Physics of Theraband Gold

Discussion in 'General Slingshot Discussions' started by Enn Block, Sep 3, 2012.

  1. Enn Block

    Enn Block New Member

    13
    0
    0
    So, when I started my introduction thread I asked which forum would be best to ask questions about making a slingshot crossbow, and I was told that the general forum was fine. If I made this topic in the wrong place, please move it. Because I have so many questions about designing a slingshot crossbow I decided to divide my questions into 2 topics: 1 on the physics/limits/potential of Theraband (mostly on theraband gold), and the other on designing a slingshot crossbow in terms of body design, materials to use, etc.<br><br>My questions on Theraband Gold:<br><br>I plan to use a pulley method (like Joerg's bullpup crossbow) so that the bands do not smack the frame.<br>*1. So I have to ask about firing ammo lighter/heavier than the crossbow is capable of: if I were to fire light ammo, would it cause a "dry firing" effect that would damage the bands? Conversely, if I were to fire ammo too heavy for the bands would it cause strain?<br>2. How much (if at all) can the bands withstand slight prestretching in order to hold them in place across the pulleys? Will there be a noticeable loss in band life?<br><br>Now on to more theoretical questions about bands:<br><br>3. Because Energy= .5 X Mass X Velocity X Velocity, I want to know what the maximum contraction speed of Theraband Gold is, in order to understand the maximum possible velocity. I know that because of tapering/stretch/band width the absolute number is not known, but any information you have regarding a range, either from Joerg's videos or your own knowledge would be valuable.<br><br>*4. On Joerg's band calculator there are settings for the amount of stretch, and the amount of tapering for bands, rated from longest life to maximum power. I was wondering how many shots you could get out of a band at what ratings. Say for example, a set of bands at 5 and 5, a set of bands at 1 and 1, and a set of bands at 9 and 9. Please post what band dimensions you use and at what ratings in these numbers if you don't have a clear number.<br><br>5. While on the issue of the band calculator, when it comes to stretch length, how do each of the numbers measure to the % in which the band is being stretched. For example: would a 1 on the scale be 400% while a 9 would be 600%? (I am just wildly guessing on those numbers and have no confidence in them, they are just examples of what sort of info I am asking)<br><br>6. Do other colors of theraband have a faster contraction speed? or is Gold the fastest?<br><br>7. If I were to set up a set of bands of another color of theraband at the same draw weight and tapering as a set of theraband gold bands, would it launch the same projectile at the same speed and power?<br>8. If so, would it be more durable or just as durable as the theraband gold?<br><br>*9. A basic physics question that I honestly do not know the answer to: if 2 objects of different mass (that have equal air resistance) are dropped, while they both fall at the same rate of 9.8 Meters/second, do they fall at the same speed? or does the heavier object fall at a faster speed x the rate of gravity?<br><br>There are multiple factors to the amount of energy imparted from a band to a projectile. The following questions are about what role each plays:<br><br>10. Is it correct to assume that the amount at which a band is stretched and the amount of tapering of a band is essentially the same force? That is, both factors on the band cause the band to extend more and therefore contract faster and contribute more strain?<br><br>11. What is the relationship between the number of bands to the speed/energy of a projectile? Let me take away a few variables to make this question easier to answer: Let's assume that Y is the energy released from a specific shape/length of band. if another band is added, does the energy released become 2Y? By adding a 3rd band does it increase to 3Y? Will the energy imparted by the bands increase by 1Y until the projectile reaches a point where the projectile accelerates to the speed at which the bands contract? Or by adding more bands will the energy imparted increase only a fraction more for each band added?<br><br>12. What is the relationship between the draw distance and the speed/energy of a projectile? Let's say that I were to make 2 sets of bands, both the same width, with no tapering, and both with lengths that if they were stretched to their draw distance they would be 1's on the calculator for length. Would these bands fire the same projectile at the same speed? If not, then:<br><br>13. How much more energy is put into a projectile by a longer draw length? theoretically would a draw length of 36 inches be greater than 24? would a draw length of 48 inches be greater than 36?
     
  2. pasquale

    pasquale Member

    282
    24
    18
    Sorry I cant answer every question.<br><br>9. If 2 objects have the same air resistance, they fall down at the same time. The mass doesnt matter.
     

  3. Enn Block

    Enn Block New Member

    13
    0
    0
    It's ok, I do not expect any single person to be able to answer every single question, that is why I put numbers next to each question, so a person could pick out which questions they can answer and just put the number next to the answer.<br>If you have made any slingshots or slingshot crossbows of your own using Joerg's band calculator, you can help answering question #4 by telling me what dimension the bands are, what they are rated in terms of the 1-9 scales for band life regarding draw distance and tapering, and how many shots you have been able to make with the bands before breaking.
     
  4. pasquale

    pasquale Member

    282
    24
    18
    I cant help you in the other question because, I have no theraband gold and so I didnt use the band calculator.<br><br>I use cheaper flatbands.
     
  5. Tysonspapa

    Tysonspapa New Member

    907
    0
    0
    on question 1: dry firing doesnt hurt rubber bands, thats why that company was able do make a bow firing arrows and steelballs with rubber, steelballs in 10 mm diameter are much lighter than an arrow, a normal bow couldnt handle that. <br>to much wieght shouldnt do any damage either, u just wont get the same acceleration and speed.
     
  6. Tobias

    Tobias New Member

    390
    0
    0
    # 4. the 1 and 1 setup made about 1000 shot and the 9 and 9 about 20 shot.<br>you can count the Strech to 9 thats no problem but at the tapering is 5 enough
     
  7. JoergS

    JoergS Administrator

    5,803
    109
    63
    <table width="90%" cellspacing="1" cellpadding="0" border="0" align="center">
    <tr><td><span class="genmed"><b>Enn Block wrote:</b></span></td></tr>
    <tr><td class="quote">
    <br>I plan to use a pulley method (like Joerg's bullpup crossbow) so that the bands do not smack the frame.<br>*1. So I have to ask about firing ammo lighter/heavier than the crossbow is capable of: if I were to fire light ammo, would it cause a "dry firing" effect that would damage the bands? Conversely, if I were to fire ammo too heavy for the bands would it cause strain?<br>
    </td></tr>
    </table>
    <span class="postbody"><br><br>Rubber is very forgiving. Too light ammo will cause handslaps on a normal slingshot, but for a crossbow, no problem. <br><br></span><table width="90%" cellspacing="1" cellpadding="0" border="0" align="center">
    <tr><td><span class="genmed"><b>Quote:</b></span></td></tr>
    <tr><td class="quote">
    <br>2. How much (if at all) can the bands withstand slight prestretching in order to hold them in place across the pulleys? Will there be a noticeable loss in band life?<br>
    </td></tr>
    </table>
    <span class="postbody"><br><br>Prestretching is OK up unto about four times the relaxed length. If you keep them fully stretched (5,5 or 6) for a longer time, they loose all flexibility and need time to recover - but if you don't go beyond factor four, things are good. <br><br>In general, stretched bands are more vulnerable against light and mechanical wear, though. <br><br></span><table width="90%" cellspacing="1" cellpadding="0" border="0" align="center">
    <tr><td><span class="genmed"><b>Quote:</b></span></td></tr>
    <tr><td class="quote">
    <br>Now on to more theoretical questions about bands:<br><br>3. Because Energy= .5 X Mass X Velocity X Velocity, I want to know what the maximum contraction speed of Theraband Gold is, in order to understand the maximum possible velocity. I know that because of tapering/stretch/band width the absolute number is not known, but any information you have regarding a range, either from Joerg's videos or your own knowledge would be valuable.<br>
    </td></tr>
    </table>
    <span class="postbody"><br><br>TB Gold can do about 150 m/s on a hot day. Extreme tapering and maximum stretch is needed. <br><br></span><table width="90%" cellspacing="1" cellpadding="0" border="0" align="center">
    <tr><td><span class="genmed"><b>Quote:</b></span></td></tr>
    <tr><td class="quote">
    <br>*4. On Joerg's band calculator there are settings for the amount of stretch, and the amount of tapering for bands, rated from longest life to maximum power. I was wondering how many shots you could get out of a band at what ratings. Say for example, a set of bands at 5 and 5, a set of bands at 1 and 1, and a set of bands at 9 and 9. Please post what band dimensions you use and at what ratings in these numbers if you don't have a clear number.<br>
    </td></tr>
    </table>
    <span class="postbody"><br><br>9 and 9, about 30 to 50 shots. 5 and 5, maybe 100 to 250. 1 and 1, thousands. <br><br>Of course I assume you have a roll cutter (new blade) and a good cutting mat. <br><br></span><table width="90%" cellspacing="1" cellpadding="0" border="0" align="center">
    <tr><td><span class="genmed"><b>Quote:</b></span></td></tr>
    <tr><td class="quote">
    <br>5. While on the issue of the band calculator, when it comes to stretch length, how do each of the numbers measure to the % in which the band is being stretched. For example: would a 1 on the scale be 400% while a 9 would be 600%? (I am just wildly guessing on those numbers and have no confidence in them, they are just examples of what sort of info I am asking)<br>
    </td></tr>
    </table>
    <span class="postbody"><br><br>1 is factor 3 plus 3 cm for the attachment. 9 is 5.48 plus 3 cm for the attachment. All other values are linear steps. <br><br></span><table width="90%" cellspacing="1" cellpadding="0" border="0" align="center">
    <tr><td><span class="genmed"><b>Quote:</b></span></td></tr>
    <tr><td class="quote">
    <br>6. Do other colors of theraband have a faster contraction speed? or is Gold the fastest?<br>
    </td></tr>
    </table>
    <span class="postbody"><br><br>Gold is actually the slowest. The thinner, the faster. But you can only notice this when you shoot very light ammo. Gold lasts longer and looks more clean, so I prefer it in most cases. <br><br></span><table width="90%" cellspacing="1" cellpadding="0" border="0" align="center">
    <tr><td><span class="genmed"><b>Quote:</b></span></td></tr>
    <tr><td class="quote">
    <br>7. If I were to set up a set of bands of another color of theraband at the same draw weight and tapering as a set of theraband gold bands, would it launch the same projectile at the same speed and power?<br>
    </td></tr>
    </table>
    <span class="postbody"><br><br>The thinner bands would have more performance when you shoot light ammo. No more noticeable difference with heavy ammo.<br><br>You would need more bands per side (or wider bands) nto get the same draw weight of course. <br><br></span><table width="90%" cellspacing="1" cellpadding="0" border="0" align="center">
    <tr><td><span class="genmed"><b>Quote:</b></span></td></tr>
    <tr><td class="quote">
    <br>8. If so, would it be more durable or just as durable as the theraband gold?<br>
    </td></tr>
    </table>
    <span class="postbody"><br><br>Less durable.<br><br></span><table width="90%" cellspacing="1" cellpadding="0" border="0" align="center">
    <tr><td><span class="genmed"><b>Quote:</b></span></td></tr>
    <tr><td class="quote">
    <br>*9. A basic physics question that I honestly do not know the answer to: if 2 objects of different mass (that have equal air resistance) are dropped, while they both fall at the same rate of 9.8 Meters/second, do they fall at the same speed? or does the heavier object fall at a faster speed x the rate of gravity?<br>
    </td></tr>
    </table>
    <span class="postbody"><br><br>In vacuum, everything falls at the same speed. In athmosphere, air resistance changes that. Dense, heavy objects fall faster. <br><br></span><table width="90%" cellspacing="1" cellpadding="0" border="0" align="center">
    <tr><td><span class="genmed"><b>Quote:</b></span></td></tr>
    <tr><td class="quote">
    <br>10. Is it correct to assume that the amount at which a band is stretched and the amount of tapering of a band is essentially the same force? That is, both factors on the band cause the band to extend more and therefore contract faster and contribute more strain?<br>
    </td></tr>
    </table>
    <span class="postbody"><br><br>Tapering means the bands stretch more at the thinner parts. So yes, your statement is true.<br><br></span><table width="90%" cellspacing="1" cellpadding="0" border="0" align="center">
    <tr><td><span class="genmed"><b>Quote:</b></span></td></tr>
    <tr><td class="quote">
    <br>11. What is the relationship between the number of bands to the speed/energy of a projectile? Let me take away a few variables to make this question easier to answer: Let's assume that Y is the energy released from a specific shape/length of band. if another band is added, does the energy released become 2Y? By adding a 3rd band does it increase to 3Y? Will the energy imparted by the bands increase by 1Y until the projectile reaches a point where the projectile accelerates to the speed at which the bands contract? Or by adding more bands will the energy imparted increase only a fraction more for each band added?<br>
    </td></tr>
    </table>
    <span class="postbody"><br><br>Heavy bands are good for heavy balls. If you shoot a light projectile with heavy bands, it will actually fly slower than shot with thinner bands. You need to find the right relation between band and ammo.<br><br></span><table width="90%" cellspacing="1" cellpadding="0" border="0" align="center">
    <tr><td><span class="genmed"><b>Quote:</b></span></td></tr>
    <tr><td class="quote">
    <br>12. What is the relationship between the draw distance and the speed/energy of a projectile? Let's say that I were to make 2 sets of bands, both the same width, with no tapering, and both with lengths that if they were stretched to their draw distance they would be 1's on the calculator for length. Would these bands fire the same projectile at the same speed? If not, then:<br>
    </td></tr>
    </table>
    <span class="postbody"><br><br>The acceleration length is an important factor. You can increase the energy either by increasing the draw weight or the draw length. Of course if you max out both, you will get the best power (see my rooftop slingshot). <br><br></span><table width="90%" cellspacing="1" cellpadding="0" border="0" align="center">
    <tr><td><span class="genmed"><b>Quote:</b></span></td></tr>
    <tr><td class="quote">
    <br>13. How much more energy is put into a projectile by a longer draw length? theoretically would a draw length of 36 inches be greater than 24? would a draw length of 48 inches be greater than 36?</td></tr>
    </table>
    <span class="postbody"><br><br>In general, yes, a longer acceleration will always increase the shot energy.<br><br>The closer you get to the max speed of the rubber, the less you will gain from an even longer acceleration.</span>
     
  8. Enn Block

    Enn Block New Member

    13
    0
    0
    Thank you Joerg! People usually get intimidated when I ask a lot of technical questions. This info really helps me out.