Should we rethink arrow spine ?

[Yew Selfbow]

The appropriate arrow spine selection for wooden longbow arrows is a discussion that crops up again and again with different people offering differing suggestions and recommendations.
The common consensus seems to be that the correct (or near correct) spine lies anything from 5lb to 15lb below the draw weight of the bow. But is the draw weight of the bow a correct indicator of the required stiffness of the shaft?
The required stiffness of the arrow shaft is a result of the magnitude of force applied to the arrow on release from the bow. Force is a product of mass X accelleration so there fore would it not make for a more accurate spine indicator if the limb speed (arrow accelleration) were the prime indicator factor and not the force required to bend the bow.
As a simplistic example, take 2 50lb at 28" bows. One has a limb speed of 1meter per second the other has a limb speed of 10 meters per second. The force exerted along the axis of the arrow is increased by the second bow by a factor of 10 so the arrows would need to be proportionally stiffer to resist excessive deflection caused by an increase of accelleration.
Of course it would only be of help if bowyers were able to provide limb speed data for there bows.

[Flying Whale]

Originally Posted by Yew Selfbow

The appropriate arrow spine selection for wooden longbow arrows is a discussion that crops up again and again with different people offering differing suggestions and recommendations.
The common consensus seems to be that the correct (or near correct) spine lies anything from 5lb to 15lb below the draw weight of the bow. But is the draw weight of the bow a correct indicator of the required stiffness of the shaft?
The required stiffness of the arrow shaft is a result of the magnitude of force applied to the arrow on release from the bow. Force is a product of mass X accelleration so there fore would it not make for a more accurate spine indicator if the limb speed (arrow accelleration) were the prime indicator factor and not the force required to bend the bow.
As a simplistic example, take 2 50lb at 28" bows. One has a limb speed of 1meter per second the other has a limb speed of 10 meters per second. The force exerted along the axis of the arrow is increased by the second bow by a factor of 10 so the arrows would need to be proportionally stiffer to resist excessive deflection caused by an increase of accelleration.
Of course it would only be of help if bowyers were able to provide limb speed data for there bows.

Draw weight can only be an indication, and a reasonable starting point. This is where an experienced bowyer sales person can help.

My experience is only from recurve, but I have 2 pairs of limbs. The first is 36lbs. I have used them all summer. I decided to buy some lighter limbs for inddors, because I do not need the cast. I bought some 32lb limbs. When I tried them out the arrow tuning was IDENTICAL to my 36lb limbs. The lighter limbs are Border TXG Golds, and I have since been told that they are notoriously fast. It seems the arrows are emerging from the bow with the same speed (confirmed by the fact my sight marks at 60 yards are identical) and the tuning is the same...quite a bonus as I diod not need to buy new arrows.

Sorry to drag recurves in to this section, but I thought it was illustrative.

[Yew Selfbow]

F.W.
I think you've made a very good point.
Were you given any advice from the retailer as to what would be an appropriate arrow spine for your new limbs?

[gwynn]

Yeah as you say - I spine mine 10lb less than the bow and they work pretty well - saying that I've shot arrows that are 'underspined' and those that are well over [according to the blurb] and the Longbow seems pretty tolerant throughout.
My ex coach, I think it was, just told me to experiment until I found a spine that suited me and my bow.
Helping a friend sort out arrows for a Scythian bow, shot with a thumbring, we found he had to 'overspine' by a large amount.
So yes I think none of this is written in stone - experiment!

[kernowtom]

Static spine is what an arrow does on a spine tester; basically how stiff it is, and that is how we buy arrows... because it can be measured. Dynamic spine is what it does on a loose, and there is no good scientific model of this. The obvious variables include:
1 - the arrow: length, mass, pile weight, fletching drag and weight, nock weight and grip, elasticity (ie not just how stiff it is, but how fast it bends and re-bends)all affect its dynamic spine. Tapering or barrelling will also affect the interaction between shaft and bow.
2 - the bow: weight is what we use to match arrows, but as said above, the limb acceleration is important and on top of that, tillering will affect the acceleration curve. For an extreme example.. compound bows have a different acceleration pattern to other bows (apart maybe from composites with recurves or siyahs), because of the cams. Bow handle width affects the paradox needed.
3 - the string: string mass affects acceleration, but also string stretch affects the acceleration curve applied to the arrow. the nocking point will also affect the vertical torque on the arrow.. wooden arrows are made to be stiffest laterally and irt seems easier to get porpoising than fishtailing.
4 - you... a clean release increases the initial acceleration - any sideways movement of the string at the release point might also be significant.

Ummm.. I am sure others will think of other things. but all that is enough to convince me that the comment about experimentation is the most important in this thread. There probably isn't a "right" spine... there is only what works for you with your bow. Ancient egyptian arrows were hopelessly underspined by modern standards, but killed plenty of people none the less.
BUT....
My challenge for today - does an underspined arrow fly left or right??????

[Flying Whale]

Originally Posted by Yew Selfbow

F.W.
I think you've made a very good point.
Were you given any advice from the retailer as to what would be an appropriate arrow spine for your new limbs?

Actually I bought them second hand. It was my local dealer who made the comment about the relative speed. It is nice to have somewhere to go where they happily talk and provide you with tea, even when you have just been and bought 2nd hand kit rather than new from them!!

[English Bowman]

Originally Posted by gwynn

My ex coach, I think it was, just told me to experiment until I found a spine that suited me and my bow.

Was that me you're talking about there Gwynn?

I have always said for an English Longbow assuming a draw of about 28" start about 2/3 of the weight of the bow, as a starting point and experiment until you get the best flight, it varies so much from bow to bow, and archer to archer that you can't draw a chart that will work for everyone.

Originally Posted by kernowtom

My challenge for today - does an underspined arrow fly left or right??????

Are you right or left handed?

If you're right handed, and using a Mediterranean Loose, the an underspined arrow will fly to the right. (usually)

Daniel

[steve58]

Hmmm, I have two sets of arrows for my 53lb Bickerstaffe, the heavy ones have 125grn piles and a 50/55 spine, the lighter ones (but only by about 10%) have a 45/50 spine and 100grn piles. 4" fletch on both sets. Outdoors they both come off nice and straight. Indoors at 20yds the lighter ones go right. If I shot this bow regulalry indoors I would use the lighter ones. Experiments are fun! I had some old shafts and built some variations, taper on the front, taper on the back, 100 vs 125 grn piles. I put 5" fletchings on and cut them down as I went along. All the shooting was at 60 yards. My conclusion was that a lighter pile, or a taper on either end, is worth one roll of the rubber band (about 1/4"). 1/2" off the fletching is worth a roll of the band. So if I had a taper (I liked the flight of the rear tapered ones best), a lighter pile and 2 1/2" fletchings rather than 4" I could in theory put the band 1 1/4" higher. This would make a significant difference to me at 100yds. I can see some money heading Hi-Force's way in the near future! Caution, what works for me may not work for you!

[robtattoo]

Slightly off topic, I know, but this is the best arrow tuning explaination I've ever read

http://www.bowmaker.net/index2.htm

I always experiment with any arrows I buy. It seems that a lot of folks in the UK haven't cottoned on to the fact that dynamic spine can easily be altered by changing pile weight & arrow length, shorter or lighter points to stiffen a shaft, longer or heavier to weaken a shaft. I see so many people at shoots with 40-60# ELBs, consistently shooting very weak shafts, when the shaft has been left at full 32" length & fitted with 125gn piles. When you mention anything, all I hear is "I can't warrant buying another dozen arrows, mate"

[nelly]

I agree, arrow length makes a big difference. I only have a 25" draw and shoot a 58# at 25" ELB. Ive long given up using any tables as they cant cope with these kind of numbers and only seem to work on average bows at average draw lengths