# does all rubber have same stretch specs?



## JetBlack

What I mean is some tubing seems to stretch further than other s,, usually thinner walled.like tex tube vs 1745.also even with thin strips of tbg does not seem to stretch as far as hygenics flats.granted again the tbg is a little thicker (at least what I have).i've heard 600 is about average for bottoming out.then i've also heard its really 900 percent.but many a tubing broke when I pull over 650.tell me what the deal is...


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## Tex-Shooter

Thera-Band gold and my .030 latex is supposed to be the same thickness. When it is not it is because of the tolerances. Pure latex will stretch further that any latex that has additives like Thera-Band, Linatex, rubber bands and etc. It will also stretch further than vulcanized cured rubbers like Gum rubber. There is a place for all kinds of rubber though! -- Tex


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## JetBlack

Thanks Tex, thought they felt different.so I may have to add a 1/4 to my active draw when using tbg.or just get more sheets from you....


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## boby

"Does all rubber have same stretch specs?"

This question has a nice ambiguity that makes me think of a range of issues.

You seem to be most interested in how far you can stretch it. If this is a question of your strength (as opposed to the rubber's), then if you double the thickness or width of a given type of flat bands, you will double the force needed to pull the band out to a given stretch factor (=ratio of length when stretched to unstretched length).

If you wonder about the strength of the rubber, there are two considerations. First, if you pull the rubber back far enough, it will eventually break at some stretch factor. This stretch factor at breakage will be about the same whether the bands are double-width, double thickness, or not. But this stretch factor at breakage is not that important, because by the time you get to this stretch factor, you are wasting a lot of force and energy and don't want to operate there. How so? As for force, as you pull back to stretch factors beyond about 5, the required force gets noticeably harder to draw further. I call this the "force wall", and I have written about it on the forum ( http://slingshotforum.com/topic/22870-force-wall-effects-in-slingshot-operation/ ). Most people find this hard to read, but the point of the analysis is that when the draw force increases dramatically near the end of the draw, the force is not very good at storing energy in the stretched rubber. So you would be better off for a given draw force to use wider/thicker bands and a lower stretch factor (a stretch factor of about 5.0 - 5.5 seems a good choice for both flat bands and tubes). Using tapering or pseudo-tapering is also helpful for reducing the effects of the force wall.

But there is also a problem with wasted/unconserved energy that gets rapidly worse at large stretch factors. I have only recently become aware of how large this effect is (e.g., 40% of energy can be lost at large stretch factors). What happens as you draw back the bands can be pictured as follows: The actual rubber can be viewed as being composed of thousands of micro-rubber bands attached to each other through sticky sides (a representation of long molecules tangled up with each other). As you pull the pouch back, all these micro-bands stretch out, but the more you pull the more some of these micro-bands lose their grip on each other and (internally) snap back. The energy it took to stretch these snapped back micro-bands is lost to heat etc. and will not contribute to the energy available for transfer to the projectile. The plot given here shows "reverse" draw pull force vs stretch factor. This means, the draw starts at full stretch and finishes at no stretch. The plot shows, for example, that at a stretch factor of 4, the force would be about 5.5 lb if the full draw had been to a stretch factor of 4 (the "+" plotting symbols), but the force drops to only about 2.5 lb if the full draw had been to a stretch factor of 6 (the upside down triangle plotting symbols). As the energy available for transfer to the projectile is proportional to the area under these curves, one sees that a large fraction of energy is lost at large stretch factors (although not much for stretch factors below 4). If energy was conserved, all curves should have been on top of each other. This energy loss at larger stretch factors makes it even more favorable to operate at lower stretch factors (more toward 5 than 5.5, *for a given force*).

Lastly, all latex rubbers are not equal. Some have less mass to get the same stiffness/force and thus would be better for slingshots. Or some may have less of a problem with energy nonconservation (as described above). My calculations (now including non-conservation of energy) are yet to be tested against reality, but they show that Dankung tubes are similar to Hygenic tubes and that Hygenic flat bands might be about 5% faster than the tubes (for a given force).


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## zwillie

Hi boby,

this what I found out, thank you.

In Germany the university of Halle is working on Superelastic polymers with an elongation of 1500%.

http://www.chemie.uni-halle.de/bereiche_der_chemie/technische_chemie/ak_weidisch/forschung/elastomere/?lang=en

Zwillie


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## JetBlack

That's what I call butterfly bands! Could you imagine a quarter inch active set,I know the math if off but still you get my point.wonder how fast it contracts....


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## Amarsbar

That would be seriously cool


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