Arrow spine differences

[tzolk]

#4133

Greetings!
I am looking to get some Easton XX78’s for my Martin Bamboo Viper Longbow and the Easton chart shows a 400 spine for a 31″ arrow at 61#. For a recurve the same weight and arrow length it shows a 300 spine. I currently shoot GT XT with 340 spines (7595) for my longbow and recurve. I don’t have the option to try the XX78’s before hand. Anyone with an opinion on why the spine differences and what I should go with?

[David Petersen]

#27601

tzolk — I haven’t a clue. But someone here surely will. And welcome! Dave

[Steve Sr.]

#29408

Key word for this is “centershot”.

The majority of longbows are less centershot than the majority of recurves and the more centershot it is, the stiffer of shaft you can use.

That said, one needs to look at the individual bow’s centershot when choosing but even then it can become a trial and error thing and shafts need to be cut long at first and worked down. Well, NEED isnt the word………but should give you the best results tuning the shaft to the individual bow and also the individual shooter.

Not used to tuning with a 31 inch arrow but even then a 300 seems a bit much. I do also know that eastons recommendation chart CHANGED about 1970. IMHO I felt that was due TO more bows being centershot and the resulting change was stiffer shafts for the entire group.

LOTS will depend on the centershot and head weight but a little experimentation is fun and educational so give it a go and let us know how it works out.

God Bless

Steve Sr.

[worldofvictor]

#30928

Steve Sr. wrote:

Key word for this is “centershot”.

The majority of longbows are less centershot than the majority of recurves and the more centershot it is, the stiffer of shaft you can use.

That said, one needs to look at the individual bow’s centershot when choosing but even then it can become a trial and error thing and shafts need to be cut long at first and worked down. Well, NEED isnt the word………but should give you the best results tuning the shaft to the individual bow and also the individual shooter.

Not used to tuning with a 31 inch arrow but even then a 300 seems a bit much. I do also know that eastons recommendation chart CHANGED about 1970. IMHO I felt that was due TO more bows being centershot and the resulting change was stiffer shafts for the entire group.

LOTS will depend on the centershot and head weight but a little experimentation is fun and educational so give it a go and let us know how it works out.

God Bless

Steve Sr.

Hi,

Can you please explain this:

“the more centershot it is, the stiffer of shaft you can use.” Why this is so? Just curious to know.

[Bounty Hunter]

#31559

Steve is so very right about the depth the shelf is cut making a huge difference in the spine of the arrow needed.

“Archer’s Paradox” is the termed used to explain what occurs to an arrow upon release of the string. You can do a search on the internet and find many explainations of Archer’s Paradox that are based on each individals perceived understanding. Basically an arrow flexes once the string is released and this flex causes it to bend away from the bow to start with and then in the opposite direction and so for until it recovers. This is actually beneficial as the arrow actually curves around the bow upon the release making most of the shaft miss the riser completely. Well that is if the arrow is spined correctly. If your arrow is too stiff you will actually hear it slapping the riser in some cases. I know I had this occur on my self bow that has no shelf until a found the right spine for the bow. The smaller (less depth) the shelf is the more flex or bend is required of the arrow to clear the bow. However too much flex/bend is just as bad as not enough flex/bend making the arrow take too long to recover or never fully recovering.

Center cut or past cent cut bows require less of a bend as there is nothing requiring the arrow to bend around. This will give you a more wide range of spines that will perform out of such bows. There is still a “sweet spine” for these bows, but spines a little too stiff will generally not drastically affect the flight of an arrow as it would in a bow with little or no shelf.

One other thing that makes a big difference in the required spine of an arrow is the amount of weight placed at it’s point end. The more weight on the point the more the arrow will flex on release. An arrow is propelled down range by a force that is applied to the nock end. This force is not applied evenly along the entire shaft to start out. The heavier the point weight is the more force must be transferred to get it to move, causing the shaft to bend more during this transfer. We know we don’t want this bend to be too much, so a stiffer spine is required to more efficiently transfer this force and maintain the correct Archer’s Paradox for correct flight. Hope this helps

[Steve Sr.]

#32139

Bounty Hunter nailed that one.

If you ever get a chance to tune an arrow to a D shaped longbow that is shot off an exterior rest or off your hand you will quickly find that spines WAY less than what are “suggested per that weight bow” will be mandatory.

Even the older Bear bows from the 50s that are not in any way centershot will be radically different.

For example I had a Bighorn TD pretty much centershot and spines I shot with (then) standard 125 grain heads was 15lbs OVER the bow poundage yet with my 50s bows, it’s common for me to shoot spines very close to the same as the bow weight and it’s not rare for me to have to leave them a little long for proper arrow flight indicating a shorter, lighter spine would also work.

I’ve mentioned before where I have altered (added to) the strike plate on one bow to allow me to shoot the same arrows out of bows of different weights but similar in design. This practice is using the exact same principal. Simply put, the less the bow is cut near centershot, the lighter spine I can successfully get great arrow flight from them.

Going extreme in FOC this information can also be used.

Picking the LIGHTEST mass weight carbon arrow increases the reachable FOC that arrow is capable of but often the spine……is way way light when going WAY down in grains per inch which often is the lightest SPINE of the shafts available.

Option here however is to use them out of a bow with zero or very little “cut out” for the rest leaving the amount of bow the arrow needs to move around at maximum OR…….building your shelf plate out reducing centershot.

Doc’s “D shaped” longbows, at least IMHO, are one of the reasons he is able to reach such UEFOC with a fine tuned arrow set up. The shaft are light spine (relative to bow weight) and also are the ones that normally are very light in GPI yet the lack of centershot on such bows makes the combo “doable” where in a centershot bow………they wouldnt fly for sour beans.

While we normally tune the arrows to the bow, a “too soft” spined arrow can often be used by tuning the bow to the arrow using this method. Experimenting with FOC found this to be normally the case for me when I reached a very high FOC end result yet found the head/shaft combo produced a shaft not stiff enough for the bow in use……..as is.

Where ya at Doc? You’re missed here!

God Bless

SteveSr.

[Bounty Hunter]

#33888

One other thing is that a lot of spine charts and spine calulators don’t take into account the different sizes of shelves longbows have. Most of them is a one size fits all setup, so be careful when using them to select arrows for your longbow.

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