# Calculating Elongation



## The Gopher (Aug 25, 2010)

OK guys, here is a question for you. In my engineering mind I consider % elongation to be calculated by: (draw length-static length)/static length*100. As an example and with an unrealistic setup to keep it simple, if my static was 10" and my draw was 20" that is 100% elongation in my book.

However, I think most people are considering %elongation to be the ratio of draw to static, so in this example they would say 20/10*100 = 200%

I know it may be semantics, but it makes a big difference and may be misleading to someone new.

Is the general consensus among the slingshot community to use the ratio and consider this the elongation?


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## Can-Opener (May 11, 2013)

I take my draw length and divide it by 5 to get a 500 percent elongation. So a 40" draw length I would use 8" of what you are calling static length I have always called it active length and it dose not include the band that is under the wrap. If I want 400 percent I divide by 4.

475 percent of stretch divide by 4.75

450 percent divide by 4.5 and so on 

600 percent which would be maximum for most elastics divide by 6


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## StretchandEat (Nov 11, 2015)

I agree with Can-Opener..


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## The Gopher (Aug 25, 2010)

thanks guys, I'm assuming this is the most common approach, and I accept that, but it isn't a % elongation, we should call it a stretch ratio or something. I guess I'm just being picky.


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## StretchandEat (Nov 11, 2015)

I just call it 5 to 1.. so i guess your right


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## KawKan (May 11, 2013)

Gopher,

I had the same issues with the semantics/math. And I came to the same conclusion regarding the common usage.

The ratio seems to be what everyone uses. Okay as long as we're on the same wavelength.


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## The Gopher (Aug 25, 2010)

Yup, I'm fine with it, I just want to be speaking the same language as everyone else. its funny, I've been into slingshots for many years now and I've always used "real" elongation calcs, I never gave it much thought, because I made my bands to what worked for me anyway. now I know the rest of the story!

By the way, I have a friend who owns a restaurant in Lawrence, its just not a city name I see much, take care, Dan.


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## brucered (Dec 30, 2015)

I always use the 4:1 or 5:1 language too.


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## Clang! (Jan 16, 2017)

Another engineer here, and I tend to tally everything up to set the band strain (engineering definition) to around 300%. I figure that if we have the training, we should use it.

Example problem.

+Distance from fork attachment to anchor = 29"

- Half the distance between pouch attachment holes = 1.5"

- Band attachment to fork (Spanish style cord loop) = 1.5"

- Band half total band loop allowance (1 inch loop at each end) = 1"

active stretched length = 29-1.5-1.5-1"=25"

active static length=25/(1+300%)=6.25"

Rule of thumb example problem:

29"/6.25"=4.64, which puts it nicely in the 4:1 to 5:1 range


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## Henry the Hermit (Jun 2, 2010)

No need to complicate things. Most slingers just divide stretched length by relaxed length to get stretch factor. IOW 36 inch draw divided by 6 inch relaxed length = 6:1 stretch factor. Keeps things simple.


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## albertdd55 (Apr 28, 2017)

If you want to do anything "scientific" regarding elastic material research and development in the interest of improving your band performance, use scientific terminology and use % elongation as indicated by the engineers above. If you are just messing around with rubber stuff to see if you can shoot things further with your slingshot, use whatever terminology you want.

yah, I'm an engineer too


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## Cjw (Nov 1, 2012)

I doubt Rufus Hussy or any of the old slingers of the past did that much calculating their band sets . And they could still out shoot most people here. Over complicating things. The problem with being so precise is rubber you buy varies from batch to batch. I've cut Thera Gold the exact same length and width with the same pouch and one set shoots perfect and the other set lobs the ball at the target. I've even purchased looped tube sets from our venders that are exactly the same cut and pouch and they pull and shoot different. It's not an exact science. I've been a competitive rifle and pistol shooter for over 30 years and ammunition varies from lot number to lot number. So when you find a round your gun shoots well you buy enough of that to last you through your competitions through the year. Rubber is way less precisely made. It's not being made for shooting accuracy in most cases. It's being made for exercise and therapy and other applications and it doesn't have to be that precise.

Sent from my iPhone using Tapatalk


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## brucered (Dec 30, 2015)

I never measure my bands any more. I eyeball it against my hand spread out when tying to the frame.

As @Cjw said, no need to overcomplicate it.

Band it up and shoot. For most of us, a 1/4 here or there or even an inch here or there, isn't going go make ot break our shooting session.

Overcomplicating, over researching, over thinking can only lead to obsessing over stuff and driving the fun out of it.


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