Thought I'd share an interesting finding, with a few prefaces:
First, I use a homemade spine tester - all credit to two-jays which I modified to have adjustable arms so that I can measure spine at any length from 26 to 34 inches (If you are asking, 'why would I do this?', read on...).
Second, I also buy a lot of gold tip 'blemished' arrows which I tend to break while stump shooting- often enough that I had 14-15 lying around of various lengths. Important to note that 'blemished' arrows often are just that- slightly flawed prints or other factory rejects- but can also include arrows that were rejected due to not meeting spine or length tolerances.
Now, if you don't do your own bareshaft testing with the ability to do two things, you are forced to be much more reliant on industry charts and guidelines when selecting arrows, usually with less than satisfactory results in my experience: Those two things are 1) the ability to change point weights - which means keeping an inventory of points (I have points ranging from 80 grains to 175 grains) and 2)the ability to use a dremel and sanding block to cut your own arrows. There are simply too many variables to get an accurate spine, length, and point weight from a catalog without doing real world testing. Everybody who says otherwise is either very lucky or simply unaware of what an enormous difference a well-matched set of arrows can do for accuracy and consistency. On this point, I know many people are uncomfortable cutting (or fletching) their own arrows. If you have the shop or store glue in the inserts when you purchased your arrows - or if you use commercial adhesives- you really can't remove them. What I do instead is use hot melt glue for all of my inserts, which means I can easily heat up and remove any insert I've put in. I've never lost an insert in a target using hot melt, but I have lost inserts using epoxy, super glue, and other adhesives.
SO after sorting the good and the bad blems, I was able to salvage 12 arrows by cutting down - in this case, from 32" (which I shoot on a 40lb Win&Win rig but will be shooting these on a 47lb Border recurve) to 30"- a length I found by cutting and bareshaft testing in 1/4" increments which took a LOT of time. However, of the dozen, I found two arrows that measured significantly weaker- nearly a full spine. So the question is: how much would I need to cut the weaker static spine arrows to behave the same as a stiffer arrow on the same bow?
There really isn't much guidance for that. But this is where dynamic spine comes in. Static spine measures the arrow at a pre-determined length: 26" with a 2lb weight using ATA standards. However, when you shoot an arrow, it flexes across the entire length of the arrow as a function of point weight, bow draw weight, and a few other lesser variables. Standard spine measurements only give you a relative measure vs a standard: a 500 spine arrow should flex .500 inches. However, if you measure arrows on a spine tester- from tip to tip rather than at the industry standard 26", a shorter arrow will show less flex than a longer arrow. This is a proxy for dynamic spine.
In principle, I should be able to get two arrows using the same point weights, but with different spines, to shoot the same from one bow, just by changing the length of the arrow.
So, will it work in actual practice?
Back to those weaker arrows. Based on my own bareshaft testing, I learned that 400 spine arrows cut down to 30 inches is what I need for my bow (using 80 grain points). So I measured the weak arrows to .650 by pushing in the arms of the spine tester until I got to an arrow length that gave me a reading of .650- which turned out to be exactly 1" shorter or 29". I cut them down to 29", added inserts and 80 grain points ,and presto- they shoot the same as the rest!
Two follow up comments: First, while in principle I should be able to buy an arrow of any spine- say 350 or 600, measure it to .650 deflection on a spine tester, and cut it- I can't go longer than the original arrow length, or shorter than my draw length.
Secondly, point weight affects spine, but it takes a lot of point to equal 1" of cut arrow- I'd say 70-80 grains of point weight. And while there are folks that say more point weight is better due to kinetic energy and penetration, my theory is that FOC is over-rated when it comes to accuracy. You just get a slower arrow which means more vertical drop at any distance, and - for my shooting style at least- using point weight alone to compensate for excessively stiff arrows gives me a less consistent, and far more finicky arrow to tune.
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