Unfletched Arrow Spin: A Mystery of Bow Mechanics

[Arrowdude]

Some friends and I have been having some discussions about why an unfletched arrow will spin right when shot from some bows and left when shot from other bows. We have taken bows and shot a series of different arrows and every arrow out of that bow went in the same direction.

So I gues I have two questions.


Any ideas as to why an unfletched arrow begins to spin when shot from a bow

Why will one bow spin arrows to the right and another bow spin them to the left

 

[davenn]

random chance ??

The way the arrow is held and released from the fingers ?

Dave

 

[K^2]

What do you mean by "spin"? Do you mean around it's own axis? If so, how do you know it was spinning? Or do you mean arrow twisting during flight? In that case, it's due to string travel not being perfectly aligned with arrow rest. On some bows, especially traditional, they are impossible to align, and then you just have to correct for it. Of course, fletching will significantly reduce the effect.

 

[Arrowdude]

What do you mean by "spin"? Do you mean around it's own axis? If so, how do you know it was spinning? Or do you mean arrow twisting during flight? In that case, it's due to string travel not being perfectly aligned with arrow rest. On some bows, especially traditional, they are impossible to align, and then you just have to correct for it. Of course, fletching will significantly reduce the effect.

I took bare unfletched shafts and placed a dot on the arrow near the nock end at 12:00. I stood just in front of a target and shot the arrows into the target. The arrow rotated on its own axis to the left slightly, the further I moved back the more they rotated. We test various bows and on some bows the arrow would rotate to the right and on some to the left. Some of the guys are trying to say the string oscillation caused so I shot the bow with a shooting release and straight off my fingers and every arrow rotated on its own axis to the left. We wanted to find out if the way the bow applies energy to the arrow and the build up of that energy forces the shaft to begin to rotate. The bows we used were compound bows not traditional.

So if I understand what you are saying the position of the string in reference to a perfect center shot is the cause, so if I move the center shot slightly right of center is should spin in one direction and left of center should spin it in the other direction.

 

[K^2]

I'm not familiar with compounds. With a recurve, I'd blame the draw, because it's made very consistent, and slightly off-center draw would definitely impart a bit of spin. But if you tried this with release mechanism and got the same results, it's probably not that. Still if the rest isn't perfectly centered, it could lead to the same effect. If the position of the arrow rest is adjustable, you can try messing with it a bit to see if it affects the spin.

 

[Arrowdude]

I'm not familiar with compounds. With a recurve, I'd blame the draw, because it's made very consistent, and slightly off-center draw would definitely impart a bit of spin. But if you tried this with release mechanism and got the same results, it's probably not that. Still if the rest isn't perfectly centered, it could lead to the same effect. If the position of the arrow rest is adjustable, you can try messing with it a bit to see if it affects the spin.

Compound bows come with a number of setups, one is a wheel on top and a cam on the bottom. They typically fall off to about 20% or the draw weight when you are at full draw. The draw curve is different than a recurve, as you draw a recurve the draw weight increases and on release it falls off gradually. With a compound the bow loads up as you go to full draw then about 3/4 of the way back it drops off to only about 20% of the full draw weight. Then when you release the bow it actually increases force as it fires.

I feel it has nothing to do with the arrow rest I have moved it left and right and have seen no change. With a recurve you can't get a true center shot with a compound you can.

I think it has something to do with the way the force of the bow firing and how it is applied to the arrow that makes the arrow want to begin to rotate but I have no background in physics so I have come here to the experts to help me out.

My reason for trying to figure this out is for as long as I can remember in archery we have had discussions on which is best for a compound shooter, Right Hand Fletching or Left Hand Fletching. Some archers swear by RH and some by LH and nobody has ever taken a scientific look at the differences so I started to look into it and found the issue with some bows spin arrows right and some left so I wanted to find out the science behind it and hopefully the physics behind it as well.

 

[Arrowdude]

random chance ??

The way the arrow is held and released from the fingers ?

Dave

Dave, I used a mechanical release that put no influence on the string and I shot with fingers, two completely different types of releasing of the string and the arrow shafts still rotated left on its own axis.

 

[K^2]

Compound bows come with a number of setups, one is a wheel on top and a cam on the bottom. They typically fall off to about 20% or the draw weight when you are at full draw. The draw curve is different than a recurve, as you draw a recurve the draw weight increases and on release it falls off gradually. With a compound the bow loads up as you go to full draw then about 3/4 of the way back it drops off to only about 20% of the full draw weight. Then when you release the bow it actually increases force as it fires.

When I said I'm not familiar with compounds, I only meant that I don't have any first-hand knowledge of firing one. I know most of the mechanical differences.

I feel it has nothing to do with the arrow rest I have moved it left and right and have seen no change.

That pretty much eliminates everything I can think of. The simple act of pushing the arrow forward will not generate spin. The string needs to physically twist at the nock to impart spin. Oscillations make sense, but I don't believe such consistency could be achieved with these, especially if you tried different release methods, including mechanical, unless something interesting is going on. String going off center could also do this, but if you've moved arrow rest, that's eliminated as a factor.

To be honest, short of bringing in a high speed camera to see what actually happens to the string and the arrow during release, I'm completely out of ideas.

My reason for trying to figure this out is for as long as I can remember in archery we have had discussions on which is best for a compound shooter, Right Hand Fletching or Left Hand Fletching. Some archers swear by RH and some by LH and nobody has ever taken a scientific look at the differences so I started to look into it and found the issue with some bows spin arrows right and some left so I wanted to find out the science behind it and hopefully the physics behind it as well.

Yes, it does sound like it can make a difference and explain why some people prefer right fletching and others prefer left.

 

[256bits]

Would it have anything to do with the twist if the string? Isn't the string multiple fiber with strands twisted? Even if not left or right handed twist, it has to be something to do with the string imparting a spin to the arrow as that is the only contact with the arrow. Under greater tension, some fibers of the string would have greater tension than others and produce a twist to the whole string which upon release transfer that to the nook and arrow.

 

[Arrowdude]

Would it have anything to do with the twist if the string? Isn't the string multiple fiber with strands twisted? Even if not left or right handed twist, it has to be something to do with the string imparting a spin to the arrow as that is the only contact with the arrow. Under greater tension, some fibers of the string would have greater tension than others and produce a twist to the whole string which upon release transfer that to the nook and arrow.

So it seems it has to be oscillation! When the bow is fired the limbs begin to move outward pulling the string back toward its at rest location. Because of the amount of force that is placed on the string it actually moves past its at rest position, when this happens it sends shock waves (for lack of a better word) up and down from the point where the arrow is connected to the string. These waves are moving extremely fast and on the last wave just as the arrow is about to leave the string the wave, whether On the way down or the way up would impart a small about of spin to the arrow as it tries to pass the nock point. Does this idea seem correct.

Sorry if I didnt explain this every well, it is a tough thing to explain when you are simple country boy.

 

[Arrowdude]

I have been assuming the oscillation would be forward and backward because of the direction the string is moving when in fact the string could be oscillating right and left as well. This would seem to be able to spin an arrow right or lefts as the oscillation cycles move up and down the string!

Thanks for all of the imput, if you think this is correct let me know.

 

[K^2]

Would it have anything to do with the twist if the string? Isn't the string multiple fiber with strands twisted? Even if not left or right handed twist, it has to be something to do with the string imparting a spin to the arrow as that is the only contact with the arrow. Under greater tension, some fibers of the string would have greater tension than others and produce a twist to the whole string which upon release transfer that to the nook and arrow.

Directions don't match. By the time the twist in the string can induce sideways oscillations, required to spin the arrow, the string needs to be spinning at high frequency. This isn't going to happen during a shot.

string could be oscillating right and left as well

Well, yes. It does. And it has to be to impart spin to the arrow. I don't understand why it's consistent. That's the puzzle.