# any conversion on draw length vs band lenth



## crapshot (May 3, 2011)

was wondering was there a size chart depending on your draw length to band length to maximize both speed and economy plus also depends on what your using tubes gum or linatex or thera band


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## Jacktrevally (Feb 14, 2011)

Crapshot, there is a calculator which has just been released by Joerg on his blog for Theraband.

It takes into account the ammo, power you want, band life and your draw.

It then calculates the dimension of your bands.

http://slingshotchannel.com/band_calc.html


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## Charles (Aug 26, 2010)

That is a very handy calculator. Thanks for the link.

Cheers ........ Charles


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## ZDP-189 (Mar 14, 2010)

Jacktrevally said:


> Crapshot, there is a calculator which has just been released by Joerg on his blog for Theraband.
> 
> It takes into account the ammo, power you want, band life and your draw.
> 
> ...


I would agree with the length dimensions as long as you use his stretch factor between 7 and 9. The lower numbers give you an unreasonably low draw, especially for anything thinner than TBG. Presumably, you'd take one factor unit off is you are drawing in cold weather.

His tapers are a bit aggressive for my tastes. I'd recommend a taper factor between three and four. Maybe I need to experiment with more aggressive tapers, but his higher taper factors are sure to cause rapid failure at the pouch tie, especially if coupled with higher elongations.

I am not sure about his width calculation. I have a hunch they would underband by about a third of the width, producing velocities somewhere in the 40-50m/s range. His tapers do not adjust when you increase the projectile mass; heavy projectiles will be slower than lighter ones, even though the width has increased.

If you use anything thinner than Theraband Gold, you will have to scale up the widths accordingly. Even for TBG, I'd add some, maybe as much as a third.

But all said, it's the only publicly available band cutting calculator I know of and it's a good starting point for your own experiments. Nobody cuts a better band than the shooter himself, once he has a little experience.


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## ZDP-189 (Mar 14, 2010)

Hmm, he also did not take into account of the dead mass of pouch and band ties plus excess rubber at the tie.

If you are curious, you should read the following blog entries:
http://slingshotforum.com/blog/11/entry-261-how-to-adjust-band-dimensions-for-different-sizes-of-ball/
http://slingshotforum.com/blog/11/entry-219-calculation-resources/

Calculation resources includes a few excel spreadsheets, including one for scaling band widths that illustrates this point about dead mass. It does not recommend tapers, nor lengths as this was beyond the scope of what I needed it for, but it is more rigorous in other areas.


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## Jacktrevally (Feb 14, 2011)

Thank you Dan.

I'm interested in reading the 2 blog entries.

Your blog is a wealth of knowledge. Have you considered making a CD format with all the blog writings and the excel calc files?

Or availaible as a zip file compilation for Download. Might be very useful. I can't run the excel sheets from my phone. If I could get that on a computer, I would defintly read all the material and try to understand how it works out.

-------

BTW, I can't dload Joerg oogle book file on my phone. he says it's on a pdf format. I would like to read his band manual. Any direct link?


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