# Spring assisted slingshots?



## ARB (Dec 31, 2009)

I know ballistas and lever/spring assisted slingshots have been discussed here before. I didn't really understand much of that discussion.







But simplifying things a bit, would it be possible to have slingshot forks spring loaded. Eg the forks are located in runners and can move back compressing coil springs. The runners and forks would be made in such a way that the forks would not move around, they could only move backwards or forward. The slingshot would still use rubber bands.

Would this be of any benefit? Are springs more efficient than rubber when it comes to releasing stored energy?


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## pelleteer (Dec 19, 2009)

The only trouble I can think of would be caused by the elastic nature of the bands. Either the springs would have to be weak enough for the bands to compress them, meaning that they'd be too weak to add much velocity, or the mechanism would have to be set up so that the bands were shorter than your full draw length, meaning the bands would reach their full extension before reaching your full draw, and then as you drew them to full draw they'd compress the springs. This might give better velocity, but it seems like it'd be very hard on both the pouch and the bands themselves, so you'd probably wear those items out pretty frequently.


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## ARB (Dec 31, 2009)

You're right, pelleteer. The spring strength would need to be chosen carefully and matched to the band strength and length. And even then it might not work and the problems you describe could occur,

Unless there was a way to pre set the springs and then trigger them at the same time as the pouch is released.


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## pelleteer (Dec 19, 2009)

ARB said:


> Unless there was a way to pre set the springs and then trigger them at the same time as the pouch is released.


Ah! Some sort of preload mechanism might indeed work. Like you said, though, it would have to release at the same time as the pouch/bands. An intriguing possibility that I'm not enough of an engineer to figure out (














), but maybe Joerg or one of our other resident designers could work on.


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## ARB (Dec 31, 2009)

pelleteer said:


> Unless there was a way to pre set the springs and then trigger them at the same time as the pouch is released.


Ah! Some sort of preload mechanism might indeed work. Like you said, though, it would have to release at the same time as the pouch/bands. An intriguing possibility that I'm not enough of an engineer to figure out (














), but maybe Joerg or one of our other resident designers could work on.








[/quote]
I'm also not enough of an engineer







And its possible that this idea has been discussed and dismissed in the previous threads about ballistas etc.

How about this one - pre set the springs, springs are now secure. Draw the slingshot and at a certain point in the draw the springs become "armed". Now at any point after this if the pouch is released, the springs release at about the same time. Would require some sort of trigger mechanism maybe with some smaller springs and cams etc. and could be connected to the pouch by means of a string. Or else the arming and releasing mechanism could use band tension. I have no idea of the practicalities of any of this


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## JoergS (Dec 17, 2009)

The first problem with this idea is the trigger. You have two choices:

1) No trigger, just pull back both rubber and string (pretty much Warhammer's suggestion to use mouse traps as fork arms).

2) Devise a trigger that has to be released at just the right time (= when you release the pouch).

The first way does not give you any additional strength other than your own body power. The tension curve of the whole setup maybe different, but my tests with such slingshots did not give me any significant bullet spead increase over conventional bands.

The second way could potentially add some power, depending on the force of the springs. But releasing the trigger at the right time ain't easy, impossible to do manually. So you have to design a mechanism, which unlocks as soon as the tension in the bands slacken. I lack a good idea for such a mechanism. Remember it would have to hold down enormous pressure, yet the release must be as light as a hair. That is some kind of challenge, as you can see when you look at powerful crossbows and the trigger design that those guys come up with.

Anyway, it is questionable how much power such a slingshot would gain over a conventional one. Remember that the springs do not directly apply force to the ball, but to the rubber that holds the ball on the other side of a highly elastic band. What it would do is it would pull out a little more than the shooter has already done. So if the springs travel 3 cm and the shooter has a draw length of, say, 90 cm, the setup has a draw length of 93 cm, that is about it. The nightmare of a crossbow designer is "slippage", which means that the string lengthens a tiny bit when the crossbow is cocked. This slippage leads to an enormous loss of power. A slingshot "string" (the bands) have all the slippage you can possibly imagine.

Also consider the shock. At the end of the spring extension there must be a "stop" of some kind. The accelerated slingshot frame (or the fork) would hit that stop with some speed, which will result in a shock. That shock may not be favorable for your aim.

I have played very much with all kinds of mechanisms, most of the tests have been failures. That does not mean no such concept could ever work, it is just that I have given up for now.

The key to a powerful slingshot is

1) a braced starship design where legal

2) a low fork for non braced slingshots

3) in any case the most efficient rubber you can find and shoot

Just my 2 cents.

Regards

Jörg


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## ARB (Dec 31, 2009)

Thanks Jörg, that is a very good explanation of the difficulties with this idea. It sounds as though rubber and springs aren't really compatible. Still, it's interesting to think about these things.


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## ZDP-189 (Mar 14, 2010)

You can use the lever action to gain acceleration.

You don't need a trigger.


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## JoergS (Dec 17, 2009)

ZDP-189 said:


> You can use the lever action to gain acceleration.
> 
> You don't need a trigger.


Well, if you want to increase the power, then you need to store the energy. Otherwise you have to reduce the draw weight of the rubber in order to make "room" for the draw weight of the spring. If you need to store the energy, then you need a trigger mechanism to release the energy at the right moment.


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## ZDP-189 (Mar 14, 2010)

If I get round to building it, I may show you what I mean. For now, I apologise for my vagueness and beg your indulgence.


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