# Calculating Active Length



## AppalachianFlipShooter (May 9, 2020)

Hey all! 
I've been shooting semi butterfly draw for a while now, and am still going guess work when it comes to what my active band length should be, especially when using different thicknesses and tapers. My usual go to thickness for shooting is .66 GZK, but I'll switch it up every now and then with something thinner, and of course when hunting season gets here I'll go thicker. My usual taper is 3/4-1/2, or 15-10mm depending on the frame I'm using. Does anyone have any suggestions as to what I can do to calculate a general active length, or is it going to be different regardless because of the thickness of the latex?

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## Grandpa Grumpy (Apr 21, 2013)

As a starting point I divide my draw length by 5. This gives me the the distance between the pouch and the fork. I add an inch or so for tying and make adjustments from there.


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## KawKan (May 11, 2013)

Here's my thinking on calculating lengths for latex.

The fewer variables that are involved, the more reliable the calculations are for predicting the performance of a new cut.

So, if we're working with the same thickness and compound or latex, I am convinced that if you double the draw length, you can reduce the taper widths by half and achieve the same performance. It works for me.

I have not tried extending the calculation, say for a 1.5 times longer draw length.

BUT - changing thickness, and brands makes it real tough in my experience. Some compounds seem to hit a wall at about 5 times the active length, and others seem to be willing to stretch to 6 times or more.


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## KawKan (May 11, 2013)

Oh, for measuring a semi-butterfly draw, I usually put my tape in my slingshot hand and pull the tape like it was a pouch.

You can also use a string, then measure that.

Most of us divide the draw length somewhere between 4 and 5 for an active length. or at least a starting place, like Grampa Grumpy said.


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## AppalachianFlipShooter (May 9, 2020)

KawKan said:


> Here's my thinking on calculating lengths for latex.
> The fewer variables that are involved, the more reliable the calculations are for predicting the performance of a new cut.
> So, if we're working with the same thickness and compound or latex, I am convinced that if you double the draw length, you can reduce the taper widths by half and achieve the same performance. It works for me.
> I have not tried extending the calculation, say for a 1.5 times longer draw length.
> BUT - changing thickness, and brands makes it real tough in my experience. Some compounds seem to hit a wall at about 5 times the active length, and others seem to be willing to stretch to 6 times or more.


I've sold everything I had that was an odd brand and I'm going with all GZK latex.

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## urbanshooter (Aug 23, 2017)

Elastics vary and some may have more elongation than others. So it depends on whether you want to shoot your bands maxed out or at a more optimal, just short of max. Generally, 5 times is a good starting point and adjustments will need to be made depending on the elastic and how you like to shoot it.


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## MakoPat (Mar 12, 2018)

All very solid advice.

I use the same calculations as Kawkan, but I only shoot a very short draw length anchored under the ear at my beard.

But as mentioned by Urbanshooter sometimes the material is just not the same... and my body is off somedays.

So my solution is as tedious as myself. I add 2" to my desired band length, 1" for tying pouch & to forks. 1" is for tuning to my preference. I shoot 10 or so shots. If the bands are too soft/weak, I untie at the forks and trim 1/4" or 1/8" off then reattach. Repeat til I am slamming how I want.

Good luck.


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## SLINGDUDE (Aug 15, 2018)

Grandpa Grumpy said:


> As a starting point I divide my draw length by 5. This gives me the the distance between the pouch and the fork. I add an inch or so for tying and make adjustments from there.


I pretty much use this method, with a little trial and error.


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## hoggy (Apr 17, 2017)

good info from one and all.


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## robbo (Jun 8, 2019)

i use an online width by breath by thickeness calculater, if you have 150mm active length by 25x15 of .60 put the specs in the calculater and the specs of some one else as a guide line to whats close to yours and ajust acordinly to get the same amount of rubber in your draw length all good fun mate putting the exact amount of rubber from a 65 inch butterfly draw to a 30inch standard draw or vise versa its got a lot to do with the overall weight off everthing.


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