Bare Shaft Question

Frank Lawler

New member
Hi All,
Just a few questions: why is it that an underspined bare shaft will strike nock left, and if overspined nock right? Also, why do we set our nocking point on the string higher than the arrow rest on the riser? Wasn't able to figure these out for myself. Thanks.
 

Desbob

New member
ck
Hi All,
Just a few questions: why is it that an underspined bare shaft will strike nock left, and if overspined nock right? Also, why do we set our nocking point on the string higher than the arrow rest on the riser? Wasn't able to figure these out for myself. Thanks.[


When an underspined arrow is shot, it bends around the riser as it leaves the string, because it is not stiff enough it bends too much and the point goes right so the nock strikes left. With an overspined arrow the arrow does not flex enough, so does not bend round the riser enough and goes left, with the nock striking right.
The nocking point is higher than horizontal because of tiller. This causes the bow to tip up slightly when drawn, so is actually now in line.
 

geoffretired

Supporter
Supporter
It isn't just the size of the bend, but the rate at which the bend changes from side to side. Stiff arrows bend from side to side at a faster rate than softer ones. A stiff arrow can bend one way and the opposite way and then back again, by the time it reaches the launch position. If it bends too quickly for the speed of the bow, it will exit the bow facing left. If the arrow is too soft, it will bend too slowly and still have a bend in it that is facing the front end over to the right.
 

joetapley

New member
Hi All,
Just a few questions: why is it that an underspined bare shaft will strike nock left, and if overspined nock right? Also, why do we set our nocking point on the string higher than the arrow rest on the riser? Wasn't able to figure these out for myself. Thanks.
As, with a good setup, you can easily move the bare shaft either side of the fletched arrow with just a small change of button spring tension it's clear that bare shaft going left or right has nothing to do with "spine". The left/right result comes from the net lateral force (actually torque) on the arrow from the string and button spring being to the left or right.

If you imagine what happens if you move the nock point up or down a foot from the normal area you can see why one nock point is better than another. :). For tuning purposes there is an optimum nocking point position (for each specific target distance) related again to the net torque on the arrow from the string.

PS

As bow set up relates to matching the arrow flex rate to the bow this by definition is correct so bare shaft going left/right has nothing to do with arrow flex rate either. :(
 

geoffretired

Supporter
Supporter
Hi Joe, You have confused me yet again.
If it is nothing to do with spine or flex rate, why don't all spine arrows shoot the same?
 

feaviolp

New member
Sorry Joe, don't agree. Not only do the text books state that spine is responsible for bare shafts impacting left or right of fletched, I have demonstrated it for myself recently. After a change of limbs I needed new arrows of a stiffer spine so I went and tested various spines to get the best match. I can categorically say that bare shafts of the same length, same point weight, different spine, same bow, same button setting, will impact left, right, or with their fletched equivalents depending upon the spine. So I am certain that spine IS responsible for the left/right impact.

Furthermore, I have had very limited success with getting bare shafts and fletched arrows to group with button tension changes. I have found that changes in button tension move both the fletched and bare shafts in the same direction, so to fine tune the bare shaft into the group would be done by winding the limbs in or out a bit to match the bow to the spine, if you see what I meam? So match the spine to the bow as close as possible initially, then (since the spines go up in steps) fine tune the bow to the chosen spine. To be honest I don't bother with the draw weight fine tuning though, mainly because I'm pulling about as much as I can manage at the moment.. So what I do is get the closest matched spine that I can using bare shafts, and then tune using the fletched arrows only at various distances ensuring the sight stays over the point of the arow and windage adjustment is not needed as distances are changed. Once I have done that I don't care if bare shafts and fletched arrows land together; I already know the spine is as close as I'm going to get and I have tuned so windage adjustment isn't necessary; I am happy. Sorry - going a little off-topic, just providing some context with my reply.

I'm sure you've been shooting longer than me so no offense intended with my reply. Just that I don't agree that spine has no effect on bare shaft left/right because I have seen it myself.
 

Easily Confused

New member
As bow set up relates to matching the arrow flex rate to the bow this by definition is correct so bare shaft going left/right has nothing to do with arrow flex rate either. :(
Is this not all assuming that the arrows are the correct spine? If the spine is incorrect then surely by definition, the flex rate will be INcorrect?
 

joetapley

New member
Hi Joe, You have confused me yet again.
If it is nothing to do with spine or flex rate, why don't all spine arrows shoot the same?
When you go through a bow set up process what you are doing is adjusting the bow so that all (spine) arrows do shoot the same. By this I mean that the flex rate of the arrow matches the bow geometry. The arrow goes through 0.75 of a cycle on the string and 0.25 of a cycle between the nock leaving the string and passing the riser. So overall the arrow goes through one flex cycle between full draw and riser and this takes 1.25 the arrow free free vibration time. Established/verified for last 60 years and forms the basis of all these arrow selection systems.

All spine arrows shoot the same. You select an arrow in the right ball park and adjust the draw weight to obtain this arrow /bow agreement. There is some wiggle room around this both with the nock/string separation and the arrow status as the nock passes the riser.

However....when you have a perfect bow set up, there is still a problem, the bow string generates a net torque on the arrow during the power stroke. This torque would result in the arrow spinning around a vertical axis as it comes off the string.(Try shooting a left hand bow right handed and see what happens, There is nothing to counter the arrow rotation from the string torque). The pressure button spring generates a torque on the arrow in the opposite sense to the string so it can be adjusted to get an approximate null net torque on the arrow. The higher the bow draw weight the higher the string torque on the arrow.
This gives rise to the well know relationship between draw force and button spring tension. (IIRC spring tension often quoted as 6 grams/lb draw weight).

If a flying arrow is rotating around a vertical axis it flies in a curve (vaguely similar to hook/slice on a golf ball). If the string torque is dominant then for a RH archer the arrow curves right (weak in the jargon). Reverse if the button torque is dominant.

For a given arrow rotation (energy) the deviation of the bare shaft arrow is larger than for the fletched arrow. So in principle if the fletched and bare shaft arrows hit at the same horizontal point then the string and button torques on the arrow balance.

Why does any of the above matter - well you can put together a reasonable argument that the forgiveness of the system (the arrow grouping) is related to the amount of rotation the arrow has when leaving the bow, and hence the string/button torque balance as well as the nocking point position.

PS There are no textbooks on archery, only books containing peoples' opinions (usually with zero supporting arguments) and these can be sensible or nonsense. The only real judge is how archers vote with their feet.

pps

Is this not all assuming that the arrows are the correct spine? If the spine is incorrect then surely by definition, the flex rate will be INcorrect?
If you cannot sensibly match the arrow flex to bow geometry then you have the wrong arrow. As for saying the spine is incorrect you could just as well say that the draw weight is incorrect or the archers arms are too long :)
 

geoffretired

Supporter
Supporter
All spine arrows shoot the same.
Hi Joe, I follow what you say this time. The bit I am having problems with is quoted above.
I feel that you are saying that in a way that I am not "seeing".
I imagine a stiffer arrow and a weaker one, being shot from the same bow by the same archer, and one going through more flex cycles than the other, because of the flex rate differences between them. So, in that sense, they don't fly the same.
 

joetapley

New member
Hi Joe, I follow what you say this time. The bit I am having problems with is quoted above.
I feel that you are saying that in a way that I am not "seeing".
I imagine a stiffer arrow and a weaker one, being shot from the same bow by the same archer, and one going through more flex cycles than the other, because of the flex rate differences between them. So, in that sense, they don't fly the same.
If the arrow was too stiff then during the set up you would increase the draw weight to get the bow to match the arrow.
If the arrow was too weak then during the set up you would decrease the draw weight to get the bow to match the arrow.

Not the best idea but instead of changing the bow you could change the arrow:

If the arrow was too stiff then during the set up you could increase the arrow length to get the bow to match the arrow.
If the arrow was too weak then during the set up you could decrease the arrow length to get the bow to match the arrow.

Wherever you start from you end up in the same place. If you can't reach the bow set up from where you're starting from then you either have to change the arrow or the bow (the arrow being the sensible option usually).

The same applies to basic tuning. If you cannot, with the pressure button spring, cross the bare shaft over the fletched arrow then there's something screwed with your bow set up. (ref. Mike Gerard favorite candidate here is poor limb alignment).
 

mrtufty

New member
When you go through a bow set up process what you are doing is adjusting the bow so that all (spine) arrows do shoot the same.....
Hi Joe,

Would it be a gross oversimplification to then say that:

adjusting limb weight/braceheight tunes the bow to the arrows flexing rate/cycle
spring button pressure effects the arrow roatation
spring button in/outness (left rate) effects the arrow direction

or jsut a little one? ;)
 

Frank Lawler

New member
It isn't just the size of the bend, but the rate at which the bend changes from side to side. Stiff arrows bend from side to side at a faster rate than softer ones. A stiff arrow can bend one way and the opposite way and then back again, by the time it reaches the launch position. If it bends too quickly for the speed of the bow, it will exit the bow facing left. If the arrow is too soft, it will bend too slowly and still have a bend in it that is facing the front end over to the right.
Thanks, that makes a lot of sense to me.
 

joetapley

New member
Hi Joe,

Would it be a gross oversimplification to then say that:

adjusting limb weight/braceheight tunes the bow to the arrows flexing rate/cycle
OK - also established at set up is the arrow launch alignment


spring button pressure effects the arrow rotation
Better put as spring button pressure (and nock point) are used to control arrow rotation at launch

spring button in/outness (left rate) effects the arrow direction
This is the tricky one. Center shot affects both the lateral string force on the arrow (launch rotation) and probably the arrow launch alignment.
Arrow rotation is adjusted with button spring not center shot (normally). Center shot is set to experienced based position and not adjusted until after bare shaft tuning completed. (Fairly obvious as arrow launch alignment cannot be seen until after any arrow launch rotation is removed).

Of the two, launch rotation and alignment, rotation has a much bigger effect on arrow grouping so at basic tuning level alignment is ignored. Arrow launch alignment (center shot adjustment) is normally left to a full blown group tune. There have been a few suggestions about adjusting center shot after a bare shaft tune for arrow alignment before a group tune is carried out (McKinney, Frangilli, Buzz Light Beer).
 
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