The general rule is subtract 5# for each inch below 28" with a standard 125 gr. point. It also calls for adding 5# for each inch above 28"; arrow length is measured from the bottom of the nock to the back of the head. Add 5# for fast flight or comparable strings. Add 5# for each 25 grains in added point weight above 125 grain. This means if you use a 150 gr. point, the spine should be increased by 5#.
Because you are shooting 42#, start with 40-45 lb spine, when you subtract or add. You may find that 45-50 also works as a base, for your recurve. Add or subtract from where ever you start.
If you cut them to 27.5" round that to 28". If you cut them to 26.5 round up to 27" before you add or subtract for string differences, or point weight.
If you draw 26.5", but cut your broaheads to 27.5, (round to 28"), and 26.5 (round to 27") for field points, you might find there is a difference, go with the stiffer shaft and if you need too, you can add a heavier point. It is harder to go lighter in point weight, because most broadheads are 125 gr. or more.
Keep in mind, that you can use a slightly higher spine than the recommended if the bow is center-shot. Your bow is close at 1/8" past center, so the arrow will need to flex as you release. The arrow pushes against the bow, bending inward, then outward as it passes the bow. This is the paradox.
The reason Traditional longbows shoot a lower spine is because they are not cut as close to center as most newer recurves.
Three Rivers has a spine chart for wood, aluminum and graphite shafts on their website.
If you are not sure, you might consider getting a test kit, with a dozen shafts covering a range of spine. If you go with the 4 shafts in each group, you will have 3 different spine ranges. Cut the shafts to the maximum length, with two fletched and two not for testing. You can use different wood as long as the weight is close. POC is lighter than fir and may shoot differently for you. Good luck
Topic: Figuring correct spine with wood arrows (Read 267 times)
