Arrow Length and draw Weight

albatross

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Hello.

Can someone tell me why it is that with wooden arrows, the recommended draw weight for a shaft increases as the length increases. I thought that increasing the length weakens an arrow's spine and reduces the recommended draw weight. By the way I come from a recurve background and carbon arrows.
 

Raven's_Eye

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Ironman
Might be best with an example, 3 longbows 40lb draw but with draws of 26", 28", 30" (Please Note in these examples spines are probably wrong, I'm just using them to explain.)
Say the 28" draw needs 30-35 spine

26" draw because you've cut the arrow you've increased the stiffness of the shaft, so you start out with a more flexible shaft so by the time you've cut it down it's the 30-35 spine, but the shaft you've bought is 25-30.

30" Draw: Because the arrows are longer than the 28" the spine has actually decreased, so you need something stiffer to get it back to the right stiffness for the bow. So 35-40

Wooden spines are worked on based on 28" arrow (I think) so if your arrow is longer the spine is lower so increase the spine, if your arrow is shorter you don't need as stiff so reduce the spine.

Understand?
 

albatross

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I understand the principle of what you are saying about spine weights.

I was wondering why the same arrow spine (shown in the scale below) can be used on a more powerful bow if the length is increased?


If I select a 5/16" shaft of 35-42 spine. At 28" it's suitable for 30-35 draw weight. But if I use it at 31" it's rated for 40-45lbs.
 

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Raven's_Eye

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Ironman
Ah, that is because you are reading the table wrong. The column on the left is the bow's draw weight (it's drawing between 35-42lbs) not the arrow spine.

The columns on the left are the spines of arrows you should be using, depending on your arrow length.

So as I was saying before the longer the arrow you have the heavier spine you'll need to get the same flex.
 

ozzy1977

Member
You are reading the chart wrong, left section is for your draw weight, follow this line across to the right section to find the spine for your arrow length. eg if you shoot a longbow draw weigh 35-42 lb and your arrow length is 28" you would order 30-35 lb spine shafts
 

albatross

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Yep. 'Slap on back of head' Gotit. Easy when you know how! I'm too used to Easton charts.

Another question. Is there any way, other than shortening, to increase the spine of a wooden arrow.
 

bobnewboy

Member
You can increase the effective (dynamic) spine of an arrow by using a lighter pile, assuming you have a heavier one to start with. At loose, an arrow with a lighter pile will bend less (as it would with higher spine) as is stands in the bow, compared to the same arrow with a heavier pile. So it depends upon your original pile weight.
 

Rik

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Yep. 'Slap on back of head' Gotit. Easy when you know how! I'm too used to Easton charts.

Another question. Is there any way, other than shortening, to increase the spine of a wooden arrow.
Add weight at the back? That's universal. But watch the balance.
 

albatross

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Add weight at the back? That's universal. But watch the balance.
Thanks Rik for the suggestion. I have been told that 'footing' an arrow weakens its spine. So. Would putting a heavier 'rear footing' (oak as an example) work. The reason I asked the question is I have just made a spine measuring jig. I used a 2lb weight and a digital vernier to measure the deflection. According to my readings a couple of my arrows show a draw weight of 30ish lbs. But they were supplied as 35-40. They have been used several times and I was wondering if arrow spine weakens during use. But my jig will be measuring 'static' spine but I guess adding weight to the rear will effect dynamic spine.
 

Raven's_Eye

Active member
Ironman
The other problem is that with longbows the chart you posted before would only be a rough guide as the wideth and depth of the bows vary that you could use a highier spine than recommended because with a thinner bow you arrow won't need to bend as much. So test your arrows with your bow and see how they fly.
 

albatross

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I think I am beginning to understand the science behind these arrows. If I am correct. The static spine measurement gets you in the 'ballpark' for the bow draw weight. But you have to find which dynamic spine (by shooting) is best suited to your bow and shooting style.
 
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