Pre-stretched vs normal band/tube

Discussion in 'General Slingshot Discussions' started by otta88sun, Nov 18, 2012.

  1. otta88sun

    otta88sun New Member

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    Hi again guys,<br>i saw in some Joerg's video that some sling-crossbow are build using a pre-stretched band/tube. Let me explain better, in his weapon there is a drawlenght of 100cm, instead to use a 25cm band/tube, he use 50cm but attatched near puchside of weapon. Then 2 pulley at the end of "barrel" take band/tube prestretching them and then u load the weapong pulling back the pouch.<br>I don't know if i've explained really good my answer, to be more clear look at this video<br><br><a href="http://www.youtube.com/watch?v=XTykf0KRYSo" target="_blank" rel="nofollow">http://www.youtube.com/watch?v=XTykf0KRYSo</a><br><br>I would like to know if using (at same draw weight, or "barrel" lenght) prestretched band/tube we can have more speed (at same bullet weight).<br><br>Thank's for reading<br>Ottavio
     
  2. zwillie

    zwillie New Member

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    Hi,<br>wow your good in physics <img src="http://illiweb.com/fa/i/smiles/icon_cheers.png" alt="cheers" longdesc="28"> .<br>I thought about the same thing.<br>Making longer (but prestreched) Bands to use the effect of (nearly) evenly power over the hole draw/acceleration-way.<br><br>Jörgs V - slingshot is made with rollers for that.<br>You get more drawway = accelerationway but it is not prestreched.<br>I will make one with alutubes and long inner sliding tubes.<br><br>Zwillie
     

  3. onnod

    onnod Im from Holland, isnt that weird?

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    i think i know what you mean, and in that case; yes it does mean more speed because you use less rubber on the same distance. right?
     
  4. otta88sun

    otta88sun New Member

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    <table width="90%" cellspacing="1" cellpadding="0" border="0" align="center">
    <tr><td><span class="genmed"><b>zwillie wrote:</b></span></td></tr>
    <tr><td class="quote">Hi,<br>wow your good in physics <img src="http://illiweb.com/fa/i/smiles/icon_cheers.png" alt="cheers" longdesc="28"> .<br>I thought about the same thing.<br>Making longer (but prestreched) Bands to use the effect of (nearly) evenly power over the hole draw/acceleration-way.<br><br>Jörgs V - slingshot is made with rollers for that.<br>You get more drawway = accelerationway but it is not prestreched.<br>I will make one with alutubes and long inner sliding tubes.<br><br>Zwillie</td></tr>
    </table>
    <span class="postbody"><br><br><br>Yeah V is not prestretched but u can have a little more energy coz the "not prestretched band" are into the slinghost and not out like other.<br>But i did not mean the V-Slingshot, but really the prestretched one.<br>This because with a pre-stretched one, u can pre-stretch 1 meter of band/tube <img src="http://illiweb.com/fa/i/smiles/icon_wink.gif" alt="Wink" longdesc="15"> and it wil be really more powerfull the V one <img src="http://illiweb.com/fa/i/smiles/icon_wink.gif" alt="Wink" longdesc="15"><br><br>When thera band arrive to my home i'll start to build it.<br><br>Hope Joerg read this and post his personal pre-stretched band/tube calc! <img src="http://illiweb.com/fa/i/smiles/icon_biggrin.png" alt="Very Happy" longdesc="1"></span>
     
  5. otta88sun

    otta88sun New Member

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    I think that pre-stretched are more powerfull coz joerg posted a video "twice long, twice more energy" or somthing like this and he showed how doubling lenght of bands/tubes we can dobule its speed. Sure, we'll not have the full power of a duoble drawlenght, but something really close i think.
     
  6. zwillie

    zwillie New Member

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    Hi,<br>okay here my thoughts about prestreched bands.<br><br>Here is a diagramm of a normal ss with 20cm Bands (deadway) streched with a 95cm drawlength and 30N weight at the angerpoint.<br>I would say it is about 12N average drawweight over the drawlength.<br><br><img src="http://up.picr.de/12525991yd.jpg" border="0" alt=""><br><br>Now we use bands of 50cm and strech them with 195cm drawlength.<br>Again we have have a weight of 30N at this point and 12N average drawweight over the drawlength.<br><br><img src="http://up.picr.de/12525992jt.jpg" border="0" alt=""><br><br><br>Now we have a closer look at the last 95cm drawway because our bands are prestreched.<br>At the starting point of the drawout we have 10N!!!!<br><br><img src="http://up.picr.de/12525993iq.jpg" border="0" alt=""><br><br><br>Now we have about 20N average drawweight over the drawlength.<br>With some kind of math (integral) you are able to calculate the energy in this area.<br>Sorry, my english is not good enough to explain this better.<br>Zwillie
     
  7. otta88sun

    otta88sun New Member

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    Hi, i did not understand what u want explain <img src="http://illiweb.com/fa/i/smiles/icon_sad.gif" alt="Sad" longdesc="3"><br>i'm a little confused on diagram too, so can u tell me if (as ur diagram says), is better a 20cm stretched to 80 or a 40cm (prestretched) stretched to 160?<br>Thank u <br>Otta
     
  8. zwillie

    zwillie New Member

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    I think it is better to have a 40cm band, prestreched to 80cm and then using the next 80cm as your drawway.<br>In the last 80cm there is a lot of power during the the acceleration.
     
  9. otta88sun

    otta88sun New Member

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    yeah, i think it too, infact in a slow motion video Joerg showed that u get more acceleration in last half of slinghost.<br>The V-slinghost, however, make me think that without pre-stretched u can earn a lot of speed (so energy) putting down the bands! <img src="http://illiweb.com/fa/i/smiles/icon_smile.gif" alt="Smile" longdesc="2">
     
  10. zwillie

    zwillie New Member

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    Here a diagramm of the drawweights over the drawlength.<br><br>Normal slingshot bandset (red) vs double length bands prestreched using the last 80cm (blue).<br><br><img src="http://up.picr.de/12529852xm.jpg" border="0" alt=""><br><br>Zwillie
     
  11. otta88sun

    otta88sun New Member

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    it seems like at max stretch them have same power but during acceleration, prestretched accelerate more...
     
  12. zwillie

    zwillie New Member

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    Yes that is the idea.<br>With stronger bands you get more power but you have also more drawweight at your anchorpoint.<br><br>Not so here.<br>We have the same weight at the anchorpoint but on the way to the fork we have more power in all sections.
     
  13. otta88sun

    otta88sun New Member

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    thank u <img src="http://illiweb.com/fa/i/smiles/icon_wink.gif" alt="Wink" longdesc="15">