# How much does shortening an arrow increase it's spine/stiffness



## CEM (Jun 14, 2010)

Anyone have a feel for how much shortening an arrow increases it's stiffness compared to say an increase in outer diameter or wall thickness? In other words, if you cut down a 2016 arrow by 2” is that equal to going up in size to a 2116, or is a 3” length reduction required, etc. Just looking for rule of thumb. Thanks in advance to anyone who responds.

-CEM


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## limbwalker (Sep 26, 2003)

There is no one answer to this question. No rule of thumb because every shaft and length arrow is different.

Best way to discern this is to check the Easton arrow guide.

A 31" 2016 puts you in the T7 box. Cut off 2" and that moves you 2 boxes (2114/2115). 3" moves you 3 boxes. 3 boxes away from a 2016 is equivalent to a 2116.

The Easton chart is your best source for comparing arrow spine and length to calculate dynamic spine.


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## Hank D Thoreau (Dec 9, 2008)

There are a couple of ways to approximate it. The one I use is the ratio of cubes where spine at length = (shaft length) **3 / (28) ** 3. where **3 is the cube of the number.

There is another method that uses standard shaft strength. Looking at my formulas, it looks like it goes like this:

Standard shaft strength = 28/spine
Shaft strength at length = standard shaft strength - (shaft length - 28) * 5
Spine at length = 28/Shaft strength at length

For my full length ACE 430 I get 0.680 using the first method, and 0.667 using the second.

The two methods are very close for my carbon arrows, but pretty far apart for my long (35 5/8 inch) aluminum's.

Full length Redline 460: 0.655, 0.646
Full length Goldtip 5575 (400): 0.597, 0.560
Full length Goldtip 3555 (500): 0.615, 0.609
Cut 30.5 inch X7 2312 (505): 0.653, 0.652
Full length X7 2312 (505): 0.827, 0.920
Full length X7 2315 (342): 0.612, 0.540

There is also the Stu Miller calculator, which I have never gotten to work, and a program from Italy that I found. These are all just approximations, but they are probably good enough.


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## Mika Savola (Sep 2, 2008)

Back in the day (30 years ago) it was so simple to choose a shaft with only a few aluminums to choose from, and only two point options, 7% and 9% NIBB for most shafts. A rule of thumb was that going from 1816 to 1916 for example, would require one inch of more arrow length, or about five pounds of extra bow weight...


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## Joe T (Apr 5, 2003)

There are two common approaches as regards arrow properties and "dynamic" spine.

One is the critical load criteria i.e. the arrow has to bend to get around the riser see http://margo.student.utwente.nl/sagi/artikel/egyptian/appx.html

The second approach is to get the arrow around the riser the rate of shaft flex has to be matched to the bow geometry see http://www.ocf.berkeley.edu/~archery/wp-content/uploads/2011/08/KNSU-paper-by-Lieu-version4.pdf


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## limbwalker (Sep 26, 2003)

> Back in the day (30 years ago)


LOL, what's wrong with today? Today IS your day...


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## Viper1 (Aug 21, 2003)

CEM -

For most arrows in the more commonly used spines and lengths, 1" translates to about 5# of dynamic spine. 
Understand that's ball park at best.


John - 


limbwalker said:


> LOL, what's wrong with today? Today IS your day...


Know what I think is wrong with today?
Too many people trying to fix things that aren't broken, and we both have enough stories of that ... :behindsof

Viper1 out.


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## limbwalker (Sep 26, 2003)

Viper, 

You can't complain about people not trying to fix things, and then complain about people not trying to fix things... LOL!


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## CEM (Jun 14, 2010)

Thank you everyone for the info.

-CEM,


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## Mika Savola (Sep 2, 2008)

limbwalker said:


> LOL, what's wrong with today? Today IS your day...


I 'm sorry, I thought that was just common expression of past times. English is not my native language...


"You do what you will and I'll do what I think"


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## lksseven (Mar 21, 2010)

Mike,

You used the phrase correctly. I think John, within the abbreviated confines of a 'Twitter philosopher', was just making a ricocheted philosophical quip. He does that to me, too!


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## limbwalker (Sep 26, 2003)

lksseven said:


> Mike,
> 
> You used the phrase correctly. I think John, within the abbreviated confines of a 'Twitter philosopher', was just making a ricocheted philosophical quip. He does that to me, too!


Yup. I just giggle when I hear people say "back in MY day" and there they are, still alive and kicking...  Just seems ironic to me, that's all.


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