge.
My question envolves the method of getting the ball to the tall goal. After they showed the softball thrower during the kickoff, that was my best idea! Any other ideas out there that you want to share? its seems to me that anything pneumatic or spring loaded would take way too long to shoot, considering the number of balls on the field. Thanks a bunch and good luck in the upcoming competition!
Ben
Team 281 - ex student
softball type thing is way to go... i mean you can keep feedin' um in and you can shoot a bunch at once. pneumatic and spring would take too long... i'd only do it that way if thats you secondary "weapon" or if your attempting to shoot from a distance ACCURATELY. i think only a handfull for teams are gonna go for that center goal too and theres gonna be a lot of defence and "battlebots" type action, except in a controled manner.
I find that really interesting you don't think many will go for the center goal. i was honestly really usure about that myself. I mean, you only have to get 1 out of 3 to make it worth your while, but it will prove to be very challenging, no doubt there. thanks for the input
I'mwithstupid^
07-01-2006, 23:54
I think that it would be easy to do a pneumatic system that would be fast. You just use a double solenoid and a hopper (like a paint ball gun) and the ball falls down into chamber and than is shoved out by a plate on the end of a pnumatic system, than automatically the cylinder pulls back and another falls in and repeats, and if this isnt fast enough for you, these aren't complex shooting machines which means you could build two of them fairly easily.
you can have alternating firing mechanisms, so that one fires while the other reloads. Also you could use a pulley system to shorten the time of retraction for your springs. If you've got any corrections plz feel free
KenWittlief
07-01-2006, 23:55
the laws of physics say that any way you use to transfer energy to your shooter is going to take some time to get the necessary energy transferred to it for the first shot, and then to spool-up again for the repeated shots
something to keep in mind. Its easy to see with something like pneumatics, where the pressure has to be resupplied, and less obvious with something like a baseball (wheel based) launcher. It will slow down after a ball is fired, and will take a specific amount of time to get back up to launch-speed.
Lil' Lavery
08-01-2006, 00:46
a trebuchet would totally win the cool award
Im trying to think of an effective way to rifle the ball in a mechanism similar to the softball laucher.
there are also many hybrid ideas to the softball laucher, including tubes with belts in them, multi-ball lauchers, "gatling guns" etc etc etc
sciguy125
08-01-2006, 02:01
Im trying to think of an effective way to rifle the ball in a mechanism similar to the softball laucher.
there are also many hybrid ideas to the softball laucher, including tubes with belts in them, multi-ball lauchers, "gatling guns" etc etc etc
I was thinking about rifling also. The main problem with putting groves in the barrel is that it would either cut up the ball or simply slow it down too much. I was thinking about adding a set of rotational wheels. If there's a set of wheels mounted perpendicular to the "launching" wheels, it could spin the ball. You'd probably have to use a set of omniwheels though. You'd also have to make sure that the barrel is relatively low-friction.
For a little bit today, we were considering a gatling gun style device. Had it not been discounted for impracticality, it would have been pretty cool.
I think that it would be easy to do a pneumatic system that would be fast. You just use a double solenoid and a hopper (like a paint ball gun) and the ball falls down into chamber and than is shoved out by a plate on the end of a pnumatic system, than automatically the cylinder pulls back and another falls in and repeats, and if this isnt fast enough for you, these aren't complex shooting machines which means you could build two of them fairly easily.
you can have alternating firing mechanisms, so that one fires while the other reloads. Also you could use a pulley system to shorten the time of retraction for your springs. If you've got any corrections plz feel free
Pneumatics won't fire at fast enough speeds. On paintball gunns the piston pulls back the bolt and pressurized gas shoots the ball.
I can't wait to see people practicing in the pits. LOL
Andrew Blair
08-01-2006, 08:20
Im trying to think of an effective way to rifle the ball in a mechanism similar to the softball laucher.
I'm not sure that you need to introduce that kind of axial spin to achieve greater accuracy. I would imagine it would be best, but very difficult. Air Soft guns use a system called Hop Up, where spin is put onto the pellet to increase accuracy and range. I do not believe that it gives spin in the direction rifling would, but rather, in a axis perpendicular to the direction of travel.
techtiger1
08-01-2006, 09:30
Ben good thread. My team after yestersday has a pretty good idea of what where going to be doing as far as the robot, however the shooting mechanism is something that was really up in the air yesterday as far as our ideas well a few arised but this one I think was the one I liked best. This idea uses the softball machine analogy, basically we were talking about using the four kit wheels with two motors and some how feeding them through the two sets of kit wheels. There are a few problems off the top of my head that I can think of with this, but just thowing an idea out there. I love the game it rocks, Shooting and shooting accurately is going to be a process to figure out.
Good luck to all the teams,
Drew
mm...no matter what the design i think the ball will need to have spin on it. if not, the ball may vibrate to a stall in a barrel. its a lot easier to get a metal bullet to slide through a gun, than a rubber ball. spin should keep the ball more stable and flying straighter.
Michael Corsetto
08-01-2006, 14:51
Just a random thought about this whole barrel getting the ball to spin discussion. Is a barrel really necessary? If you look at a pitching machine, once the ball goes through the one or two wheels that propel it foreward, it doesn't go through anything else. I understand that a barrel would perhaps put a corkscrew spin on the ball, thus making it more accurate, but when the wheel(s) make contact with the ball, wouldn't it negate any spin that was there before? And if you are left with just the spin that the wheel(s) produce on the ball, isn't that a constant spin that you can factor in when you aim you shooter? That's just my two cents.
Michael C.
Ian Curtis
08-01-2006, 15:14
Just a random thought about this whole barrel getting the ball to spin discussion. Is a barrel really necessary? If you look at a pitching machine, once the ball goes through the one or two wheels that propel it foreward, it doesn't go through anything else. I understand that a barrel would perhaps put a corkscrew spin on the ball, thus making it more accurate, but when the wheel(s) make contact with the ball, wouldn't it negate any spin that was there before? And if you are left with just the spin that the wheel(s) produce on the ball, isn't that a constant spin that you can factor in when you aim you shooter? That's just my two cents.
Michael C.
That might be true shooting heavy small objects, but we are launching large light objects. With the spin the ball expeiriences being launched, and with the lightness of the ball, those balls will be going every which way immediately after they go through the launcher. The game will come down to who can shoot their balls with the highest accuracy percentage.
Seekthematrix
08-01-2006, 16:05
well they give us those super cims(thats wat our team are calling them) anyone actually know the names of those motors. Those things could probley power a scooter. But a double belt system working on one of the motors or a one belt up inside a large barrel it would be fast accurate and easy to feed, with very little fear of jamming.
If you do a two-wheeled shooter (or two belt or whatnot), you could run the wheels at different speeds to put a backspin on the ball. Not all spin has to be like a spiraled football.
Making sure you get your wheels back up to speed before putting another ball it will be huge. High RPM and low torque wheels will slow down.
Who's taking a field trip to a batting cage?
Barry Bonzack
08-01-2006, 17:38
I mean, you only have to get 1 out of 3 to make it worth your while, but it will prove to be very challenging, no doubt there. thanks for the input
Thats only assuming you can make one of three in the same amount of time it takes another robot to get three one point goals. Speed scoring and defense will be a factor as well as accuracy.
DonRotolo
08-01-2006, 17:54
Im trying to think of an effective way to rifle the ball in a mechanism similar to the softball laucher.
Simple: Use two wheels (not just one like the softball launcher), but put them at a slight angle to the direction of travel - for example, the top one is canted 15 degrees to the left (CCW) and the bottom one 15 to the right (also CCW). This is how football launchers work.
With the spin the ball expeiriences being launched, and with the lightness of the ball, those balls will be going every which way immediately after they go through the launcher.
I disagree, and I see no need for a 'barrel'. In a protoype I built today (see photo), the balls went very very repeatably to the same spot. The biggest variable will be the supply voltage (= motor speed), but the astute student will figure out how to compensate for that ;-)
I think that it would be easy to do a pneumatic system that would be fast.
Umm, using what energy source? A tennis ball launcher uses what is essentially a vacuum cleaner, blowing instead of pulling, and shoots the ball out of a 'barrel'. Longer barrels shoot further and flatter, and a screw-thingie on one side imparts spin. But, a 7 ball needs a LOT of air volume. Do the math with a 2' long barrel - what is the volume of the barrel in cubic feet, and how many cubic feet per minute at (60?) PSI can that KOP compressor supply? That gives you your best firing rate, BUT there is no place to store that volume of air at 60 PSI on a legal robot.
The way I see it, the only viable options are wheel-based and spring-based launchers. (A trebuchet is spring-based [EDIT: No, it is powered by a falling mass[/EDIT], and I agree, it would be soooo cool).
In the photo, that is an 8" wheel. At about 1300 RPM it launches a 7" soccer ball about 10 feet (from a table 40" from the floor), and in ten tries they all landed within an inch of each other.
The biggest issues are balance of the wheel - it wobbles a lot at 1300 RPM, it's not made for that - and the huge black marks the rubber leaves on the ball.
Don
Kevin Sevcik
08-01-2006, 18:29
Thats only assuming you can make one of three in the same amount of time it takes another robot to get three one point goals. Speed scoring and defense will be a factor as well as accuracy.
Am I the only person thinking about ball availability? If you're on offense, and the opposing team is on defense, they have no reason to put balls in play where you can get them after you score them. So if you have 40 balls and score them all as one pointers, you get 40 points and have to wait until they give you your balls back. If you make 3's, you get 3 points. the ones you miss are still available to you if you have a ball scoop, so if you start with 40 balls you could get a max of 120 points before you've run out of balls.
FreakyAntelope
08-01-2006, 18:43
The way I see it, the only viable options are wheel-based and spring-based launchers. (A trebuchet is spring-based, and I agree, it would be soooo cool).
I hate to call you on a technicality, but this is a bet of a pet peeve. So, for the enlightenment of all,
A trebuchet is *not* spring powered. A trebuchet derives it's energy from a falling mass. Remember the really cool looking things that threw the giant rocks from Minas Tirith in the Return of the King? Those were Trebuchets.
If it uses elastically derived energy it is another kind of catapult. So, if you use a rubber band, spring, or the bowing of wood (or something else) to throw the object it's a catapult.
To clarify, a trebuchet is a *kind* of catapult. One that uses a falling mass.
More info here (Trebuchets) (http://en.wikipedia.org/wiki/Trebuchet) and here Catapults (http://en.wikipedia.org/wiki/Catapult)
:rolleyes:
- Toby
haha, i was going to also mention this toby, but you got there first. but yes, a REAL trebuche would be totally awesome!
DonRotolo
08-01-2006, 21:19
A trebuchet is *not* spring powered. A trebuchet derives it's energy from a falling mass.
Yes, you're absolutely right and I stand corrected.
Don
greencactus3
08-01-2006, 22:59
to the guy saying a simple cylinder pushing a ball out... not even close to enough. i tried a 3/4in bore cylinder at 100psi pushing a tennis ball. didnt get far at all. AT ALL.
so. to increase speed use simple machines. pulleys. lever arms... i htink thats the best bet... wondering about 60psi tho.... wish they let us use higher pressure ones.and 2 tanks... not enough for multiple fires.
im gonna think about motors..
Rickertsen2
09-01-2006, 00:05
to the guy saying a simple cylinder pushing a ball out... not even close to enough. i tried a 3/4in bore cylinder at 100psi pushing a tennis ball. didnt get far at all. AT ALL.
so. to increase speed use simple machines. pulleys. lever arms... i htink thats the best bet... wondering about 60psi tho.... wish they let us use higher pressure ones.and 2 tanks... not enough for multiple fires.
im gonna think about motors..
We did some experimenting yesterday and built a number or prototypes. One of which was a pneumatic direct acting system that acheived a ball velocity of 8 m/s @60 psi. Its possible, but we have some tricks up our sleeves. ;) We have a much more powerful version planned for monday or tuesday once some parts arrive.
Arkorobotics
09-01-2006, 00:23
Are we allowed to create a chamber where the air compresses safely and the chamber is a cone with the ball that rolls back into it?
----------\--------|
~~~~~O ~~~~~~ <-- Air in
----------/--------|
------ ^------^
----- Ball ---Chamber
I like the idea with two spinning wheels, like a hot wheels launcher. The important thing is that if the wheels aren't spinning at the same speed, the balls will fly off course.
Just food for thought.
Jeremiah Johnson
09-01-2006, 01:16
I see no reason why you would need to shoot quickly if you have an accurate launching system. A barrel is probably necessary though. In my opinion, a pneumatic system would work best. There's less chance of unpredictable velocity... maybe. I donno though. I'm not the mechanical type. I just throw out ideas.
Michael Corsetto
09-01-2006, 02:25
I like the idea with two spinning wheels, like a hot wheels launcher. The important thing is that if the wheels aren't spinning at the same speed, the balls will fly off course.
Just food for thought.
Not necessarily. If you spin both wheels at the same speed, you end up with a ball without spin coming out of the shooter. And just like a knuckleball in baseball, the ball will zig zag in any and every direction. One of the solutions we came up with is to have the two wheel mounted one on top and one on bottom. You purposly gear down the top wheel a little more than the bottom one, which results in a slight backspin. This produces a shot that is very straight, with little to no sideways curve. If you go with one wheel instead of two, i would recommend mounting the wheel on the bottom, with a solid surface above it for the ball to pass through.
Both of these ideas are direct knock-offs of baseball and softball pitching machines, so google up some images if you need a visual reference
I personally would recommend going with the two wheels, just so that you can minimize spin. With a one wheel system, whatever spin you have, be it topspin or backspin, will always be directly related to the tangencial speed of the wheel. A two wheel system has the ability to greatly reduce this relation between spin and speed.
Michael C.
artdutra04
09-01-2006, 02:49
There are many more ways of quickly launching balls than it first appears. There are also a lot of "side effects" that people may experience from a particular idea. Some ideas will prove better than others, depending on the hardware and software behind them and how a particular team implements them. What works for one team might not work for another.
Earliar today, I had an ingenous idea on how to shoot balls accurately and quickly into the center goal. I already have it prototyped in Vex parts, and it launches the one raquetball every second about halfway across the room by [bleep] [bleep] [bleeeeeeeep] [bleepity] [bleep] and [bleeeeping] [bleepity] [bleep] [bleepment]. It's truly amazing. ;)
Cyber Punk 234
09-01-2006, 07:45
Truly Im not 100% sure but I cant wait to build.
Nitroxextreme
09-01-2006, 07:55
During our brainstorming session yesterday we came up with a few ideas as to how to shoot the ball.
However, there was one that we came up with that I just can't see working. We thought of using a spring, sorta like a pinball machine, and having a winch pull it back. I just see this being extremely slow, needing a long pull length and not being consistent unless we use a hookean spring.
yes, you are correct. a spring would require many mechanisms to get it to work, including a pull back and releasing mechanism, along with a way to load the balls. also springs do not scale up well. if we were shooting pinball balls it would be much more feasable.
Mark Pettit
09-01-2006, 15:33
I am no engineer and I certainly am not a ballistics expert, so correct me if I am wrong here. . . .
Rifiling works excellently to stabilize the flight of a cylindrical object like a bullet or missile but would it not cause a spherical object like a Poof-Slinky ball to curve? Somehow that doesn't sound like it would be more accurate to me.
Masterfork
10-01-2006, 21:49
I was thinking about rifling also. The main problem with putting groves in the barrel is that it would either cut up the ball or simply slow it down too much. I was thinking about adding a set of rotational wheels. If there's a set of wheels mounted perpendicular to the "launching" wheels, it could spin the ball. You'd probably have to use a set of omniwheels though. You'd also have to make sure that the barrel is relatively low-friction.
For a little bit today, we were considering a gatling gun style device. Had it not been discounted for impracticality, it would have been pretty cool.
LOL WELL WE LOOKED AT rifling but the thing is it is to complex but if u are trying to get a spin on the ball of that sort try doing what some paintball barrels do. which would be cut holes in a spiral sequences around the barral to let the air flow give it some spin, that was one way i could imagine doing it farely but rifling it by cutting or putting grooves would slow it down tramendously but that is what happens in a gun. i would also imagine that actual rifling would cut up the ball. also one way could be to sort of making a slight elavation in the inside surface of the barrel and spiral it to do the spinning. something I would also like to say is that it would prolly be a waste of supplies and time to try to rifle it becuase the rifling works so well with a real gun is because the bullet is actaully sealing the barrelfrom the charge so it can use the propulsion of the powder or gas made by it and grip the grooves of the barrel . which means applying it to this game would mean those balls would have to be a tight fit to grip the rifling or spin as well as a bullet.
and shooting balls with rifling well at least when i muzzle load with lead balls the rifling makes the balls rise(not curve it stays on a straight path it just rises at a distance) to a small extent at like 50 yds on a muzzleloader but very acurate still just keep that in mind if u found a way to impliment it because it might be affected in the same waybut if successful please tell me how because i would really like to see how that would work and how well it would too but i would not recommend the waste of time it might involve
Masterfork
10-01-2006, 21:54
During our brainstorming session yesterday we came up with a few ideas as to how to shoot the ball.
However, there was one that we came up with that I just can't see working. We thought of using a spring, sorta like a pinball machine, and having a winch pull it back. I just see this being extremely slow, needing a long pull length and not being consistent unless we use a hookean spring.
and u have to take in to play the rule about stored energy cus we were thinking along those lines and just think of how much energy it would use to pull the spring back in a match and how many times during it too, to be affective
sorry about the 2 posts in a row
just found a new thing to comment sorry
Kevin_547
10-01-2006, 22:29
it seems that most ppl are toying with the softball pitcher idea to shoot that balls. which i think is a very good way of doing so but how is everyone thinking how u are going to get the balls from ur holding area to the wheel to shoot without over loading ur gun or shooter or w/e u want to call it. :confused:
how is everyone thinking how u are going to get the balls from ur holding area to the wheel to shoot without over loading ur gun or shooter or w/e u want to call it.
That remains to be figured out, assuming teams use a holding area. One of our engineers had an idea, but what that is is a secret.
Masterfork
10-01-2006, 22:35
That remains to be figured out, assuming teams use a holding area. One of our engineers had an idea, but what that is is a secret.
well that is what peoples creativity comes in which will be interesting
Kelvin Ng
10-01-2006, 22:57
my dad had some awesome ideas. I just wonder about jamming. Is it possible that a pile of balls may interfere with each other's ability to load? Kind of like how 2 balls may get stuck in a funnel and neither actually falls into the hole that is designed to fit one? If that is unclear, sorry I'm not the best person at explaining things.
Is probably best for the shooters if the balls are stored single file to prevent jamming. Boy, I want to go present all my ideas to my old school's team, but as an alumni(us) (which is singular?) I want to leave all the creativity to the students. I just hope they ask me for ideas =).
BTW.
if anybody does not want to use baseball-style guns to fire, I experimented with a scaled down catapult made out of lego. It uses a couple elastics (the more elastics/springs, the more stored energy), to store energy, and the accuracy is surprisingly good. I was able to score little blocks of lego in the 1 foot diametre lego basket from approximately 2 metres away at nearly 100% accuracy. However, there was some awesome recoil from that thing.
Is there a limit to the amount of force we are allowed to have stored in a spring on the robot? or does the projection of the ball by that method just have to follow the 12m/s rule?
WebWader125
10-01-2006, 23:13
Has anyone figured out how much power it will take to keep a softball-style shooter going? Seems like all that freewheeling would kill the battery.
Masterfork
10-01-2006, 23:29
Has anyone figured out how much power it will take to keep a softball-style shooter going? Seems like all that freewheeling would kill the battery.
we thought about that but u have to put in to thought that once u get it going the momentum is what will save the battery life because it will take less energy to keep it going
Flywheel and a good PID will prevent your motor from overworking. Its not that bad. And, just shut it off if you're not using it =)
My question is, is 12m/s muzzle velocity? if so, if we move the entire launcher assembly by some v, we'll have a ball traveling > 12 m/s but with a muzzle velocity of < 12 m/s..
I just wonder about jamming. Is it possible that a pile of balls may interfere with each other's ability to load? Kind of like how 2 balls may get stuck in a funnel and neither actually falls into the hole that is designed to fit one? If that is unclear, sorry I'm not the best person at explaining things.
it is possible to have two balls jam. the teams will just have to design a way around that. stored single file, yes, or a take a field trip to some factories. i did some temp work on an assembly line one summer and was amazed by all the machines used to move things around. a few that come to mind as aplicable would be a pinwheel on the bottom of the hopper to break up jams if they were to occur or a slowly contracting space (like a funnel but with steeper sides).
Eria4044
13-01-2006, 12:12
One of my teammates had the idea to have two spinners (like in old Hot Wheels tracks) in the barrel to accelerate the ball and prevent spin from killing accuracy.
Stu Bloom
13-01-2006, 12:27
Actually a spin on the ball will improve accuracy, probably backspin or spiral would be the best. In baseball the erratic behavior of a knuckleball pitch is due to the fact that it has little or no spin. It has to do with turbulence of the air around the ball, but since I don't use that stuff at work (and its been a LONG time since I was in school) I am unable to regurgitate any specific details.
Eria4044
14-01-2006, 09:59
Really? I assumed that, if you spun the ball one way, it would veer off to one side once out of the barrel. Or you could make a long barrel, which would have problems of its own. Maybe it was my analogy at one meeting that the balls were like musket balls. Maybe I just need more sleep.
I agree with Eria, because when you think of spin on something like a football, that helps to prevent different faces from being on the front, and thus pose different amounts of drag, causing the ball to tumble even worse. In the case of one of these balls, it is unclear if it will act like a baseball, because these have far less mass per unit of volume, and thus when accelerated have less momentum keeping them going forward. As the physics aren't totally clear, I asked my physics teacher about it, and he said to stay away from spin on this kind of ball. He didn't see the exact ball though, and I think the most definitive way to know how the ball will react to spin is to mock up a shooter and try it, which is what my team hopes to accomplish this weekend, among a million other things.
Mark Pettit
14-01-2006, 12:22
More video footage of Poof Balls being launched through a flywheel.
Our team borrowed the volleyball serving machine from our athletic department and you can see the footage HERE (http://faculty.brophyprep.org/robotics/AimHighPage.htm).
Squirrelrock
14-01-2006, 13:30
617 has so far made a prototype with AC drills driving the top and bottom wheels on the softball-style shooter, with the top one smaller than the bottom wheel. The bottom wheel was the 12ish inch pneumatic wheel from two years ago, and the top "wheel" was two 5 inch diameter wheels duct taped together on a shaft so that there was a depression between them. The speeds that we shot at were right around max depending on the speeds of the wheels. We got some major backspin and can hit the goal's center consistantly from midfield.
Good luck to all.
<edit>
we have a 4-inch wide channel to feed the ball through the shooter also. that probably helps with the straightness of the shot.
</edit>
im thinking of a hot wheels sort of thing where u pick the balls up they go down into a basket or some sort of holding device and then shoot up between 2 wheels kinda the same thing but w/e
ditto
a trebuchet would totally win the cool award
Im trying to think of an effective way to rifle the ball in a mechanism similar to the softball laucher.
there are also many hybrid ideas to the softball laucher, including tubes with belts in them, multi-ball lauchers, "gatling guns" etc etc etc
We thought about rifling which could be introduced by a series of small, slightly angled, opposing wheels, but realized there is a problem. These balls are not equally balanced. They always roll to their heavier side where the injection point is. Spinning the ball will most likely produce an unpredictable wobble depending on where that heavier side is when it goes through the shooter.
The Trebuchet would definitely be cool, but difficult to load! The counter weight is a problem too. Maybe a dangling compressor.
whilst we are not using the catapult it would be pretty cool if someone did
and it wont be that bad of mechanism
it will be VERY percise, but will only shoot I guess 2-4 balls in 10 seconds
the thing is we wanted to do it like a gear in a car, it shoots, but the motor leaves the catapult when it hits the angle, so it falls back down (after it hits a bumper or whatever also)
we didn't go that way eventually
but it would be cool to se someone do it =)
617 has so far made a prototype with AC drills driving the top and bottom wheels on the softball-style shooter, with the top one smaller than the bottom wheel. The bottom wheel was the 12ish inch pneumatic wheel from two years ago, and the top "wheel" was two 5 inch diameter wheels duct taped together on a shaft so that there was a depression between them. The speeds that we shot at were right around max depending on the speeds of the wheels. We got some major backspin and can hit the goal's center consistantly from midfield.
We have some major backspin as well, but without a wheel moving on top... In our case the backspin also causes a very accurate shooting mechanism. There was no need for extra barrel.
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