Has an effect on the spine of an arrow? I think so. If you have 2 seventy pound bows but the ATA is different doesn't the longer one increase poundage more gradually.

I haven't shot any of the shorter bows so I have no experience to go on, but my 38.5 ata bow with a 340 arrow for the most part always seems like it's to stiff.

If there is some truth to it that would help reduce the guessing about which arrow/spine is best.

Don't know the physics but when I moved myG5 from my 36.5 to 31.5 I was shooting 30yds off my 10 yrd pin. The shorter axel seems alot faster to me.

Speed has also got to have some thing to do with it. It would make sense that a heavier arrow would effect spine.

I don't know how they come up with spine charts but If I did oneI'd err on the side of stiff/heavier just for saftey, then again I think GT's are the opposite to get the shaft lighter (their sales pitch).

They sell arrows according to poundage and supposed speed/cam, a heavier arrow effects speed. If speed is a factor then picking the right spine should include that not just what cam you have.