Hi!
There are a lot of topics about spine issues, but there is one I haven't found. I have bought some very cheap aluminum arrows on ****. These arrows are 8,3mm in diameter, and 0,42mm wall thickness. This gives me roughly a 2117 arrow, or a 21(16.5) to be exact. Is it possible to calculate the spine (deflection) from this? They were sold as 400 spine, but I am not sure this is the case.