Spine adjustment?
 

There is a lot of talk about the importance of correct spine, yet I rarely see this sharacteristic published in manufacturers data. Is this just grains / inch. I thought that was weight. If I need to increase my spine, do I just increase my grains / inch. If not do most web sights advertise individual arrow spines or is this something you measure on your own.

Also, when an arrow manufacturer adverstises + / - 2 grains per dozen. Does that mean 2 grains per inch per arrow or if you weigh a single entire arrow, it may be off two grains per inch from a second arrow. If my calculation are correct, a 30 inch arow could be off 60 grains in total weight. That seems like a lot. What tolerance should I be striving for?

RE: Spine adjustment?
 

Each arrow manufacture has their own standard for measuring spine. Take Beman for example. 340's are stiffer than 400's. You just have to look at a arrow chart. Generally arrows that are stiffer are heavier. This is simply a rule of thumb. +- 2 grains is for the entire arrow. 2 grains means almost nothing, it's not worth worrying about unless you are a target archer that is really really good.

RE: Spine adjustment?
 

As I thought about that question i realized it was alttle silly. Obviously arrows would have to be closer tolerance than 60 grains. So if you have a 240 grain arrow, then out of a dozen some may be as low as 238 and some 242, right?

RE: Spine adjustment?
 

Why does changing the length of an arrow or tip weight change spine? Obviously it must have some correlation with total arrow weight.

RE: Spine adjustment?
 

Because you are changing the stiffness of the arrow.Putting on a heavier brodhead will make the arrow less stiff,and conversely adding a lighter weight broadhead will make the arrow stiffer.

RE: Spine adjustment?
 

Ok when a manufacture puts their number on the arrows say beman with the 340's they measure static spine. They put a wieght on the arrow and how much the arrow bends, determines the static spine.

When you shot that 340 out of a bow that is dynamic spine. When you shot your arrow bends (Pretty dramatic if you have never seen this). Think of it this way, if you take your pincil and break it in half it breaks pretty easy right? Now do it again. It gets harder and harder to break. The same way with an arrow the shorter the arrow is the harder it is to bend. The same way with point weight think of a long pole picking up a weight. The heavier the weight the more bend you will get in the pole.

Here is the great things about length and arrow spine. I have a custom PSE bow I put hoyt command cams on. I tuned the bow to Beman 300's full length. I also tuned it with 400's that were as short as I could go. So the lengths alone allowed me to tune the bow to 3 arrows. 300,340,400.

Maybe someone can explain this better than me, it's kind of hard to explain. Keep the questions coming

RE: Spine adjustment?
 

Thanks for the analogy, that clears things up. Can I expect a dozen "middle of the line" arrows to have fairly consistent spine?

RE: Spine adjustment?
 

Ewolf,

What are your feeling on carbon aluminum composite arrows?

RE: Spine adjustment?
 

Thanks for putting that into perspective ewolf, I never did really understand that. So, I guess my 26 1/4 inch 2117 with a 100 grain head, is PRETTY stiff for a 55 lbs bow.

RE: Spine adjustment?
 

Nah, I think you did a pretty good job ewolf. I would only clear up the other question. Grains per inch has nothing to do with an arrows spine. All it determines is the arrows aoverall weight when you're done cutting it to length. What determines an arrow's spine is a combination of diameter and wall thickness. Large diameter, thin walled shafts can have the same spine as a small diameter, thick walled shaft---the difference being that the thin walled shaft will usually be lighter in wiehgt (grains per inch).

RE: Spine adjustment?
 

As for the differential in arrow weight, this is negligible on 20 and 30 yard shots and for a hunting setup there is little concern. It's when you shoot competition at longer yardages that the weight becomes a critical issue. I assemble my own arrows with each being cut at precisely the same length with all components identical. I'll then weigh all the completed arrows, without tips, and find the lightest. Then I screw in the tips and check the weights again using the lightest as my base. I'll grind the base of the tip until it matches the lightest completed arrow and do this for the entire dozen. Each arrow is exact in spine and weight. This will make a substantial difference in accuracy and tuning even on close shots. The length of the arrow effects spine. The longer the arrow the weaker the spine or the more the arrow will flex on release. In turn, a shorter arrow will stiffen spine. One helpful hint is to make sure your arrow is long enough so that the insert that holds your tip is NOT on the rest. If your insert rests on the rest or cushion plunger for fingers shooters it deadens the rest or plunger. Make sure you have at least 1/2" of the insert in front of the rest prongs or cushion plunger.;)

RE: Spine adjustment?
 

God bless this forum. Ten years ago who would have thought some much information could be available so quickly and cheaply.

Thanks everyone.

RE: Spine adjustment?
 

If you reduce the tip weight by grinding it, the effective spine will be stiffer. Although they may fly very close to the same, the spine would not be exact on each arrow. In addition, each arrow starts out with variable spine within the group. So, although you can get weight to be very close to exact, matching spine can be very difficult - especially on some arrow brands. To match spine to even a somewhat close measurement, you'd have to pick arrows with very consistant spine on each side and them cut shafts until you had matching effective spines. I don't even know how this would be measured unless you devised a spine tester that had a movable end-support, so you could test each arrow with the ends resting on the supports.

If I had an arrow that measured .330 spine and another in the group that measured .340, the stiffer arrow is also likely to the heavier one. Cutting the heavier one, will make it even more stiff, resulting in even greater spine variation among the group. If you're shooting the whole group on the stiff side, then flight differences (with broadheads) might not be that great, but if spine is borderline, then fairly large variation in flight may occur.