differences in arrow lengths and head weights

[mspink]

Hey guys got a question for you. I have recently made 6 arrows out of red oak and four out of poplar. The red oak arrows are all around 550 grain all spined the same and around 28" long in total length. They fly perfectly. The four poplar arrows fly all wacky in comparison, They are at least 3 to 4" longer but exacly the same spine and weight. The heads are the same weight as the heads on the red oak. About 100 gr. Does the length make that big of a difference or is it the ratio of the length and the weight of the heads that is throwing it off? The red oak arrows are perfect but the poplar, not so much. What is the prob? And if it is the length then what weight should the heads be in comparison? Is there a formula or something to go by? Thanks for your help.

[Pat B]

For each inch over 28" you can subtract 5# os spine weight so at 3" to 4" longer you have reduced the effective spine weight by 15# to 20#.

[mspink]

so if i was spining the red oak arrows at say 55 lbs then i would spine the longer poplar arrows at 40 lbs or 60 lbs roughly?

[Pat B]

They will still read the same spine weight on a spine tester but the extra 3" to 4" will
 effectively reduce the spine weight by 5# per inch.
  This is an AMO standard. It also includes a 125 gr arrowhead. More weight will decrease the effective spine and less weight will increase it.