Arrow Spine

[Alexander Traditional]

I've got a little spare time on my hands,and been doing a lot of thinking. I don't think my question will matter one way or the other,and it is what it is.

I'm shooting a different bow of late,and tuning some aluminum arrows for it. I have 2217 doing the best and 2018 could work,but can't get that one to do just right.

What I'm wondering is 2217 and 2018 aren't that far off in spine. If all things are equal the 2117 is stiffer. I'm wondering if spine stays the same as the arrows are cut down. Could shafts change and reverse themselves as they are cut down? Like could the 2018 at one point in time get stiffer than the 2117? I'm asking with any two shafts? Or is this impossible?

F

[rustyspike]

It will increase the spine. Shorter you go the stiffer the arrow/shaft.

All the spines you see on shafts are calculated 28" draw.

Rich K.

[Alexander Traditional]

Yeah Rich I get that it will stiffen the spine,but I was wondering if a stiffer one at a longer length could get reversed as you cut the shafts down and for instance say the 2018 would be stiffer at some length? Probably thinking too much,and I know it doesn't matter.

[rustyspike]

I think you got it. Longer more flexible. Shorter makes it stiffer. About 3 lb per inch. More wight weight on the point will weaken the spine.

Rich K.

[McDave]

When you compare cutting 2217's and 2018’s, two things are happening:  first, the dynamic spine of each shaft increases proportionately to the amount that was cut off, so I wouldn't expect to see the relationship between the two change if this were the only thing that was happening.  In other words, I wouldn't expect the one that started out stiffer than the other one to become weaker than the other one as equal amounts were cut off each shaft. 

However, the 2217 is wider than the 2018, so the angle between the 2217 shaft and the centerline of the bow will increase more as the shaft is cut than the angle between the 2018 shaft and the centerline of the bow increases (assuming both were outside the centerline of the bow to start with).  This means that the 2217 will gain slightly more stiffness for the amount cut off than the 2018 will.  So if the 2217 started out slightly weaker than the 2018, I suppose it is possible that it could cross over and become stiffer at some point as more was cut off.

[Alexander Traditional]

When you compare cutting 2217's and 2018’s, two things are happening:  first, the dynamic spine of each shaft increases proportionately to the amount that was cut off, so I wouldn't expect to see the relationship between the two change if this were the only thing that was happening.  In other words, I wouldn't expect the one that started out stiffer than the other one to become weaker than the other one as equal amounts were cut off each shaft. 

However, the 2217 is wider than the 2018, so the angle between the 2217 shaft and the centerline of the bow will increase more as the shaft is cut than the angle between the 2018 shaft and the centerline of the bow increases (assuming both were outside the centerline of the bow to start with).  This means that the 2217 will gain slightly more stiffness for the amount cut off than the 2018 will.  So if the 2217 started out slightly weaker than the 2018, I suppose it is possible that it could cross over and become stiffer at some point as more was cut off.

Very interesting! I don't understand the dynamic spine thing at all,but you answered my question as to where I can understand it. I keep forgetting about one being fatter and not being the same centershot. I sort of figured if one started out stiffer it would remain,but just got to thinking about this and thought I would ask.

[McDave]

I think my reasoning above was a bit faulty.  What I said would be true if the arrow were drawn to its full length each time it was cut shorter.  However, the more likely case would be that all the cutting would be from the overhanging part of the arrow, and the draw length would stay the same before and after the cuts.  Therefore, there would be no change in the angle between the arrow and the centerline of the bow as the arrow was cut.  In this case, the dynamic spines of both arrows should change at a consistent rate as they were cut shorter, and there is no reason I can see why the spine of one would overtake the spine of the other.

[Kelly]

Never heard of a 2217?

2117 spines out at 82/83# on 26” centers rated to 28”. The 2018 is about 67/68# spine on the same spine testor.