Simplest real compund slingshot

Discussion in 'New project ideas' started by krnico, Jan 18, 2015.

  1. krnico

    krnico New Member

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    Hi, this is my first post on this forum and i wanted to show you and idea that came to my mind.

    this is not one of those over complicated designs in fact it is very simple and theorycally it can boost the speed by 2, thats mean if rubber can make 300 fps this will make 600fps.

    youtube video showing the concept:
    [ame="https://www.youtube.com/watch?v=VrZbgOlSJcU"]https://www.youtube.com/watch?v=VrZbgOlSJcU[/ame]

    the idea is to attach a cord to your shoulder and use a roller pulled by the slinshot.
    [​IMG]

    the rubber needs to be half shorter but 2 times stronger, the cord should ideally be made of Dyneema and the roller small and light weight has posible.

    what do you think?
    i would love if someone could make it real.
     

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  2. TheNewSlingshotGuy

    TheNewSlingshotGuy AKA- "The Raccoon"

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    I don't know where to start. These concepts never work, as It's all about the max retraction speed of the rubber. It seems we've been over this a hundred times here on this forum. You're welcome to try it, but I doubt you'd see any increase In performance. When it comes to rubber, simpler is better. Good luck though!
     
    Last edited: Jan 18, 2015

  3. krnico

    krnico New Member

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    why

    why do you consider it wont work? the max retraction speed of the rubber will be aplied to the roller and the projectile will move twice the distance of the roller in the same time meaning twice as fast.
    the problem with others concepts that never work you refer is the loss energy in friction and moving parts such as arms, this dosnt have any of that since only use one roller and low friction cord such as fishing line
     
  4. TheNewSlingshotGuy

    TheNewSlingshotGuy AKA- "The Raccoon"

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    The problem with that concept in the video is that the only thing the rope is doing is making it a little easier to draw maybe? Either way, all he's doing is drawing to a set distance using a rope. I would get the same speed by just drawing it with my hand.
     
  5. TheDaddy187

    TheDaddy187 Administrator

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    as tnssg mentioned, usually these ideas are alot more effort for not much of a power increase...

    this particular idea has the issue of whatever pully you use will be flying towards the hand holding the shooter...
    regardless how small or light they are, they will hurt... even bandslap hurts after a while

    team that up with the fact you would need 2 pulleys and 2 strings from the shoulder and i think you will end up looking like a tangled puppet with sore hands...
     
  6. krnico

    krnico New Member

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    i think this idea is diferent from those for using only one pully and a very light one, the pully wont hit anything since it will be in the middle of the slingshot as shown in the image so there is no need for a second one as you propose. the only waste of energy is moving the pully at rubber speed and moving the cord at 2x rubber speed.
     
  7. Bert the Welder

    Bert the Welder New Member

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    .....and I think you're actually just gaining mechanical advantage, not rubber power. The pulley is making easier to pull the bans but that doesn't affect the power of the rubber. It may, MAY, allow you to pull heavier bands, but that's not increasing the power of an existing band, as I think you are thinking. The power of a given band is a static. The effort required to draw is a dynamic number. Think of it in terms of lifting a 1000 lbs with pulleys. More pulleys makes the load easier to lift. It doesn't decrease the 1000 lbs, that will always be 1000 lbs.
    But it's a fun method of drawing back the shot you've come up with, and having fun with it is what's important. And you get to learn so physics to boot. Welcome to the forum. :)
     
  8. TheDaddy187

    TheDaddy187 Administrator

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    the inertia will cause the pulley to continue through the fork...

    only one pulley would cause the ammo to never shoot straight...

    i am assuming that the pouch would be attached to the string?

    also i see issue in using half the rubber but twice the thickness... alot of the speed from a shooter comes from the length of acceleration...

    long thin bands with a mean taper shoot faster than short thick bands as the rubber has more distance to accelerate the ammo...

    remember, rubber stretches more than just 1 direction...
    the only way to stop the pulley would be to use the string to limit its movement to level with the forks... but this will cause attachment of the pulley to wear at a much faster rate...

    my only suggestion is that if you are sure your idea will be a big improvement then put your money where your mouth is and prove it... make a fully working proof of concept, then compare to a normal setup...

    you will need 2 identical shooters, 2 identical bandsets (one folded for your idea, one left long) 2 identical pouches, and a chrony...

    you can line up empty cans and see which setup penetrates furthest, but chrony will give more acurate readings...

    i am willing to put money on there being little to no improvement in power thats worth the extra hastle of attaching strings and pulley(s)...
     
  9. krnico

    krnico New Member

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    the projectile will be at the end of the cord, im sorry for this big confusion.
    The aceleration distance is the same has a normal slingshot dispite the fact the rubber is half long, the drawweight is halved aswell. thats why the speed is dobled.
    Stoping the pulley with the cord is a good idea, will do the test, thank you all for your comments.
     
  10. allu

    allu Junior Member

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    Maybe it's just me but I'm failing to understand how that is even possible. As earlier mentioned, for those extreme speeds you need heavily tapered bands and a long draw which you'll get from using the butterfly shooting style. If you half the band set and use a cord and a pulley, how could you theoretically get twice the speed? To me it sounds like you'd only reduce the accelerating force and the the length of it and therefore reduce the speed instead of increasing it? It'd make more sense if you used the same band set and then used that cord thing. Though then one should be a giant to be able to pull it back to full draw! The idea itself sounds pretty cool and it's pretty innonative for sure, but I doubt it'd give you any sort of an advantage over tapered bands and full butterfly shooting style. I'm curious to see what you'll come up with though!
     
    Last edited: Jan 18, 2015
  11. Arturo Borquez

    Arturo Borquez Administrator

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    No, in fact will be slower than a standard slingshot as it will need to pull the string ... rubber retracts around 350 FPS (max, no load) at standard room temp.
    Arturo
     
  12. krnico

    krnico New Member

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    you are right the aceleration force will be reduced by half, but the lenght of it wont since you draw the projectil(at the end of the cord) exactly to te same position that you will normally draw it to, and it will acelerate from there to the end of the slinghot, what will have half the lenght would be the rubber with the roller at the end.
    the speed of the projectile will be twice the speed of the roller, and since the roller is acererated by the rubber itself and will be very lightweight it should be at around 300fps therefore projectile will get the 600fps
     

    Attached Files:

  13. krnico

    krnico New Member

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    yes it needs to pull the string and the roller/pulley and loss energy rolling it too, but this can be reduced by using very lightweight parts using wax on the string.
    the gain in speed is still huge since the max retraction speed of the rubber will be aplied to the roller and the projectile will move twice the distance of the roller in the same time meaning twice as fast.
     
  14. bigdh2000

    bigdh2000 Administrator

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    Jörg has done extensive testing over the years and proven that the extra stuff hinders more than helps. He mentions this frequently.
     
  15. Will

    Will Thread Hijacker

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    I just think this is self-defeating. You shorten the draw length with rubber and you lose power at an exponential rate. Even with compound bows it is the same thing. My buddy and I shoot the same exact bow, and poundage of draw. However, his draw length is 25 inches and mine is 29. You would not believe the difference in power.
     
  16. Arturo Borquez

    Arturo Borquez Administrator

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    I don't want to discourage you, do your tests but use a test bench and eye protection, it is very likely that thing will hit the pulley or forks or RTS the projectile direction could be unpredictable ... be safe
    Arturo
     
  17. allu

    allu Junior Member

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    It seems my phone messed up and didn't send my reply, oh well let's try again.

    The problem I see with this is that when you reduce the amount of rubber you also reduce the amount of potential energy. The idea works in theory, but have you taken account the mass of the projectile? The tip of the string might theoretically whip at the speed of 600 fps (minus all the friction forces plus the mass of the pulley and string, remember the faster the speed the bigger they are). When you add the mass of the projectile it won't accelerate in an instant due to it's momentum. I doubt that you'd reach the maximum retraction speed with half of the force available, so you'd need to add more rubber in it. Eventually you'll come to the point where nobody could draw the bands used in your slingshot back with a normal shooting style. Due to the roller you'd be pulling only half of the force, so then your design might have an advantage, but that's not a fair comparison anymore. That's why I think the butterfly style is superior to this. It has the maximum amount of rubber AND a long draw length. But I still see this design has some potential, but it'll never reach 600 fps or even get close to it, no way.

    What I still don't get is that how much longer the draw length would be after all. Let's think the draw length with a full butterfly is 100 cm. Maybe the bands would be 20 cm in length. So we'll end up with 80 cm long power stroke. Now if we half the bands and use a roller and a string; the bands are 10 cm long and because you're pulling them only half as far, they'll be 50 cm long when pulled back. The string will add that needed 50 cm, so 100 cm in total. So in this design the rubber bands have only 40 cm long power stroke. Thanks to the string and the pulley the projectile will be accelerated for 90 cm. So you gain 10 cm longer power stroke but lose all of the benefits for the friction and momentum, probably more than just the benefits.

    Let alone how much more complex (to shoot mainly) and dangerous the slingshot would become for its shooter. But I might be completely wrong, so please correct me if something in my speculations is completely wrong! :p I'd still like to see you try it out though!
     
    Last edited: Jan 19, 2015
  18. BillHays

    BillHays Member

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    First of all, latex by itself already has a very close to perfect inverse relationship... meaning double the draw length at half the draw weight sends the projectile at approximately the same speed as double the draw weight at half the draw length... 60" draw length at 15 lbs draw weight will shoot at about the same speed as 30" draw length and 30 lbs. draw weight.
    To enhance speed, tapering bands and reducing pouch weight are usually the easiest things to do.... but, everything comes at a cost... tapered bands don't last as long as straight cuts, and so forth.
    There's other things you can do that aren't as obvious, such as not using a pouch as all and using "<" shaped ammo on the bands or a tiny loop of string instead... so for the sake of argument we'll just say that's what you do since you're going to be using string anyway... plus by doing it this way you'll be able to simplify the whole device even more by only having to make one side instead of a fork.. in fact you can get away with using an inline one band style stickshot slingshot rifle.. similar to Jorg's Christmas tree one a few years back...

    The problems you're going to face with a pulley system are friction and weight.... The weight of the pulley, the friction/weight to set it in action and the weight/friction of the string, which don't contribute any elastic potential are your main problem... so if you can get your device to propel a projectile at or above the actual contraction speed of medical latex, which is around 400 fps at 90º f. , then your project has merit because it can be faster than latex itself is.

    A couple of things you might try are... maybe using shock cord instead of string and using more than double the latex draw weight (just make sure the pull length of the latex part is at 5 to 1 or greater)... after all, as I understand it, your goal is to shoot faster than the inherent potential of latex not necessarily more efficiently than just latex, which of course you won't be able to beat.

    Something like:

    [​IMG]
     
    Last edited: Jan 21, 2015
  19. watcher_by_night

    watcher_by_night New Member

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    Using a stock definitely sounds easier/safer for at least getting the kinks worked out. Archers who do flight shooting often use strings that are so light-weight that the string intentionally breaks every shot or so. So no harm if stuff tends to break when testing the idea out (as long as all body parts are out of harm's way). The ComBow guy used small pulleys, and before pulleys he used lube (which he said was pretty messy), so maybe it would be possible to use a very limp fishing line running through a ring with a good coat of lube.
     
  20. krnico

    krnico New Member

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    I love this concept you have improved my idea so much, i imagine that if i more than double the latex draw weight i will need to cock it has a crossbow maybe and use some kind of triggering mechanism? it would be great, cant wait to build it.
     
    Last edited: Jan 21, 2015