Slingshot physics

Discussion in 'General Slingshot Discussions' started by Mycophage, Jan 22, 2014.

  1. Mycophage

    Mycophage New Member

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    Has anybody done a write-up explaining all the geometry and physics behind homemade slingshots? Here are the assumptions and observations I've come up with (obviously for maximum power!):

    1) Theraband gold only contracts at 150m/s, so no matter how strong you make the draw, your projectile will never move faster than this (without effectively making a compound bow, and using lever arms to magnify the speed... which isn't really a slingshot anymore)
    2) Because speed is your limiting factor, to get the most energy delivered to the target, you must go with the heaviest load possible e.g., a 1/2" lead ball @ 150m/s will have twice the energy of a 1/2" steel ball moving at 150m/s
    3) The height of the forks, without a forearm supporter, will limit your draw power, because taller forks make a longer lever to pull against your wrist with. Therefore, you want to have the lowest forks possible, yielding the highest risk of hitting yourself.
    4) The width of the forks limits your maximum velocity, as the further apart they are, the less the rubber is pulling in the direction you're firing. For maximum efficiency, you must have the most narrow forks possible, also yielding the highest risk for hitting your hand/slingshot with the ammo.

    Am I right in these observations, and assumptions?


    Questions:
    1) What do you guys use as extra heavy ammo? Cast your own enormous lead balls? I was thinking of taking some 1" (or greater) iron rod, and cutting it into 1" long segments, yielding a somewhat suboptimal cylinder, instead of a sphere... I can't imagine it'd be worth the time and difficulty trying to file it round, though. Large steel nuts (for bolts) were also a consideration, as they're readily available, albeit an inappropriate shape for accuracy.

    2) What has better penetration for soft, and hard targets? Is lead, at twice the energy, but much softer better, or is it better to use steel, as it won't deform, at the cost of possibly ricochetting? I supposed if you were hunting small game (rodents), it wouldn't matter, but lead would be more lethal, and not penetrating the entire animal and ruining the meat would be an upside, but for larger game, say, um, a possum or racoon, the steel may be better, as you'd be trying not to crush the entire animal to kill it, but penetrating to hit vital organs. I suppose the best idea, which likely isn't worth anyone's time, is to make full metal jacket lead shot. Also, how about penetrating hard targets, like plywood? I'm thinking lead would be better. Thoughts?

    3) What is the benefit to tapering the bands towards the pouch? Seems like you're just cutting away rubber for no reason to me, and losing power and durability as a result.
     
    Last edited: Jan 22, 2014
  2. dolomite

    dolomite Banned

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    As far as assumptions go, #2 and #3 are correct. Compound slingshots just don't improve speed very effectively as they introduce friction that doesn't outweigh the gains. Tapered bands shoot faster but also wear faster. If you hunt, headshots are mandatory. Some guys and gals(Ruthie) use cut down rebar.
     

  3. Erlkonig

    Erlkonig New Member

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    Shooting the slingshot without a projectile accelerates the band assembly to a maximum speed. The slingshot will never be able to shoot any projectile faster than this maximum velocity.<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:eek:ffice:eek:ffice" /><o:p></o:p>
    <o:p> </o:p>
    The weight of projectile and pouch slows down the shot. The pouch end of the bands gets accelerated over the whole length, while the end at the frame doesn’t move at all. In average, half of the bands weight is accelerated<o:p></o:p>
    Using normal bands, doubling the pull force of the bands is only possible by doubling the weight of the bands, too. Tapered bands have more of their mass on the prong side, so less than half of their mass is accelerated. That’s why they shoot faster.

    <o:p>
    2006, by Melchior Menzel<o:p></o:p>
    </o:p>
     
  4. JoergS

    JoergS Administrator

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    Melchior Menzel is pretty full of himself, really. He feels like the elven king of slingshots. But in reality he is just a thin armed little plinker boy who never once shot a powerful slingshot in his life, as he is simply too weak.

    Anyway, tapered bands are faster because of the "payload" being lighter at the pouch side. But another reason why tapering works is that the tapered part of the bands is stretching more than the rest.

    Bands that are stretched to factor 7 are simply a lot faster than bands stretched to factor 3. While you can not stretch the whole band set to factor 7, you can do that with a tiny part, the one close to the pouch - and that brings you a serious speed advantage.

    But it also tears at that point, quickly.

    Also 150 m/s is a very theoretical limit, only achievable with very extreme band sets. Massively tapered and overstretched, most likely heated or shot in extreme summer weather. Only good for five shots maybe.

    I like to shoot around 60 to 80 m/s with 15, 20 or 25 mm steel balls. Plenty of fun, lots of destruction.

    Energy is also probably not the only value you want to maximize. As our member Boyntonstu pointed out recently, momentum is probably more important.

    Everybody has to find the best compromise, based on personal issues like body strength, arm length, shooting capabilities, and money (how much rubber do you want to afford?).
     
  5. Grumpyoldgit

    Grumpyoldgit Junior Member

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    I was experimenting with folding a bandset as opposed to two separate bands ie double band set today. 20mm wide TBG straight cut.
    8 degrees C. 16mm steel ball 254.6 grains mass. 6 shots per bandset.

    PFS double bandset secured to forks and pouch,
    8" active length = min 116 - max 123 fps

    PFS folded bandset ie passed through pouch holes and tied to forks only
    8" active length = min 132 - max 147 fps.

    The only difference ( in my not so humble opinion :D )
    is the weight of the rubber used to secure the pouch.

    Maybe the 'energy' in the folded bands was able to be released
    better because the band set was effectively self adjusting.
    Anyway, just thought I'd share my findings.
     
  6. JoergS

    JoergS Administrator

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    So many different influences, most importantly the time you take to aim.

    If you draw out and release IMMEDIATELY the shot will be 15% faster compared to waiting just half a second.

    You are shooting at such slow speed anyway that tests are random at best. Anything under 150 fps is very unreliable. Go down in ammo weight or up in rubber strength.
     
  7. Achso_42

    Achso_42 Senior Member

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    #2 is correct, #4 is not. The fork width has no influence on the power bcs the not-unidirectional force means less dead length.
    #3 mostly correct, but very low forks don't always have the highest risk of hand hits bcs of the flipping motion.
    For the tapered bands, listen to Jörg.
     
  8. Mycophage

    Mycophage New Member

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    Wow, that was a lot of detailed info, thanks for the response, guys!

    How does one go about cutting rebar easily? I had also considered that instead of steel rod... is rebar soft enough to be snipped with some bolt cutters, or will I need the angle grinder for it as well?
     
  9. NyamNyam

    NyamNyam Member

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    Hi!

    Seriously, is getting ammo this way really worth the effort? I mean - If you buy a pack of steel balls/marbles/gumballs and use it with a catch box there isn't really a big loss of ammunition over time.
    Shooting with spherical ammunition is more "true" and safe too.

    Maybe you should consider this one time investment.
     
  10. Mycophage

    Mycophage New Member

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    Hey NyamNyam,

    If I wanted to settle for "normal" levels of power, I never would have found Joerg's youtube posts, and wouldn't have joined this forum :D

    I want to fully understand all the parameters going into design, and see what the absolute maximum energy I can deliver to a target is with a traditional slingshot design.... mwahahahahahahaha

    Joerg, thanks for the reply! I originally planned on shooting butterfly style, as the longer acceleration time should allow for maximum velocity, but after your post saying it only takes 1/2 a second to lose 15% power, it seems like the traditional pull may have the advantage since you can probably do it more quickly. Thoughts?
     
  11. coyote-1

    coyote-1 Member

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    Rebar is easily cut with a hacksaw. Cut it at a steep angle, to create as severe a set of penetrating/slicing edges as possible.
     
  12. dolomite

    dolomite Banned

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    Best bet is a starship with butterfly and a quick release, huge power potential.
     
  13. ruthiexxxx

    ruthiexxxx ruthiexxxx

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    I'm afraid I'm lazy and cut the rebar with an angle grinder. One can make a lot of ammo very quickly. I think the ideal for me is 14mm cut into one inch lengths, the cuts being at 45 degrees. This ammo does a lot of damage !!
     
  14. kohlqez

    kohlqez Accident-Prone

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    You'll need an angle grinder if you want to cut 1" rebar as you'd previously mentioned, you can get one for about $20 at harbor freight but the cutting wheels are kinda expensive. If you want some good ideas about rebar rounds search lobotomizer at the search engine at the top of the forum. Someone (I can't recall who) mentioned making rebar lobotomizer ammo in a thread I read a while back
     
  15. 4foruglenncoco

    4foruglenncoco Member

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    [ame]http://youtu.be/pVEo_mckBIc[/ame]
    [ame]http://youtu.be/1v4TEX2erog[/ame]

    Your welcome :D
     
  16. JoergS

    JoergS Administrator

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    The butterfly style rocks.

    The delay time starts "counting" at full draw. You can draw back slowly, but when you arrive at the max draw length, you must IMMEDIATELY release if you want max power.

    The warmer the environmental temperature, the less dramatic the loss of energy is. Likewise, the colder the temp, the more severe the loss. In winter, slingshot crossbows perform poorly in comparison.

    I have done a series of videos about that effect, by the way.

    My most powerful slingshot is a massive draw extended wrist braced monster, shot in the heat of the South Carolina summer (that thing is illegal in Germany anyway). Notice NO delay at max draw. 25 mm steel ball ammo.

    [ame]http://www.youtube.com/watch?v=pAwTfAMHCG0[/ame]
     
  17. studer1972

    studer1972 scooter trash

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    Love that slingshot.
     
  18. dinosaur

    dinosaur New Member

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    very interesting topic!

    However, there are a couple of things I'm still wondering:

    1. Thicker bands or wider bands means higher drawforce and so you can shoot heavier balls.
    A larger drawlength means higher speed.
    But is there any relation between the drawforce and the speed?
    For example: if you increase the drawlength, will you reach a higher drawforce? If yes, that means that you can shoot heavier balls with the same bands at the same speed as lighter balls?

    2. Is there a maximum drawlenght a band can reach? I mean, what is the ideal drawlegth to get the max speed and so power?

    3. If I want to shoot light balls, 7 or 8mm steelballs, do I need TBG? Or can I reach the same energy with TBbleu or black?

    Thanks everyone,
    greatz Dinosaur;)
     
  19. JoergS

    JoergS Administrator

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    Draw length and force are interchangeable in a way. Short draw length and heavy draw weight can get you the same speed as long draw length and lower draw weight. Of course high draw weight PLUS long draw length is most powerful (= Smaug killer).

    The bands can be extended to about 5,5 to 6 times their relaxed length. Then they will have their best performance. However then they won't last long. If you only extend them to 3 times their relaxed length, they will last hundreds, even thousands of shots, but performance will be lower. Compromises are called for.

    Power maximizing calls for butterfly shooting and draw extensions (= starships with arm rests, where legal). You want the longest possible draw. But many prefer a compact frame for portability.

    You never need TBG. You can always also use blue or black, simply use more of it. 7 or 8 mm steel balls are so light that you will need very little rubber to maximize performance anyway. TB blue will even outperform gold on such lightweight ammo as its retraction speed is a bit higher.
     
  20. dinosaur

    dinosaur New Member

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    realy realy realy thanks,

    This makes a lot of things clear for me. I am just home from the gym and they gave me 1m of thera band black. I'm gonna start experimenting this weekend in therms of the amount of rubber. It's my first slingshot so I'm gonna start with a simpel and (in my opinion :)) safe design.

    I keep you guys tuned.

    Is it possible to use the TBG calculator for TBblack? Or do I have to use a coëfficient to increase the amount of black rubber with respect to the gold?

    Greatz Dinosaur ;)