spine weight change

stack · June 17, 2015, 12:29:00 PM

Is there any formula that can tell me if I cut a full length shaft from 32" to 27" what the spine will be close to. I am trying to figure out what spine shaft to buy for my 39lb bow at 25.5" draw.
Should I get 35 to 40 spine or ???????????????????

Thanks
Bill

Hot Hap · June 17, 2015, 12:42:00 PM

As you cut the shaft down it will get stiffer. The accepted formula is 5# per inch-Hap

stack · June 17, 2015, 01:12:00 PM

So if I cut 5 inches off a 40 spine shaft it would go to a 65 spine shaft???

June 17, 2015, 01:26:00 PM

The particular bow, shaft material, amount of center shot and string will make a difference. If you post what model bow and what arrow material you want, you will get some ready made opinions and options. Here is one, my wife's draw and release is from 26" on a short draw to 26 & 1/4" on a longer draw. She shoots 27" back of point 1816s with 145 grain points out of her 38 pound at her draw, Lost Creek NAT. She also uses 40-45 pound tapered cedars 27" bop with 125 grain points.

stack · June 17, 2015, 01:38:00 PM

My bow draws 39lbs at 25.5. It is a hybred longbow that my son and I built from a binghams form. It is not center cut. I am wanting to use poc shafts with 145 grains up front. I would like to have my arrow at 27". I am using a Flemish twist string 12 strand.B97

Fletcher · June 17, 2015, 01:49:00 PM

The actual bare shaft "static" spine of the shaft will not change when cut, but the dynamic spine will change. Shafts are sorted and bought by static spine. Before making adjustments for depth of shelf cut and point weight, we generally reduce the static spine needed by 5 lbs for each inch of shaft less than 28" BOP. For your 39 lb and 27" BOP, I would start with 34 lb and then adjust for depth of cut and point weight. This will get you close.

stack · June 17, 2015, 02:00:00 PM

thanks for all the input.

June 17, 2015, 02:21:00 PM

I would guess that your bow is approximately 1/8" outside of center. With a 25.5" draw and a strong release, that is you are not losing too much draw length in the release process, with 1&1/2" of arrow in front of the bow at release a tapered Wapiti cedar with 145 grains up front, a spine in the 37 to 42 pound range would be right. If your release is softer 35-40. Make sure that your arrow nocks are not too tight. For my wife, her bow likes 40-42 spine, it is quite easy with a spine tester and some sand paper, to reduce a cedar shaft spine a couple of pounds and take some extra grains off. Quite often the stiffer shafts in a dozen are also a few grains heavier. I stick the shaft in a drill and take some material off, then take the cross grain sanding out with sanding strokes with the grain, being careful to sand evenly.

June 17, 2015, 02:34:00 PM

I should have added that I put the drill in a clamp and run the nock end of the arrow in hole in a clamped wood block to keep things stable. It goes pretty fast, it is easy to take too much.

JRY309 · June 17, 2015, 03:45:00 PM

Arrows are spined at 28" centers for aluminum and carbon.So a .500 spined carbon is a .500 spine at a 28" length,at say 32" it is not a .500 static spine.Usally wood arrows were spined at 26".You may want to try Stu's spine calculator or 3Rivers calculator?Or you can always pick a starting static spine and trim and tune for the dynamic spine.

old_goat2 · June 17, 2015, 09:36:00 PM

Why don't you buy some full length arrows and cut them down a 1\\4" at a time till they fly correctly. Arrows are the length they need to be too spine correctly for the bow \\tip weight you plan to shoot not some preplanned length! Just my opinion!