Very cool stuff. I am really surprised to see a full 42 fps from the 30" draw to the 45" draw. I would've never guessed it was that much difference.
Size matters- especially length
I played with some goofy ideas over the holiday including building a prototype slingshot rifle to try some wonky ideas out. Not a well finished piece by any means but I had some fun testing a few things with it.
It has been a nice test bed for testing bands and tubes that allows good control over draw length. I also put pulleys on it to see about "compounding" to get a longer draw while keeping the apparatus a bit shorter than the draw.
First up was re-inventing the wheel by testing how much draw length affects power. I set it up with various lengths of 2040 tube from simple shot and drew all to 5x the active band length. And in the spirit of re-discovering the already well known, I found that draw length makes a dramatic difference. Bummer for people like me with short arms who don't butterfly. The table below shows active length vs velocity using 3/8" steel with all of the bands drawn to 5x active length.
I haven't done much with tubes and the 2040 wasn't as powerful as I had hoped- a bit overwhelmed with the 3/8 but the relationship between draw length and velocity is still clear. And in fairness, it did get the 3/8" over 200 fps at longer draw lengths.
Also included are a couple where the tubes went over the pulleys to test if this could be compounded to get a "longer rifle" with a shorter frame. And as expected, there was a cost to turning the wheels. The system worked well but it was pretty much a straight trade between draw length gained vs power lost to turning the wheels for the 2040 tubes. I will note that I built this with wide wheels that were pretty heavy. I did this so I could also test compounding with stronger flat bands. While the 2040 tubes lost a lot going over the wheels stronger tubes may gain some actual advantage- something to test another day when I rig the apparatus up for flat tubes. A 20mm wide band will stay on the wheels so this should be a fun test when I get around to it.
A few thoughts gleaned on the compounding. I put posts on top of the barrel to catch the bands so they didn't fly around and come back to the anchor point under the barrel. I'm not always blessed with great foresight but did manage to ponder the possibility of a return to sending if the ball stayed in the pouch and it wrapped around back toward the shooter. Also, compounding is limited to stretch factor of the band- i.e. a 40" barrel cannot be compounded the full length on the bottom or the stretch factor will be reduced to just 2. So in actuality the length added on the return loop is only 20% of the top length if keeping the stretch at 500% is desired. So a bit limited on the long draw with a short barrel idea.
I can't remember who put it up but there was a video of kids in India (or Pakistan?) shooting really long slingshot rifles. Someday I may test if stronger bands that turn the wheel better can get to long band power with a shorter body.
So here it to those blessed with long arms or the skill to shoot butterfly!
It has been a nice test bed for testing bands and tubes that allows good control over draw length. I also put pulleys on it to see about "compounding" to get a longer draw while keeping the apparatus a bit shorter than the draw.
First up was re-inventing the wheel by testing how much draw length affects power. I set it up with various lengths of 2040 tube from simple shot and drew all to 5x the active band length. And in the spirit of re-discovering the already well known, I found that draw length makes a dramatic difference. Bummer for people like me with short arms who don't butterfly. The table below shows active length vs velocity using 3/8" steel with all of the bands drawn to 5x active length.
I haven't done much with tubes and the 2040 wasn't as powerful as I had hoped- a bit overwhelmed with the 3/8 but the relationship between draw length and velocity is still clear. And in fairness, it did get the 3/8" over 200 fps at longer draw lengths.
Also included are a couple where the tubes went over the pulleys to test if this could be compounded to get a "longer rifle" with a shorter frame. And as expected, there was a cost to turning the wheels. The system worked well but it was pretty much a straight trade between draw length gained vs power lost to turning the wheels for the 2040 tubes. I will note that I built this with wide wheels that were pretty heavy. I did this so I could also test compounding with stronger flat bands. While the 2040 tubes lost a lot going over the wheels stronger tubes may gain some actual advantage- something to test another day when I rig the apparatus up for flat tubes. A 20mm wide band will stay on the wheels so this should be a fun test when I get around to it.
A few thoughts gleaned on the compounding. I put posts on top of the barrel to catch the bands so they didn't fly around and come back to the anchor point under the barrel. I'm not always blessed with great foresight but did manage to ponder the possibility of a return to sending if the ball stayed in the pouch and it wrapped around back toward the shooter. Also, compounding is limited to stretch factor of the band- i.e. a 40" barrel cannot be compounded the full length on the bottom or the stretch factor will be reduced to just 2. So in actuality the length added on the return loop is only 20% of the top length if keeping the stretch at 500% is desired. So a bit limited on the long draw with a short barrel idea.
I can't remember who put it up but there was a video of kids in India (or Pakistan?) shooting really long slingshot rifles. Someday I may test if stronger bands that turn the wheel better can get to long band power with a shorter body.
So here it to those blessed with long arms or the skill to shoot butterfly!
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A perfect short summary to my many many paragraphs. And also why I am looking forward to taking up butterfly someday when I have enough time to learn how to do it well and make it hit where I am aiming. Until then, I'll be happy that current elastics are plenty snappy to be useful with shorter draws.Kokopelli.. Nice job! All those facts and figures make my head swim. But one fact hits home, longer is better, assuming you can put it where you are aiming. Otherwise it's a moot point.
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