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Which diameter for trebuchet axle?

I‘m making a trebuchet for a high school science project, and was wondering if I should use a bigger or smaller diameter for the axle on which the throwing arm spins. Should it be smaller to reduce the amount of friction maybe? Or bigger to have more arc length to have longer acceleration time. Also, what materials do you think I should make this axle out of. The axle can be any material, but the walls which it is attached to and the throwing arm that spins on it are all made of cardboard, but they are very strong 60 layer thick cardboard, so it‘s almost like solid wood. Thanks in advance for anyone that helps me.


First, don't worry too hard about friction. Use a smooth axle material and do a lot of shooting; this will give a burnished surface in the hole for the axle to run on. Just don't make the hole tight on the axle. In reality, actually, the axle only contacts the hole in one spot. Some things to think about: the axle needs to be big enough to be strong, so it doesn't bend. That depends on how much counterweight you are using and how much distance between the supports you have. I've had good luck with EMT (electrical conduit pipe). Use a 1 hole for 3/4 pipe. 15/16 is too tight. I've stood on an axle made of this and it held up, and I'm over 200 lbs. Half-inch might be strong enough for your purpose, but I forget the hole size, maybe 5/8. You might be able to get s short scrap piece free from an electrician. Less than 2 feet long would be plenty.
May 28, 2018
It just means you tire went flat and you haven't jacked the car up enough. You started jacking at the point of the flat tire.
May 28, 2018
The tire and shocks will expand back out as you jack it up. Try jacking higher next time.
May 28, 2018

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