# Draw force vs. Elongation



## vladpills (Jan 30, 2015)

Hi guys, slingshot newbie here with a first post, so incase im asking a very frequently asked question, please dont be too harsh, and point me in the right direction where i should look.
Feel free to redirect this post.

So heres the story. Im fairly handy with my home made slingshots, which are all equipped with TBG. Ive killed alot of small game which was then eaten, but recently started thinking "what if i somehow miss a headshot, and just hurt the animal", thus, deciding to make a slingbow, which would seem a little more humane for hunting, especially with a small games arrow tip, which drops the animals fairly fast.

Ive purchased a barnetts cobra, because i figured an arrow must be heavy to deliver enough impact to the animal to kill it, and it must be shot at a relatively high FPS, thus if im using a heavy arrow and need high FPS, an arm brace would be a must in my case.
Also the barnetts cobra, has fairly low forks, and a very flat large area with a screw hole, which would let me securely attach my home made whisker biscuit.

The project is still not finished, but ill put pictures up when im done.

For the "strong tubes", ive ordered some Theratube silver (the newer ones). I cannot anywhere find the draw force per elongation of the tubes though! All i can find are the bands!

So the question being, if im shooting, a roughly 450-550 (30-36grams) grain arrow, and the draw length will be a max of 30" as the arrows length, what would be the length of the TTS needed to deliver the maximum FPS to the arrow? I of course understand that i will go through a stage of trial and error with the band adjustment, but i want to hear other peoples opinion first.

Also if anybody has some sort of information on the Theratubes as to draw weight per elongation, please share! 

Thanks in advance,

VP


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## kobe23 (Jun 28, 2010)

Polymers are nonlinear elastic material, it would be much easier if you could get a datasheet of force vs. elongation or force vs. displacement from the manufacturer.


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## vladpills (Jan 30, 2015)

Thank you for that kobe, but im afraid i cant locate anything regarding the TTS specifically :/ Ive tried my luck finding an archery calculator, which would perhaps allow me to calculate the FPS, when i have a known draw force, length, and projectile weight, but still to no avail :/

Still looking for help guys!

VP


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## pgandy (Aug 15, 2011)

My suggestion would be to make a bandset of a longer length than necessary and since the Cobra uses tubes take a few shots and then trim. Continue doing so with the same bandset until content with the results. Note the length and make future bandsets accordingly. Good hunting.

An afterthought; You might want to be thinking on how you are going to hold onto that arrow with the heavy draw weights. It is not as simple as holding a more or less round projectile that you pinch in front thereby holding it in place until released.


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## vladpills (Jan 30, 2015)

Thank you pgandy, that was kind of the response i was expecting, as in, "trial and error" 
I made sure to order extra tube, incase i might mess up the first trim adjustments.

This project is going to be a slingbow only, so regarding holding the arrow, instead of using a pouch, im going to connect the two pouch ends of the tube, with paracord, attach a D-loop, and use a mechanical release aid 

Will post pictures of the finished project, and some arrow groupings soon!

VP


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## Henry the Hermit (Jun 2, 2010)

Check out this chart.

http://slingshotforum.com/index.php?app=core&module=attach&section=attach&attach_rel_module=post&attach_id=20

TBS is not going to be very fast, because of its mass. It is also very heavy draw. Look at the chart and you will see that TBS requires 21 pounds of force to stretch to 3 times its relaxed length. From much experience we know that most rubber works best when stretched to at least 5 times its relaxed length. You will have two strands, so you are looking at about 100 pounds of force to stretch TBS to 5 times its relaxed length, and it's still going to be relatively slow. I predict you will be much happier and get better results with TB Green.

You can also find the slingbow guys here.

http://slingshotforum.com/forum/130-slingbow-forum/

Good luck and welcome to the best slingshot forum in the world.


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## vladpills (Jan 30, 2015)

Thanks for that Geezer Gang!  Its nice to finally be able to comment on here after such a long time of just browsing as an anon 

Regarding the draw weight, I didnt take the 5:1 into account :/ Completely forgot about it! Although the chart youve linked, shows only the BANDS, or is it the same for TUBES?
100lb will be quite too much alright :/ I shoot 80lb from a recurve bow, so i might have to settle for 80ish lb draw, and generously increase the arrow weight......thank god theyre hollow aluminium....*fills them with lead* 

If all fails, ive still got a set of Barnett Magnums (reds) that came with the cobra, which if im guessing correctly, are somewhere in the region of Thera tube greens.
If THAT doesnt work, well guess im going to havta backup to using TBG, as im completely broke now and wont be investing into any tubes for awhile :/

Thank you for the links, ill make sure to have a browse!

VP


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## Henry the Hermit (Jun 2, 2010)

That chart is correct for either tubes or full width lengths of bands.


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