Hello All, I'm new to bowhunting (loving it!). Seems like there are more arrow options than hand loads. Is there a chart or formula to calculate new spine after cutting my arrows to length? I understand graphite pretty good as I work for a fishing rod manufacture in the states. If I buy 32" shafts with a 400 spine, what is the spine after cutting down to 27"? How can I calculate the increase in stiffness? I appreciate any feedback.
Good question. The best way to account for the length is to use a spine chart or program. One of the variables will be the arrow length. So the recommendation for the spine of the arrow the lenth is factored in. For example. Say you shooting 65 pounds with 28" arrow and 100 grain head, you will very likely be a 350 spine. However say you shooting the same set up but with a 26in arrow, you may very well need a 400 Spine arrow.... just a hypothetical example.
Yes, as trial153 said. A spine chart works pretty well as long as you use a "standard" weight insert with a 100 or 125 grain tip. Here is a link to a decent spine chart. https://www.huntersfriend.com/carbon-arrow-spine-charts-sizing-deflection-research.html However, if you're after a higher Front-of-Center (FOC) arrow and plan to build with a much heavier insert or heads over 125 grains, then a calculator will serve you better. The reason for that is because adding weight to the front of an arrow weakens the spine fairly quickly. Here is a spine calculator, all you have to do is enter your numbers and it will spit out what spine you need. I still like to reference a chart when using a calculator though, just to see if I'm on the edge between two spines. If I am, always go with the stiffer spine. This is the GoldTip site, just click on the "Find Your Spine" button. https://www.goldtip.com/Resources/Spine-Chart.aspx
Been a Bow pro shop person for years. Cutting a 400 spine down does not change the spine ! As trial153 and Mod-1t stated, what spine you need all depends on. Length of arrow, draw weight of bow, as the chart suggests. I personally like a bit heavier arrow for hunting, I prefer (KE) over super speed. With the speeds of modern compounds even using a bit heavier arrow, you can get kill zone on deer on one pin out too 25yds, maybe more depending on your poundage. I'm a big fan of the "Kiss" method. All my hunting bows only had 2 site pins. Normally from a treestand your longest shot is 30 yds or less in woods, so only having to deal with one pin, sure makes things easier. I allowed myself a 3" high or low kill zone on a deer. Good luck this season.
I always make sure I can get one pin to 25 yds and I go hunt. So maybe I have said wrong, but shorter arrow does make it stiffer..I mean yeh, a 400 will always be a 400, but the shorter it gets doesn't it get stiffer?
Yes it does get stiffer. But spine applies more too length and bow draw weight. Things have changed a bit since when we first started shooting aluminums aka 2117, 2217 etc. Took me a bit in shop to get it right and still had to refresh my ol brain occasionally. I read an interesting statistic yrs ago that the majority of Archery taken deer are less than 15 yds. I'm sure with modern bows that has increased a bit, but most should not even try shooting a deer beyond 25 yds. Good luck this season.
Your question was answered at the end of the latest 2 Bowhunt or Die videos. I forget which one but PJ answered it in his tech question clip. I believe the answer was, yes it does affect the stiffness of the arrow shaft and that is why the charts are there for us. It’s all figured out due to the final length the arrow will be cut to.
Hi All, Again thanks for the many replies. A little follow-up I discovered, and many of you may already know this, but to any newcomers like myself: Most shaft makers use an industry standard of 1.94 lbs. to measure spine (resistance to flex under force = stiffness). We use similar method in the fishing industry to measure power (heavy, med heavy, etc.). Most arrow shaft manufactures also use industry standard of measuring at 28", not the entire uncut 32" you might be buying. This eases my concern of cutting a 32" down to 27". This of course poses the question, if your a "big and tall" and use 32" what is your actual spine? Based on what appears to be the industry standard for measurement and parameters, it is something less than what is advertised. Maybe not enough to be concerned with, but a fun question.
I received a lot of good feedback from some experienced folks on this thread, and it appears the charts / recommendations are mostly based on length and draw weight. This makes since to me as all chart matrix's basically are exactly that (length and lbs.). I understand the need to understand the spine and flight effect, seems simple. How they selected their recommendations is beyond my skill set, so I will trust the pros at this point.
