Spine Formula

[Mechslasher]

after years of experimenting, this is my formula for calculating a guesstamated spine for cane arrows, but it is a top secret, proprietory equation that must not duplicated or reproduced in any way.  use with care and i'll take full credit for your arrows hitting were you aim.

start with an arrow matched to your bow's weight.  for example, 55# spined arrow for a 55# bow.

55# spine + amount of taper (5-10#) +/- length (add 5# for every inch over 28" or subtract 5# for every inch under 28") - center cut (subtract 5# for every 1/4" from center) + recurve limbs (add 5#) + head weight (add 5# for every 25gr. over 125gr.) = adjusted spine weight

[aero86]

YAY!

[El Destructo]

                                                          ..........Huh.............

[DanaM]

Chris do you subtract 5 lbs for every 25g that the point is under 125g??? Also would you add 5 for highly reflexed bows?

Bow Weight   =   Draw weight of Bow
Amount of Taper   =   add 5 to 10 lbs for taper, based on degree of taper
Arrow length   =   0 for 28" +5 every inch over and -5 for every inch under
center cut      =              0 for center cut, -5 for every 1/4" from center cut
Point Weight   =   0 for 125g, +5 for every 25g over 125g, minus 5 for every 25g under 125g
Recurve      =              0 for straight, +5 for recurve

So for a 50# straight self bow shooting off the hand and 3/4" from center, 28" DL shooting 30" tonkin arrows with 160g points Spine

Bow = 50
Taper = +5
Arrow Length = +10
Center Cut = -15
Point weight = +5
Recurve = 0
Arrow Spine = 55#

[mullet]

 Dana, the heavier the point, the more it will reduce the spine.

[DanaM]

Thats what I thought eddie but this is what Chris wrote
"head weight (add 5# for every 25gr. over 125gr"

[zenmonkeyman]

By that reasoning, if you had a 500gr arrowhead, the shaft should be a wet noodle. 

[mullet]

Zen, which is what should happen. The arrow would bend more to push the extra weight.

[jackcrafty]

 

[El Destructo]

That is correct...if the Point is say....125 grains...and the perfect flying Arrow spines out at 60 pounds...then you increase your Point Weight to say...175 grains...the additional weight of the new Point...will lower the Spine of the Shafts to 50 pounds on you...so you would need a Stiffer Arrow to get the same Spine with the Heavier Points

[Badger]

    I do it exactly the way Machlesher does and it seems to work out pretty welll. Excellent place to start at least. I usually cut my arrows to 29" and spine them the same weight as the bow with 125 grain heads, Most of my arow passes are cut about 1/4" short of center. Steve

[jackcrafty]

Sorry about the yawn Mech, it's a Sunday.   

This is a good topic. The only thing I'm not clear on is the "degree of taper".  How does one measure that?

[mullet]

  Eyeball it and guess.

[riarcher]

By that reasoning, if you had a 500gr arrowhead, the shaft should be a wet noodle. 

Actually,, it would be a broom handle if you want to use it. 

Heavy head / heavy spine.

[mullet]

riarcher, you can also use it to reduce the spine in an arrow to shoot out of a different bow if you need to.