I finally got my mini-mill back together after several years of neglect, and started on my long stagnant Big Shot project. The problem is that I've now forgotten all the fine details. Can anyone tell me what some reasonable design parameters would be for the stretching of the tubing? I am using an aluminum painter pole and some latex tubing that is something like 14mm OD with a 2mm wall thickness. The tubing slides onto 1/2" steel U-bolts.
Oh, I was wondering about choosing a length for the tubing and estimating the necessary pole length. Attachment options for the pouch would also be of interest. I expect to build the pouch out of 2-inch tubular webbing and am collecting the material I will use to build the trigger. I don't need to be able to shoot very high.
Basic math needed for 5/8” od x 3/8” id latex tubing
I found a book that addressed the kinetic energy calculations that bases “horsepower” on tubing size that is elongated to various percentages of elasticity.
It warps my mind but the results are easy to follow.
I use a 40 pound bag of salt to stretch each band and measure its extended length compared to its relaxed length for a single band launch tube so a dual band sling shot will have twice the power.
10” band pulled to 20” long is at 100% elongation and has a little power.
10” band pulled to 30” long is at 200% elongation and has decent power.
10” band pulled to 40” long is at 300% elongation and has the most power.
I aim to fall between 300% and 400% as elongation becomes very hard to pull and you approach the breaking point at 500%.
Determine your draw strength in pounds that you can pull easily.
Determine your elongation distance of your devices arm.
Then you can determine the relaxed length using:
(drawn length-relaxed length) divided by relaxed length