https://www.3riversarchery.com/dynamic-spine-arrow-calculator-from-3rivers-archery.html

 

Shorting arrows increases dynamic spine.

 

Yes

 

But deflection decreases, lol.

 

THIS IS A ROUGH CHART ILLUSTRATING HOW AN ARROW SHAFT SPINE CHANGES WITH LENGTH.
HANK D THOREAU did some calculations using a formula to come up with these.....so I saved it.
They should be ball-park close to right.
It shows that at 400, each 2 inch increase in length bumps spine by almost .100 .
Then, for .500s, from 28" - 30", it's more like .115, then 30"-32" weakens by almost .150.
Then, for .600s, 28" - 30" weakens by almost .150, and 30" - 32" weakens by around .160.

Calculated
Length 28 29 30 31 32
Spine 400 444 492 543 597
Spine 500 556 615 679 746
Spine 600 667 738 814 896

 

What about going the other way? I see arrows listed at say .500 and they come stock 31 inches. The spine is supposed to be calculated at 28" I think.

When they list an arrow at 500 are they assuming a 28 inch arrow or is it 500 at the stock length?

 

yeh arrows are marked for spine at 28".
the farther you get away from 28" in either direction, the more it changes, if that makes sense. a 20 inch chunk of .500 might spine .100 haha who knows.....
I did not care about shorter than 28" so I did not copy the entire table.

 

Im trying to figure how much spine Im losing by shortening my arrows. Thanks Kirk

What is your shortened arrow length?

 

You need to buy a archery software program like Archers Advantage, The Archery Program (TAP) or ON Target etc to model the bow and arrows. There is no simple equation that will deal with the variables of length, stiffness, front and rear shaft loading. Spine is a defined term using AMO(ATA) for wood beingthe deflection measured at 26" span between shaft supports with 2# weight. ASTM standard is used for carbon/alum arrows with 28" span and 1.94# weight. As long as the shaft is longer than the distance spanned by the shaft supports (26" wood, 28" carb/alum) the deflection reading will not change. Once you move the supports to a shorter distance the value obtained is rather meaningless as it no longer complies with a standard and the same reading can be obtained with multiple solutions.
The above archery software will allow you to model an arrow that flies well, then model different static spine rating shafts, with different lengths, point weights etc to find an equivalent well flying arrow.

For example I have a DAS21 with Border CVX-H limbs at 36# and 28" draw that will shoot either a 28.5" 600 spine carbon with 100 gr points or a 30" 500 spine carbon with 125 gr points well. Both arrows will land in the optimal spine area for the setup modeled in Archers Advantage.