Spine strength

[HoggPhan]

Ok. Not sure if the right forum for this article but i am sure someone will let me know if not.

Just in general, they say as a rule of thumb for choosing your arrow's spine strength that for recurves it should be +10# and for longbows it should be the same poundage as your bow.

If this is correct, as it is how i have been selecting my arrow shafts, i was getting arrows with a 50-55# spine strength because my Hummingbird longbow and recurve is 51# & 44# respectively. i had a thought the other day, i cut my arrows to 29". I don't have a spine tester so i don't know what the spine strength is now. I would think that the spine strength would change and that it would be stronger the shorter the shaft. Should i be getting a different spined shaft to start out with such as maybe a 45-50#? Or does it matter?

Thanks in advance.

[Orion]

Depends on how centershot the bow is, the bow design, the type of string you're shooting, the type of wood, the weight of the broadhead and length of the arrow, and perhaps a few other things.

Generally, because recurves are centershot or cut close to center, they will accept a wide range of shafts above the weight at your draw.   You need to spine a little closer to the bow's draw weight with less centershot bows, which most, but not all, longbows are.  

Then, you generally add spine for a number of other factors.  For example, 5# of spine for every inch you draw beyond 28 inches, 5# for fast flite as opposed to dacron string, 5# for heavy broadheads, i.e., 160 grains or more.  Also, because hardwoods are physically heavier than cedar and spruce, and because of their nature, they recover from paradox more slowly, one usually adds another 5-10# of spine for hardwoods.  And finally, if it's a high performance bow design like some Morrisons, ACSs and others, one could add another 5# of spine.

Most bows will shoot a range of spines quite well as long as you don't underspine.  I shoot most of my 1/8 inch from center longbows with arrows spined 10-15# heavier than the weight at my draw.

If your arrows are shooting well for you, no need to change anything.  If they're not, you might consider going up in spine a little.  Hope this helps.

[HoggPhan]

It does help but i should be starting from a spine strength from a cut arrow not the raw arrow shaft you buy from the seller, right? I am not the best shooter but love to shoot with my friends as probably all on here do, i am just trying to tighten up my group just a bit. I am sure my concentration before, during and after the shot needs to be tightened up a little too....

[Fletcher]

The measured static spine of the arrow won't change with the length, but the dynamic spine will.  Spines are measured between 26" centers with a two pound weight.  The amount of deflection determines the "spine" for a 28" bop arrow with the formula:
26/deflection in thousandths of an inch = spine in pounds.

In your example above, for a 29" bop arrow, you would need a 55-60 spine.  Then add or subtract as Orion describes above.  Simple, huh!   :eek:

[aromakr]

Hogg:
Fletcher gave you the basics. The spine standard is as he said, however you need to know as the arrow length gets longer than 28" you loose 5# of spine for every inch. So your 29" arrows are 5# lighter than what you bought. i.e. you bought 50/55 they are now 45/50. And it is true that the depth of the shelf will be a factor in selecting the proper spine. It makes no difference if its a longbow or a recurve. the deeper the shelf the more spine you will need, because as the shelf gets deeper the arrow does "NOT" need to go through as much paradox, but if you don't increase the spine it will. For a right hand shooter this will cause the arrow to hit to the right of your point of aim. A bow with little or no shelf the arrow will need to bend more to clear the riser, the deeper the shelf the less the arrow needs to bend, that will require a stiffer spine.
Bob