Main Discussion Area > Arrows
Compressing Shafts.
Diligence:
I think I have to both agree and disagree....and I want to be clear that I am not trying to act like an expert.....I am certainly not when it comes to archery, but I really like topics such as this as they provide insights into the "why" something is the way it is....
For wood, the modulus of elasticity (bending, not rupture) is really the measure of stiffness within the elastic zone. (i.e. how well does the wood react to loads applied perpendicular to the grain, loads small enough to not permanently cause set)...and could even be considered the resistance to shearing along the parallel grain orientation.
If I am not mistaken:
I agree that deflection caused by bending (classic beam theory) is highly dependant on the moment of inertia "I" (related to the relative geometric dimensions) and thus when the diameter is changed, so does the moment of inertia decease, and the deflection increases. (--sand away the shaft and the arrow spine gets lower)
However, when wood is compressed (ie. burnished) the density of the affected zone increases. Density of wood and strand/grain orientation is closely linked to strength and stiffness and increasing the wood density does increase the modulus of elasiticity. (---increase the density of the wood and the arrow spine gets higher)
Further, an arrow shaft without a burnished layer will be generally the same density of material (aside from ring density differences). But a burnished arrow is really a composite member, albeit with a very thin layer of more dense material. (think, a hollow cylinder of compressed wood filled with un-compressed wood)
I have to wonder if this denser layer, with a higher modulus (E) will decrease the deflection of an arrow shaft (during spine testing), more than the diameter change will increase the deflection of the shaft (during spine testing). Then again, maybe neither of these are as important as proper grain orientation when measuring arrow spine...?
It might be that the change in modulus resulting from the density increase is so small that it cannot counter the spine effects caused from decreased moment of inertia as a result of changing the diameter.
If changing the density of a wood had no effect on modulus, then heat treating the belly of a bow would have no noticeable affect either.
what do you think? Have I got it all mixed up?
Cheers,
J
ZanderPommo:
Hell I don't know ???
however it does say in the 3riversarchery catalog that the compression block reduces the spine of your shafting
mullet:
I'm grabbing a box of popcorn and pulling up a chair. ;)
CraigMBeckett:
Diligence
E = modulus of Elasticity = Young's modulus = tensile modulus = the ratio of tensile stress to tensile strain. Now why would a fibrous material like wood have an increase in E just because it is compressed laterally. The same fibres are there to resist the tensile strain. If any change were to happen because of compression beyond the elastic limit, which must have happened for the wood to remain compressed, it would be because the wood fibres were damaged and the modulus would reduce.
I suggest that increase in weight due to heat treating is because of changes to the wood's cells not the change in density. Also as heat treating actually drives moisture out of the bow the structure becomes less dense not more dense.
Have you weighed a bow or arrow before and after burnishing? I would suggest that all burnishing does is compress the surface layer to a much smoother one than it had originally and thus make it less likely that a splinter will lift, it does nothing to the strength of a bow or the arrow.
Craig.
ken75:
modulus..... what did you call me ?? ;) ;)
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