# Catapult lower ball help

Catapult experts - need help. We are using a 2 ball catapult … the top ball is fine wrt to power, but the bottom ball lacks power and cannot tilt the flag. I understand that based on physics, the outer ball should have more power. In slow-mo it shows that the lower one is hitting the top one soon after release … which may be affecting it’s power and angle. Any ideas or thoughts ?

how many rubber bands are you using?

most likely it will be either due to not enough rubber bands or the angle of release is not right.

That’s not necessarily true, on my catapult the lower ball has more power/force. You might just need to re-rubberband your catapult. When my catapult didn’t work with 14 rubber bands, I just made my catapult 2 holes longer and doubled up my rubberbands(I also changed the brand of rubber bands). Now it works perfectly with 3 rubber bands. I’m pretty sure that for maximized efficiency you should try to keep the stress of the rubber bands as constant as possible(you could make your rubber bands into a triangle). I can’t say I’m a catapult expert though( I’ve only built one).

This triangle rubber band method that many people has been talking about is actually uniform-tension rubber band system.
The main purpose is to ensure that roughly the same amount of tension is applied throughout the entire process of the lift going up (and down), so that there won’t be a situation whereby the lift will suddenly “shoot up” or accelerate.

So for this case of catapult, there is really no real advantage in using uniform-tension rubber band approach.

https://vexforum.com/t/maximizing-efficiency-of-2-ball-catapult/50845/1

Make the catapult longer, adjust the angle, add more rubber bands, make sure your catapult is light, hit the right stop on the flag. These are all things that could help, start with the easiest first.

Do you try to create some space between the balls ?

We did add more rubber bands … and it broke the sprocket used for rachet

I try to keep the balls as far towards the end of the catapult as I can without them hitting one another. That allows for maximum energy transfer.

@Got a Screw Loose - how do you take care of “without them hitting one another” ? high level tips ?

You can use short standoffs to move the balls away from each other.

The transfer of angular momentum is greater the further you are from the axis of rotation. Have you tested increasing the radial distance of your balls? Start by testing one at a time, and ensure that your position holds. i.e. Make sure that one ball does not affect the other. The release angle might have to change depending on how increasing the distance changes the final trajectory.

It helps to take slo-mo videos of your 2BC firing. Notice at which point they leave the catapult and where (if) they hit each other. It’s in the trajectory, really. You just watch to see where they line up, make modifications as needed, and roll with that.

I’ve looked into making a custom clamp-divider, but I didn’t have to. It would have clamped into the sides of my carriage to separate the balls with high precision, but I found that standoffs correctly spaced worked out just fine.