# Understanding arrow spine and draw weight



## Wolfenstein (Sep 21, 2010)

ust to fully understand the concept, an arrow's spine (including tip weight, nocks, fletching, etc) should match the weight I draw to correct? In other words I'm looking for the perfect spine to my bow. Drawing 30-31" on a 60# recurve @ 28" I should make an arrow spine to be +/- 1 or 2 lbs of my actual draw weight? (Approx 65#)


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## Viper1 (Aug 21, 2003)

Wolf - 

This probably won't answer your question directly, but here goes:

There are two types of "spine" we deal with. 

First and simplest is call static spine, and it's what's measured by putting a weight in the middle of a shaft and seeing how much it bends. That doesn't change regardless of length, head weight, you or anything else.

Next is "dynamic" spine and that's how much the arrow will flex from a given bow when shot by a given shooter. EVERY thing you can think of factors into that. (Arrow length, head weight, nock weight, bow's center shot, bow speed (not weight, directly), and the shooter!) That's what we are really dealing with. 

To really confuse things, static spine for wood arrows isn't measured the same as for aluminum and carbon - go figure.

Best (easiest) thing to do is know what static spine roughly corresponds to what draw weight and then adjust or "tune" accordingly, that will get you a dynamic spine that works for YOU. That's why I set up this chart. 

Now, since you're using a 30 - 31" arrow, you're about 2" OVER the std 28 - 29" length for spine ratings, you might need to go as much as 10# STIFFER than listed, that's where the dynamic spine thing comes in. In your case you'd need a arrow "spined" to about 75# (depending on the bow, of course). In the case of a modern 60-65# recurve, a 29" 2117 would work, if you had a std draw, but YOU would need a 31" 2216 to get in the ball park.

Clear as mud?

Viper1 out.


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## centershot (Sep 13, 2002)

Give this a try: http://heilakka.com/stumiller/ Lot's cheaper than buying arrows to try on your own.


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## Wolfenstein (Sep 21, 2010)

No worries Viper  I understand static and dynamic spine and you give a very good explanation. I guess I should have included more of my intentions associated with understanding this topic. I recently downloaded Stu Miller's calculator (as you noted above centershot) and as you know, you select a static spine and incorporating all other factors it will give a dynamic spine estimate. But when choosing, I was unsure whether I should aim for a static spine that when paired with the rest of my equipment yields a dynamic spine that is equal to my draw weight or one that exceeds it slightly. And I am assuming it's best to end up with an arrow slightly stiff?


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## Viper1 (Aug 21, 2003)

Wolf -

Any "calculator" over complicates or confuses the issue. Use something like the chart I provided, the OLD (pre-1983) Easton charts (yes, it's still out there) or the one on the www.bowmaker.net web site. (They are all basically the same, btw.) 

The problem with dynamic spine is that it's just that dynamic. I can present two bows with identical draw weights and the exact same parameters and they will not like the same arrows. Why? Because one is 10 fps faster than the other. That's why I said, bow weight doesn't matter directly. We can use it because it usually correlates pretty closely with limb acceleration, just not always. 

Short answer, there is no perfect arrow. There are arrows that can be tuned to fly perfectly - subtle difference. Corollary: an arrow that's pretty close can be make to act slightly weak or slightly stiff. 

Anywho, if you have to err, err on the side of slightly stiff. That's what most Oly types do and it seems to work for them 

Viper1 out.


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## Wolfenstein (Sep 21, 2010)

That's a very good argument. I never thought about limb acceleration and the actual velocity of the arrow. I just wanted to get an arrow that works very well for me without spending money on different arrows. I intend to go with the aluminums and a fairly heavy tip (just b/c its the tip I want, and they range from 140ish to 315 grains). I guess I should invest in the $6-10 for a set of tuning tips for my arrows.


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## Viper1 (Aug 21, 2003)

wolf - 

145 gr isn't out of reason for a broadhead on an aluminum arrow (actually used to be the norm), but 315 is downright bizarre - and just not necessary with these arrows. 

Here's the routine. In the "off" season, take your best guess at what arrow to use. The odds are it will be pretty darn close, using the references I cited. Over the following months, you can decide if they are slightly stiff. weak or OK. If you want to slightly correct it, use the *static spine charts* the go up or down slightly. With aluminum arrows, you have a lot of options. Again, changing spine and diameters will require retuning. It really sounds worse than it is.

Viper1 out.


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## Wolfenstein (Sep 21, 2010)

Yea and I don't mind tuning, I love shooting my bow and want to make sure all of my shots are the best they can be especially when hunting. I will definitely look over the chart and get some arrows and start shooting


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## Sm0lder (Aug 9, 2010)

I have a .PDF article called "Conquering the HIL by Jon Dudley", and it outlines a different way of looking at tuning. Instead of tuning a bow, one tunes the arrow. It makes a lot of sense since every bow is different. He does mention that using the arrow makers' arrow calculators will get you about 80% of the way. I can't seem to find the .PDF online but I have it on my computer and wouldn't mind emailing it to you if your interested. Just PM me to let me know.


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## BowmanJay (Jan 1, 2007)

You have to focus on both to have the most accurate combination in my opinion. You cant just focus on the arrow and neglect the bow....


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## Robrecht (Apr 1, 2010)

Do not forget that the with of the handle where the arrow needs to pass the bow also has an effect on what the perfect spine is. The wider the handle, the farther the arrow has to bend to "paradox" around the bow properly.

...


I love making things even more complex then they already are :59:


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## rattus58 (Jul 9, 2007)

Robrecht said:


> Do not forget that the with of the handle where the arrow needs to pass the bow also has an effect on what the perfect spine is. The wider the handle, the farther the arrow has to bend to "paradox" around the bow properly.
> 
> I love making things even more complex then they already are :59:


And hence you need a weaker spine to get there. One thing I like about the two metal riser recurves I now have, is that they shoot my 70# arrows with 45 and 50 pounds respectively. Personally, and I don't really like metal just cuz.... an over centershot riser has a lot of benefits in my opinion.

Much Aloha... :beer:


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## eugenec (Sep 27, 2011)

What is the scientific reason for using an arrow that flexes to match the bow? It appears to have something to do with the initial contact to the arrow rest and off center shot setup. Enter drop away rests. Do those considerations still apply if the rest is centered and drops away upon launch??


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## Thin Man (Feb 18, 2012)

The extreme example would be a longbow with no shelf and shooting off the hand. The arrow will not be in a straight line from the center of the string through the bow, but rather quite offset to the left (for a RH archer). This offset, combined with the less-than-center roll-off of the string from the opening fingers of the draw hand, and the inertia of the weighted tip, induces the initial flex in the shaft ... the beginning of the archer's paradox that must have a "wag", or oscillation, sufficiently wide to clear the riser. You might say that the arrow wants to go straight, but can't, so it's imperfect squeeze against the side plate induces a reactive bend in the shaft.

The offset of the thrust combines with the resistance of the point's weight wanting to move more slowly, adding to the oscillation. I guess you'd say that the tail of the arrow wants to move faster than the inertia of the heavier point, so combined with the angled thrust of the shaft against the riser, the bend takes place. This is why adding weight to the tip of the arrow causes an even weaker dynamic shaft flight with more bend than a lighter tip would produce ... a common tuning tactic.

If the arrow is too stiff, the oscillation will be narrow and the shaft will not clear the riser properly, often clicking against the riser, and will tend to shoot leftwards. (The riser contact can also induce a right shot as the shaft bounces away on the strike). If the arrow is too weak, the oscillation will be overly broad, and the arrow's flight may be corrupt as well as having a tendency to fly to the right. Extreme cases would have the arrow breaking due to the broad oscillation. 

Fletching is used to stabilize the arrow in the event perfection is not achieved in equipment matching or the archer's loose (ain't that the truth!).

As the shaft moves closer to the center of the riser, as on a cut-out shelf, the paradox can be appropriately more narrow, requiring a stiffer shaft. At a point, if you have the shaft laying dead center of the string's thrust, you can run into flight issues due to the fact that the string is still rolling off of the draw fingers in a less-than-centered manner, yet the shaft, being centered or even past center into the riser, is not allowed it's "bounce inducing paradox implementation" against the side plate. 

So all stickbows utilize the principles of shaft flex in combination with at least some outwards-of-center-line angle against the side plate. 

Compound bows shot with a mechanical release obtain a dead-center loose of the string, and therefore the shaft can be centered down the line of the string and shot through a drop-away rest. The stickbow shaft's paradox does not apply in the same manner to this type of shot, for the compound's release is "perfectly straight" in thrust compared to the offset release of a finger shooter. I believe adjustments can be made in this area on a compound, but I think the tuning is mainly done for nock height placement, rather than windage (don't quote me here, I ain't a compound shooter). A compound finger shooter would certainly need to account for paradox due to the nature of the draw hand's release, though. 

Scientific reason? Well, unscientifically put, if a stickbow's shaft don't bend around the riser properly, the archer ain't gonna hit the bullseye.

I hope this helps a bit, and others waiting in the wings will certainly have better scientific presentations that I.


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## eugenec (Sep 27, 2011)

That seems to agree with what I think of the "arrow spine myth". I believe for modern compound bows with fall away rest, arrow spine still play a role in safety (especially for carbon arrows), but not much in accuracy because there is nothing for the arrow to clear from.


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## bradd7 (Oct 17, 2008)

Look up Archer's paradox.


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## Viper1 (Aug 21, 2003)

Thin -

Really not a lot to add. 

I would just emphasize that any finger shot shot will have paradox due to the roll of the string around the finger tips, where any release shot bow will only generate paradox due to offset from center. 

Viper1 out.


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## LBR (Jan 1, 2004)

> an arrow's spine (including tip weight, nocks, fletching, etc) should match the weight I draw to correct?


Yes



> I should make an arrow spine to be +/- 1 or 2 lbs of my actual draw weight? (Approx 65#)


We aren't that precise. You will have to do some experimenting to find the right spine for you. Carbon arrows generally cover a wide range of spines with the same arrow. The better your release is, the wider the range of arrows you will be able to shoot.

It's not that complicated really.


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## WillAdams (Jun 6, 2009)

Sm0lder said:


> I have a .PDF article called "Conquering the HIL by Jon Dudley", and it outlines a different way of looking at tuning. Instead of tuning a bow, one tunes the arrow. It makes a lot of sense since every bow is different. He does mention that using the arrow makers' arrow calculators will get you about 80% of the way. I can't seem to find the .PDF online but I have it on my computer and wouldn't mind emailing it to you if your interested. Just PM me to let me know.


Do you mean the file:

http://www.eastonarchery.com/uploads/files/71_horizontal-impact.pdf

FWIW, I find the dynamic spine calculators a useful tool.

I was especially pleased to see that 3 Rivers Archery has adapted Stu's Excel spreadsheet and put it on-line:

http://www.3riversarchery.com/spinecalculator.asp

Interesting to experiment w/ (especially on a rainy day when one can't shoot) and much cheaper than a testing kit. I've always managed to get at least a tunable shaft, and in a couple of instances, using the calculator would have saved me from buying arrows which I couldn't get tuned to match a particular bow.


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## WillAdams (Jun 6, 2009)

Viper1 said:


> Use something like the chart I provided, the OLD (pre-1983) Easton charts (yes, it's still out there) or the one on the www.bowmaker.net web site. (They are all basically the same, btw.)


FYI, that seems to be a dead link --- it's now a parked GoDaddy site.


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## Viper1 (Aug 21, 2003)

Will - 

This thread is 3 years old. 
I think some one posted a link to it's new home a while back. 

Viper1 out.


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## WillAdams (Jun 6, 2009)

Okay.

For those who're curious, the new link seems to be:

http://www.acsbows.com/sitebuildercontent/sitebuilderfiles/tuninglongbowsandrecurves.pdf


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## eugenec (Sep 27, 2011)

It still sounds to me that very few of the "rules" about spine apply when *a release *is used and the *drop away arrow rest *is *centered*. I don't see why shooting a stiffer arrow hurts accuracy other than it may drop lower due to the weight, but that can be taken cared of by adjusting the sight to match the arrow.


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## Viper1 (Aug 21, 2003)

eugenec -

The same "rules" of Physics apply, but the parameters are different. 

A bow shot with a release can be tuned to true center shot, but most compounds are set slightly off to allow for some "user error". 
The effect is much greater with a finger shot bow, since it's the roll of the string around the fingers that initiates the paradox and the offset from center that "tames" it. 

Viper1 out.


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