More Elastics vs More Stretch. Harder vs softer elastics? Which is better for a catapult design?

Hi All. I am a first year parent volunteer/mentor to my two son’s VEXIQ team. The team has been reiterating a catapult robot for this year’s Pitching in challenge and it has been working pretty good so far. They are working on getting the robot as reliable and consistent as possible.

They have experimented with both either using more stretch on their elastics vs using more elastics to get the appropriate power into their catapult. Both works but the question is which is better from a consistency and reliability aspect. Using more stretch to get the power or using less stretch and more elastics? Which method would yield more consistent power over time? i.e. will not lose power over time do to loss of elasticity Which method would be less prone to elastics breaking? (I assume more elastics would be less prone to breaking?) What are the pros and cons to both methods.

Also, since the sizing of the elastics is the only restriction in the Vex parts list this year, (i.e. can use 3rd party elastics as long as the size is legal), what hardness of elastic bands would give the most consistent results? I know when looking for elastics on Amazon there are hardness ratings.

Thanks in advance for the help.


It sounds like all your questions will require your students to do experiments to find out what works best for them. Unless you’re students are using the “Fling” trainer, every design is unique and needs to be tested on it’s own merits…anybody else would just be guessing.

The testing of rubber band systems is good stuff for your engineering notebook: could lead to a design award for your team.


Thanks for the advice about the engineering notebook. They have been testing the different methods but can’t draw a conclusion which is better. The difficult part is comparing it over time. Their testing is maybe like 100 shots at a time. So far they haven’t had an elastic break but I guess the last thing they want is it breaking during competition. I was hoping someone may have reference or info of some sort of relationship of loss of elasticity over time as a jumping off point and whether it even matter.


The average event is 8 qualfication runs and then a teamwork final. They should calculate how many shots they can take in a match and then use that for their projections. The basic “Fling” can do ~8 a minute if you just hold the button down. Your teams robot will vary from that.

You got some great advice for the team to “science it” with doing data gathering on the different rubber bands (32, 64 and I think I saw 117 are now allowed (@lacsap help!) and then present that in their notebook. That is the kind of thing that the judges do love).

Have them think about how to design the experiment. Dry fire? Free by hand, load direct, etc. Does battery charge levels mean anything?

We want the roboteers to do the work, it’s to be student led. You can talk about experiements and the general rule “only turn one knob at a time”, but let them do the work.

Remember, the judges are good, and they ask teams "hey how did you come up with this experiment. “Well Mom told us what to do” is always the wrong answer.

Good luck with your experiment. Post back how they did, we all love seeing how the VEXIQ approach testing things like this.


Thanks Foster. Interesting idea. So instead of seeing which is better. Just determine how many shots generally will be taken in a given tournament and see if a specific set up will last that many shots and whether it will be consistent still near the end of the cycle. Makes sense. I will get the kids to figure that out and run an experiment to see. I guess if they also build in some buffer room with their total shots, that would give them confidence that it will work in a variety of different competition events.



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