Question on arrow length and spine

[Jjpso]

Hello there.

I have been reading about things that affect spine and i got a dougth
If you arrow spine is, for example 45#, this is only true if the arrow length is 28 inches. But If your arrow is for example 30 inches long the spine will only be 35#.
Is this rule true, despite you draw length?! For example, my draw length is 25, if i have 40#-45# spine and are 30 inches long, in reallity they will be 30#-35# spined?

I hope you can understand what i mean...

Thanks,

Joao

[Pat B]

Yes, that is correct. The shaft will bend at its center, not the center of your draw length. Tip weight will also affect spine. The 45# spined arrow should have a 125gr point on it. Heavier head will reduce the spine, lighter will increase it.

[Danzn Bar]

Jipso,
I think your confusing true/static and dynamic spine....
True or static spine is what the shaft is when it has been placed on a spine tester no matter how long the shaft is.
Dynamic spine is the way the arrow responds when shot.
When we say spine is reduced 5# for every 1" over 28" we are talking about dynamic spine.
Hope this helps
DBar

[duke3192]

D. B. If spine is reduced 5# for every inch over 28", why are spine testers set up for 26" centers.

[Danzn Bar]

Duke,
Not all spine testers are....Remember for static or true spine.....
Modern shafts (carbon, aluminum) are spined according to the modern standards (ASTM F2031-05) an arrow's spine deflection is measured by hanging a 1.94 lb. weight in the center at 28".
Traditional/Primitive shafts (wood, cane, bamboo, shoot etc.) is usually expressed using the old AMO standard of 2 lb. and 26".

Why, I don't know.  But when we talk about reducing 5# for every inch over 28" we are talking about how the arrow reacts when shot using 28 just as a starting point.  There is really no correlation to the 26" used for static spine and 28" when expressing dynamic spine.

I hope I answered your question   
DBar 

[Jjpso]

Pat b and Danz bar, Thanks for the help, my dougth came because my draw length is just 25, 26 the most, and i was not sure that the rule applied the same way. Now i get it.

Thanks, this is really a Nice forum!

Regards from Portugal

Joćo Pedro

[BrokenArrow]

Does the 28 inch spine referred to here include the length of the field point?

[Grasshopper Mouse]

Arrow length is typically measured from the throat of the nock, where the string rests, to the back of the point.

Guy

[Buck67]

Static versus Dynamic Spine.  There's always something new to learn.