# The Mechanics behind slingshots



## NightKnight (Dec 16, 2009)

So, I have been researching the different types of rubber out there, and I found a page that compares some different elastics.

Here is the link:
http://www.polyurethane-1.com/tensile-strength.htm

Based on what I am seeing here, it looks like the "Urethane Elastomer 55 Shore A" (Red line) would be the best choice? It looks like it would have the lowest strain while achieving the most elongation. I would also assume that it would snap back fastest since is would have a lower Modulus and thus have greater Elastic Potential Energy.

Thoughts?


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## NightKnight (Dec 16, 2009)

It looks like the stress-strain curve is the best way of selecting a material for our use. It can give us what the Elastic Modulus is as an output, which could be used to quickly evaluate how good the material is. However, it seems to me that the best material would have a graph that would show these three things:
1. High Tensile Strength (how much it will stretch)
2. Low Tensile Strain (how much pressure it takes before breaking)
3. Consistent Tensile Strain Level

We could then increase the amount of the material to match how much we can pull (strain). This in turn would increase the Elastic Potential Energy which is ultimately what we are really looking for.

Am I off-base here?


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## JoergS (Dec 17, 2009)

I probably would have to ask my brother who is a physicist. I have never ever tested other material but natural rubber (latex). Others have, though, with discouraging results.

But technology advances, and I am not ruling out the chance that a better material comes out some day soon! We need to keep our eyes open.

I still find it amazing that I can shoot a 36 gramm lead ball through a coconut, with nothing but the same muscle that would not even dent the nut should I through that ball at it with all the force I can muster. The conversion capabilities of natural rubber (slow strength into fast speed) astound me very much.

Jörg


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## Flatband (Dec 18, 2009)

I've tested Urethane Elastomers,namely Vulkollan a product made and licensed by Bayer in Germany. The return to form was excellent on the product number I tested and they do have formulations with more elongation and a softer durometer (which is what we want). The bad thing that I noticed is the wear characteristics were'nt too good. They (Bayer) have all different mixes that you can get. The problem is blending them. You have to be licensed by Bayer to mix them. I did find a company in the U.S that is licensed but they didn't use a Polyurethane elastomer with the low Durometer I wanted. I tried to contact Bayer but they never did get back to me. I guess a square meter wasn't a decent sized purchase? There are also Polymer Elastomers that have extraordinary elongation (over 1200%) but they are very soft almost a gel consistency. The 55 shore A is a little too stiff for our use. I would love to get a sheet with a Durometer of around 30 and see what she can do. Maybe Joerg can contact Bayer for more information. I think he runs a company over there. They might take him more seriously coming from another company rather then an individual. Experimentation? I love it!! Flatband


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## Warhammer1 (Dec 27, 2009)

That math assumes a constant force(like gravity). Here some additional mathematics that might be helpful to some. http://www.intmath.c...near-motion.php



AaronC said:


> So, I have been researching the different types of rubber out there, and I found a page that compares some different elastics.
> 
> Here is the link:
> http://www.polyurethane-1.com/tensile-strength.htm
> ...


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## Scary (Jan 2, 2010)

What I wonder is, is it possible for the manufacturers of some of these "decent" synthetics to extrude/form the bands in a way where we would not see the typical failures caused by the formation of those tiny holes that keep growing. I don't know a name for the phenomena, but they almost look like little entrapped bubbles that start to rupture and the in crease in size rapidly.Scary


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## Warhammer1 (Dec 27, 2009)

Thats what happens when ya over extend them past their rec. optimal range. Its a good way to trash your bands. When that happens (and I do it) I call them "pooched" meaning they have permantly lost a percentage of performance. Either dont pull back so far (you can actually feel it happening), or just add another inch or so of rubber.

If that dont give you satisfactory performance, then you need to upgrade to a heavier band...



Scary said:


> What I wonder is, is it possible for the manufacturers of some of these "decent" synthetics to extrude/form the bands in a way where we would not see the typical failures caused by the formation of those tiny holes that keep growing. I don't know a name for the phenomena, but they almost look like little entrapped bubbles that start to rupture and the in crease in size rapidly.Scary


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## Scary (Jan 2, 2010)

Thanks Warhammer, but I was refering to the use of purely synthetic elastomers. I have been shooting for 25yrs and have yet to find a synthetic that performs as well as the natural latex rubbers. I have never had holes form in latex the way they form in synthetics. I have a feeling it has to do with the elastomer chaining pattern.







Scary


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## mr.joel (Dec 21, 2009)

how do you think linatex compares in this regard?


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## Flatband (Dec 18, 2009)

Linatex is good stuff provided you get a good cut piece. I had bought some a few years ago and the measurements were all over the map. I mean bad too. We're talking in somepieces, differences of over .008" in a 12" piece of rubber! I made it work to my advantage and it was actually a rough cut gem in disguise. I put the thicker side of the rubber at the front of my tapered cuts and the thinner at the back,so in effect I had a "Double Taper " going. the bands really rocked! It made a total mess of the sheet though until I thought why not put the thicker at the back (pouch end ) for longer lasting sets.A little slower but what the heck! So I did and evened up the sheet! The sheet I have presently, I bought direct from the manufacturer (you have to buy a big piece) is awesome and is dead on at .062" thick (1/16").I believe the elongation percentage is stated at 870%. I have had that thickness problem happen on other rubber too. Make sure you measure and if off, make it work for you! Flatband


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## Scary (Jan 2, 2010)

I might have to give linatex a shot, I always thought it was similar to the synth blends used for large vegetable rubber bands.







Scary.


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## crazyslingshot (Dec 25, 2009)

Ballistic theory and knowledge needed. But I believe in practice more


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