What spine you will require will depend a lot on the make of your bow, string material and how the riser is cut. Unfortunately, the standard equation for calculation of required spine (see below) is no longer valid for the typical recurve of today: it only gives you a minimum spine requirement that is likely to be as much as 20# lower than what it will take to get good arrow flight.
The equation is based on a 28" arrow, so for a 55# bow you would start with 55# spine; but you must add 5# of spine for each additional inch beyond 28" arrow length (length being valley of nock to back of point, not to point of your broadhead or field point) (subtract 5# for each inch under 28"). With a DL of 29.5" you will want a 30.5" arrow (at least 30.25"), so add 12.5# for an arrow longer than 28" (2.5" x 5#). So now you are up to 67.5#.
Have a FF or comparable "low-stretch" string?: add another 5# - my guess is that the low-strand-number FF-like strings (usually 8-12 instead of 16 or greater used 10 years ago) require even more arrow spine.
If you will use more than a 125 gr BH or FP, add another 5# for each additional 25 gr.
Now comes that most problematic issue: How your vertical shelf is cut on your bow riser. Older recurves were not cut to center or past center and required a weaker-spined arrow to "bend around" the bow as the it was released (archer's paradox). Newer bows, made of much stronger/efficient material, are now typically cut to center and usually further, especially if you have a costom bow (on a bow "cut to center" the string will line-up vertically with the edge of the riser that your arrow rests on). Also, modern bows are made with limb materials that give the bow much better cast, i.e., a 55# bow of today will throw an arrow of a given weight much faster/farther than one of yesteryear. This all requires a healthy increase in spine!
My guess is that the "equation" was most applicable to POC shafts; if you are using a heavier wood, such as Douglas fir, this will probably require some additional spine.
To give you an example, my 65# recurves (Schafer Silvertips) are cut past center and I use a 12-strand FF-like string, and arrows 30.5", with 125-gr heads. I use Doug fir shafts. Based on the old equation I would require an arrow with a spine of 82.5#:
65# +12.5# (for 2.5" beyond 28" arrow length)+5# (for FF string) = 82.5#
In reality, however, the best spine group (after much trial and error) for my bows is 100-110# (one good thing about bows cut past center is that they will generally shoot a broader spine group than the traditional 5# range).
The other thing you have to be careful about is how the shafts/finished arrows you are buying have been spined!!! The above is based on the traditional method (AMO?) of the amount of deflection (bend) in the shaft when a 2# weight is placed on it in the middle when the shaft is resting on 2 points 26" apart. It is important also that the grain of the shaft be aligned with the force of the weight (i.e., vertical when being spined, perpendicular to bow string when nocked). What you want are shafts that have been "hand spined" in this manner (usually sold as premium" shafts, but make sure you check). I just sent back 2 doz fir shafts that obviously were not (machine spined?) that came in 10-20# under what I ordered. Also, if you ordered shafts that are tapered on the last 8-10" of the nock end down to 5/16", be aware that the spine measurement is usually what the shaft was BEFORE tapering. Such tapering, while improving clearance of the shaft when shot out of your bow, reduces your spine 8-10#! So order extra spine if you are going to have them tapered.
In summary, based on my experience, when using wood shafts for the first time out of a bow, it is important to first order a set of test arrows that cover a range of spines, usually in 5-pound increments. For the info you gave me (without knowing your string material, how your shelf is cut and the weight of your points) for a minimum of 67.5# spine, I would use a range that starts at at least 70# (probably 75# is too low unless you're using an older bow), and extends to 95 or 100#, especially if your bow riser is cut past center and you use a FF or similar string (of course do not use such a string unless your bow was made for the added stress of these strings). If you have a custom bow, it is a good idea to ask the bowyer what he suggests for his bows.
Sorry to make this so complicated, but as you probably guessed I had to learn the hard way. Once you find your "sweet spot" when it comes to spine, wood arrows shoot like darts, and they're all I use.
Answered on 12/31/2012 by Anonymous