Dual catapult help

We have used the cho cho mechanism but the kids want a dual catapult design. (I am no help) any ideas for them to look at? We looked at Ben Lippers robot but can’t understand his catapult design. Thanks in advance

1 choo choo, 1 arm with 2 spots for a ball to sit before launch

We have the same problem as you. My kids designed their arm but can’t figure out the gearing to make it work.

I made mine work essentially with two 24t gears on the motors to 48t gears on the catapult. The layout can be a little tricky, so I’d suggest also using this video as a guide. I also made a robot reveal for the catapult robot if you wanted to see more of how it worked.

For further guidance, you can check out the Fling build instructions and build that as a model.


FRC973 has some good designs, basically what you want to have is 1 choo-choo mechanism in the middle and essentially two arms linked together horizontally to hold 2 balls at a time.

Interesting I’ll tell the team and see what they think. Thanks

We are having trouble with the gear popping out any help guys

In the Vex IQ VEX IQ Builds - Fling they use double gears and a 1x to lock it in place.

Edit: Adding reference link:


In addition to what @Hudsonville_Robotics said, I would suggest getting rid of the orange spacer between the gear and the beam to make it flush with the beam, providing the most support for your catapult. Also, make your the teeth on the gears are aligned and not skipped.


So they got the twisting of the gears but now the linkage for the Choo choo keeps coming loose any ideas?

Can you be more specific? A picture / video would be really helpful.

Review this thread

Thanks for the video. It helped me understand the problem.

First, if you haven’t already, try using new pins where it breaks. You can find them on the field at the bottom left and right of the big clear plastic sheets.

If that isn’t enough, try using less rubber bands, and adjusting the geometry of your catapult. This issue is unfortunately one with many causes and consequently many solutions, but the same solution won’t always work for each case. You can also try reviewing the thread mentioned above for more solutions.

1 Like

Thanks we will try all that

I would definitely explore if having a mechanical stop the catapult’s upward motion (a bar above it) alleviates the stress on this joint.

1 Like

What do you mean by the geometry of the catapault, as I have replaced new pins and changed rubber bands, but it still breaks. Yet if it does work, the rubber bands are too weak to shoot into the high goal. Any suggestions?


Welcome to the forums!

Thank you for reading through the topic before posting.

For me, the things I mentioned and you tried were enough to make mine work, but different catapults are different and there isn’t a “one size fits all” solution. To fix this problem, there are two main solutions that I’ve found to work. You can either make the catapult stronger or divert the force somewhere else. Since you’ve already made the catapult stronger, I would suggest making the force go somewhere else not into the catapult. If you look at Fling (starting on page 35), you’ll notice that there is an axle above the catapult that stops it just before it reaches the point where it would fully extended and put all the force on the catapult mechanism. So if you put something like that on your robot to put all the force into an axle right before it would usually break the mechanism, it should help, and you should be able to put enough rubber bands on to make it into the high goal.

I would guess here that Vex-2021 is saying that they already tried more rubber bands, and that it breaks itself when they do that.

1 Like

This topic was automatically closed 365 days after the last reply. New replies are no longer allowed.