Originally posted by Blue Tick:
I'm curious as to where the 5 is coming in from for the recurve, centercut and so on. I've seen others use this method and I'm just curious as to what it is and where it came from.
Well, basically the spine weights of wood arrows are based off of a longbow, not cut to center, using a b50 string, using a 125 grain head, and drawn to 28".
So, theoretically, a 50#@28" Hill drawn to 28", for example, will take a 50# spine arrow with a 125 grain point. Any variation to the standard setup will require an adjustment in spine. There are several of these variations that will affect the spine.
The very first thing to consider is the actual draw length. Typically, adding or subtracting 5# for every inch over/under a 28" draw is necessary. A 26" draw will need -10# in spine, a 30 will need +10.
Second is the limb type. A recurve or d/r longbow is much more efficient than a straight-limbed longbow, and will require a stiffer shaft. The limb design itself might require a 5-10# increase in spine. The more efficient (faster) the bow, the more spine required. A selfbow might not be as efficient as the standard, and require a weaker spine.
Third, the centercut needs to be considered. If the standard is 1/4" from center is standard, anything over/under that needs to be accounted for. I can't recall exactly, but I think it might be 2# for every 1/8". So a bow cut to center will need 4# or so over a 1/4" from center.
Fourth, the string type. A skinny FF string will require as much as 10# more spine than a B50. A regular FF string might only need 5# more.
Fifth, the arrow length. The standard is for a 29" arrow. Longer length effectively reduces the spine. That's why trimming a shaft works so well for tuning. Usually, this is accounted for when the adjustment for the drawlength is made. However, I've found that if you want to shoot a shaft much longer than 1" over your draw, that extra length needs to be accounted for. If it didn't matter, trimming a shaft wouldn't work. I use 5# per inch.
Finally, the point weight. Added weight over 125 grains, weakens the shaft. I add 5# for each standard graduation over 125 grains. So, 145 needs +5, 160 needs +10, etc.
There you have it. This is typically how shafts were selected before arrow charts and calculators came about. Stu Miller's calculator is a much more refined version of this basic system. It'll get you in the ballpark. In fact, I've found it works exceptionally well for my Hills. I can pick any one of them off my rack and the shafts that shoot best out of it conform to the above very accurately. It's still pretty close with my r/d bows, but I usually use carbon out of those bows, and carbons are a different animal than wood.
The most sure way, however, is to get a set of test shafts in a wide variety of spines and actually shoot them to see which one works the best.
Topic: A little spine help... (Read 284 times)
