# Rubber thickness, length, and draw



## JoergS

I found out something really interesting.

While experimenting with short bands, I found that the length of the bands has nothing to do with the draw force.

So if you use Thera Band Gold (or any other kind), the draw will always be the same as long as the draw factor is identical and the band width is the same.

So let us use hunterbands as an example. 6 cm fork, 4 cm pouch, thera Gold.

If you use 20 cm active band length, the draw is about 13 kg at 100 cm draw (factor 500%).

Now you use the same band dimension, but with an active band length of just 4 cm, then you draw out to 20 cm - the draw weight is still 13 kg.

Even using the Novitzkenegger with a draw of 300 cm (3 meters), the draw is the same as long as the band width stays.

Of course the energy will be much different, but the draw weight has NOTHING to do with the band length.

I don't understand the physics behind that, but my brother (who is a physicist) said that was to be expected.

Did you know that?

Jörg


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## Itamar J

i knew that 
i think i read it on the thera band web site a long time ago


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## Jax

JoergS said:


> I found out something really interesting.
> 
> While experimenting with short bands, I found that the length of the bands has nothing to do with the draw force.
> 
> So if you use Thera Band Gold (or any other kind), the draw will always be the same as long as the draw factor is identical and the band width is the same.
> 
> So let us use hunterbands as an example. 6 cm fork, 4 cm pouch, thera Gold.
> 
> If you use 20 cm active band length, the draw is about 13 kg at 100 cm draw (factor 500%).
> 
> Now you use the same band dimension, but with an active band length of just 4 cm, then you draw out to 20 cm - the draw weight is still 13 kg.
> 
> Even using the Novitzkenegger with a draw of 300 cm (3 meters), the draw is the same as long as the band width stays.
> 
> Of course the energy will be much different, but the draw weight has NOTHING to do with the band length.
> 
> I don't understand the physics behind that, but my brother (who is a physicist) said that was to be expected.
> 
> Did you know that?
> 
> Jörg


How does the energy change when you increase the draw length? Is it linear or does it increase more rapidly?


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## JoergS

This means it is possible to come up with an online calculator. You type in your Thera Band type and the band dimensions plus your draw length, and the calculator tells you the draw weight you will see.

Amazing!

Jörg


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## stelug

i beth this is why dankung tubes are so short. Chinese knowledge


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## JoergS

Don't get me wrong, shorter means less energy at the same draw weight.

We all know that butterfly shooting will give you the best speed at any given draw weight, but it is fascinating that the length of the rubber is irrelevant to the draw.


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## NaturalFork

Would this remain true on non tapered bands? I guess it would right?


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## Gwilym

Assuming that the band acts like a hookean solid and you stay in the elastic range that is exactly what physics predicts.


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## Gwilym

If you want make a calculator just use Youngs modulus rearranged for force 
F=(E*A0*(L2-L1))/L1
With f being force e being Youngs modulus( calculated from experimental results) ao being cross sectional surface area before stretching( thickness of rubber times width of band times number of bands used) and L2 and L1 being length of band when drawn an length of band at rest respectively. This won't work for tapered bands as that's more complex and I'm too tired to do the maths


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## Dan the Slingshot Man

that is very interesting I thought it would be harder


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## Gwilym

No not difFicult at all could even calculate e from joergs results if we know the thickness of the theraband allowing further calculations. Oh f should be in newtons not kg as it's a force not a mass but this is easily done by multiplying kg by9.18


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## dgui

It seems that the length or the shorter the bands the more prone you are to have hand slaps.


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## ZDP-189

It's simple: The peak force is the force required to extend the elastic to full elongation. It doesn't matter whether that's 10" or 10'; force is a static measure. How much energy depends on the area under the force-distance profile (*on contraction not elongation but that's another story). Thus, if you have a short bandset and pull it with a certain force, you'd get the same % elongation as a longer band, but less energy because the impulse is a shorter duration. If kinetic energy is similar to elastic potential energy (*see above) then as kinetic energy = 0.5 x mass x the square of velocity and as energy is less then the velocity is less. For example if you half the distance, then you half the energy and velocity goes down approximately 29%. but by now nobody's reading sooooo....

In short....
*
A short draw is just as hard to draw as long draw, but the shot is less powerful.*


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## Jax

ZDP-189 said:


> It's simple: The peak force is the force required to extend the elastic to full elongation. It doesn't matter whether that's 10" or 10'; force is a static measure. How much energy depends on the area under the force-distance profile (*on contraction not elongation but that's another story). Thus, if you have a short bandset and pull it with a certain force, you'd get the same % elongation as a longer band, but less energy because the impulse is a shorter duration. If kinetic energy is similar to elastic potential energy (*see above) then as kinetic energy = 0.5 x mass x the square of velocity and as energy is less then the velocity is less. For example if you half the distance, then you half the energy and velocity goes down approximately 29%. but by now nobody's reading sooooo....
> 
> In short....
> *
> A short draw is just as hard to draw as long draw, but the shot is less powerful.*


ZDP, a BIG Thank You to you !!
Reading your post I realised that I nearly dismissed a great concept without testing







. Now studying your small print (yes, somebody did read it!) I'm pretty sure that force applied to a projectile by the pouch must be acting on a lot shorter distance than the distance from release point to the point where projectile leaves the pouch. The word impuls is the key here. Now thinking about it and watching Joerg's slow motion videos again I think it must be even much shorter than half of that distance. Just a short impuls getting stronger with longer draw but yet only a fraction of the entire draw length.
Is that right?
If that's true then a very unique slingshot might see the light of the day and no it's nothing to do with bare back style







but until then -


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## ZDP-189

Jax said:


> ZDP, a BIG Thank You to you !!
> Reading your post I realised that I nearly dismissed a great concept without testing
> 
> 
> 
> 
> 
> 
> 
> . Now studying your small print (yes, somebody did read it!) I'm pretty sure that force applied to a projectile by the pouch must be acting on a lot shorter distance than the distance from release point to the point where projectile leaves the pouch. The word impuls is the key here. Now thinking about it and watching Joerg's slow motion videos again I think it must be even much shorter than half of that distance. Just a short impuls getting stronger with longer draw but yet only a fraction of the entire draw length.
> Is that right?
> If that's true then a very unique slingshot might see the light of the day and no it's nothing to do with bare back style
> 
> 
> 
> 
> 
> 
> 
> but until then -


Yes, the force declines as the band contracts; so the acceleration is greatest at first and declining to zero as the band returns to its original length. Wide short bands can have the energy of narrower long bands if draw strength is not a consideration, for example with a slingshot gun or a Y fork drawn my a power lifter. Lesser mortals who want proportionally more energy for less peak force should consider a longer draw. How long? Find the balance between how long you can draw and how fast the ball needs to be at your a draw weight at which you remain accurate.

A very detailed description of the physics is given in my blog and Mel's paper. There's lots of other pertinent information in the blog too, such as how hard you can draw at different draw lengths, how to find what the maximum accurate draw force is, etc. It's all in there.


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## Jax

ZDP-189 said:


> ZDP, a BIG Thank You to you !!
> Reading your post I realised that I nearly dismissed a great concept without testing
> 
> 
> 
> 
> 
> 
> 
> . Now studying your small print (yes, somebody did read it!) I'm pretty sure that force applied to a projectile by the pouch must be acting on a lot shorter distance than the distance from release point to the point where projectile leaves the pouch. The word impuls is the key here. Now thinking about it and watching Joerg's slow motion videos again I think it must be even much shorter than half of that distance. Just a short impuls getting stronger with longer draw but yet only a fraction of the entire draw length.
> Is that right?
> If that's true then a very unique slingshot might see the light of the day and no it's nothing to do with bare back style
> 
> 
> 
> 
> 
> 
> 
> but until then -


Yes, the force declines as the band contracts; so the acceleration is greatest at first and declining to zero as the band returns to its original length. Wide short bands can have the energy of narrower long bands if draw strength is not a consideration, for example with a slingshot gun or a Y fork drawn my a power lifter. Lesser mortals who want proportionally more energy for less peak force should consider a longer draw. How long? Find the balance between how long you can draw and how fast the ball needs to be at your a draw weight at which you remain accurate.

A very detailed description of the physics is given in my blog and Mel's paper. There's lots of other pertinent information in the blog too, such as how hard you can draw at different draw lengths, how to find what the maximum accurate draw force is, etc. It's all in there.
[/quote]
Thanks ZDP, this is VERY helpful


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