Spining arrows for short bows?

[redhillwoods]

Hey everyone
I've just started using this forum and love it. What a great community!
I've made many arrows in the last few years and spined all of them for a 28"draw using the standard method. What's the formula for spining arrows for a short-bow with a 21-22" draw?

[bradsmith2010]

i dont know of a formula,, I just use trail and error,,
I find the length and spine that shoots well  by trying lots of arrows, and then duplicate that arrow ,,,
was shooting a short draw bow the other day,,, bout 50@ 20 inches,, I shot a cedar shaft that was about 55# spine 28 inches,, with no tip,, it shot great,, go figure,, 
I feel like I could have put a light stone tip on it and had good results,,,

[Hawkdancer]

Welcome aboard!  There was a discussion in "Arrows" just recently about that.  These fos have a wealth of knowledge about bows and primitive skills!
Hawkdancer

[Pat B]

The standard formula is for each inch under 28" add 5# of spine weight. I don't know how this relates to short bows.

[aaron]

I think we are running into some terminology issues here-
Spining arrows is the process of measuring their flexibility by some standard means (for example the AMO standard). This method is the same no matter what length the arrow is, what length the bow is, etc. You can use the AMO standard, or another standard or make up your own as long as you always do it the same.

Finding the right spine arrows for any particular bow is another matter. Lets say we have an arrow that is 50lbs spine by AMO standards and a bow that draws 50 lbs at 28 inches. Lets say also that this bow happens to be one inch wide and 72 inches long. We shoot the bow and find that the arrow flies pretty well. If we were to draw the same bow and arrow to only 20 inches, the arrow would be too stiff. If we were to make the bow wider, the arrow would be too stiff. If we were to make the point lighter the arrow would be too stiff. If we were to make the arrow shorter it would be too stiff. If we were to tiller the bow down to 30 lbs at 28 the arrow would be too stiff. The "dynamic spine" of an arrow is the AMO spine modified by all these factors. If we were to shorten the bow and retiller to 50 lbs at 28 the arrow would still work the same (mostly).

 Each of these factors affects what stiffness arrow would work with any bow.

Most of us here just go by feel and what flys good. Some of us use "rules of thumb" like what pat B says below. Crazy people like me input measurements into a program called a spine calculator to really understand the interplay between all these factors. I use one called "Stu's dynamic spine calculator".
To answer your question, going from a 28 inch draw to a 21 inch draw would lower the draw weight by roughly 15-20 lbs and so an arrow 15-20 lbs lighter would be required, all else being equal.
Of course, another way to arrive at this is through trial and error- shoot a lot of arrows without fletching and keep the ones that fly good.

[willie]

AAron

do you think that stus calculator will work well so far out of range? (BTW, in a recent thread, Knoll pointed out that 3rivers has an online version of stus calculator on their webpage.)

I agree that trial and error is the final test. what else is there to consider besides nock/fletching clearance at the handle?

[aaron]

not sure what you mean by "so far out of range". With stu's calculator you can input all the above variables and more.
Besides nock and fletching clearance, the other thing to watch for is arrow flight. Any arrow leaves any bow with a bit of wobble. This wobble should straighten out within the first 10 yards. The arrow should stick into the target straight- perpendicular to it's flight.

[redhillwoods]

thanks so much for all your responses everyone. It seems like trial and error is the way.
Man this is such a valuable forum