Spine to length relationship of cedar

[slow reflection]

I recently decided to increase arrow spine from 50-60 to 60-65 to get a little extra length for broadhead clearance. I was surprised that to achieve the same spine I was only able to gain 1". I have not seen this published anywhere. Is this about the same results others have experienced?

[The Whittler]

I may get this wrong as it's been a while. If you have a 50-55 28" arrow and you increase that arrow one inch you should have 45-50 you go down in spine.

For every inch longer your arrow is then you go down one spine and just the opposite for every inch  shorter then 28" arrow. Hope this helps if I got it right.

[Biathlonman]

Yep, 1" equals ~5 lbs.

[Hud]

The spine for wood is based on 26", aluminum and graphite on 28". If you look at a spine chart, the type of bow, speed, degree of center shot, string material, and weight of head are variables to consider.  Check out the one 3Rivers has online.

[slowbowjoe]

Yup, go up one spine range for each additional inch of arrow length. So, if you were shooting 28" arrows at 55/60, you'll get the same dynamic spine with 60/65's at 29, or 65/70's at 30", etc.

And you will be getting heavier arrows, from both the extra inch of shaft, and the fact that (generally) grains per inch of the shaft also goes up with heavier spines.

[slow reflection]

Thanks for the replies

[Sam McMichael]

Does this refer to static spine, kinetic spine, or both?

[starshooter]

5lbs added for every inch beyond 28  and minus 5lbs for every inch less than 28".
Also,most spine testers are designed for arrows at  default 28".
Not sure about that one. Talk me down if I m wrong . Any arrow smiths out there ??
.

[starshooter]

Retraction...for every inch beyond 28 you lose 5lbs and for every inch less you gain 5 lbs.
My bad...Sorry.

[Pat B]

...and for ever 25gr of tip weight over or under 125gr changes the effective spine by 5# also. Over reduces, under increases.

[Orion]

Stuff is starting to get discombobulated. The 5# rule has been around for quite some time, i.e., add 5# of spine for every inch over 28 inches, subtract 5# for every inch under 28 inches.  Doesn't always work out that way, but a good place to start.

This refers to the static spine.  Dynamic spine is how the arrow behaves when shot.

The AMO spine measurement is done at 26 inches.  The arrow is suspended between two posts 26 inches apart and a 2# weight is hung on it and the defection determined.  

Easton developed another system using a 1.94# weight and posts separated by 28 inches for carbon arrows. The deflections the two systems produce are different, but one can be converted to the other.

Very clear, huh?

[Don Stokes]

Wood spine testers have the supports 26" apart, for determining the spine of 28" arrows. I find the 5#/inch rule to be accurate, as well as the roughly 25 grains/inch adjustment. Other things can also affect effective spine, like the type of bowstring, whether you use lots of silencers, or use a bow-mounted quiver. Everything you change in your setup affects the bow/arrow dynamics.

[Pat B]

...for a 28" arrow with a 125gr head!
I've used this method with cedars, spruce, ash, poplar, rear tapers, full taper and parallel shafts, sourwood shoots, dogwood shoots and different canes. It worked well for me with all different materials.

[Kelly]

One important thing most do not think about with wood shafting that the total length of the shaft needs to be used. If a 28" BOP arrow the wood shaft is 29" long so you need to use that total wood length in you calculation.