Yes, I understand it doesn't matter how long the shaft is that is being spined and also the dynamics of spine...

But how in the world does one spine a 26" arrow without it falling off posts that are 26" apart. What length would the shortest shaft be when sitting on 26" centers? I suspect that length would be 28".

Are you saying then... that a 28" woodie spined at 26" centers with a 50# spine is actually a 42# spine at 28"? I don't think so... [] or I'm confused...

I believe 28" is the standard from where all calculations of spine dynamics originate, with the woodies that is.