Arrow spine

[James Park]

I would like to try to clarify the definition of arrow spine.
My understanding is that the spine of an arrow is a measure of its stiffness. That is, when suspended between supports at a particular distance and loaded with a particular weight in the centre, how much does it bend. I think everyone would agree with that.
Where I am unhappy is that many people say that you can change the spine by adding point weight. This cannot be correct, because if you put it on your "spine tester" the deflection will still be the same. That is, the so-called "static spine" will be identrical, no matter what the weight added at the point (or the nock) end.
Maybe, people are talking about a thing called the "dynamic spine"? But "spine" is still "spine" and if it means "stiffness" then it will be the same as it was before.
What I think people are actually talking about is not stiffness but in fact "natural frequency". That is: how fast does it vibrate when flexed? This frequency of vibration certainly does change as the point weight is changed (it gets slower as weight is added).
I have measured this frequency of oscillation for many arrows, and regard it as the important parameter in tuning a bow, not the arrow stiffness. The stiffness is just one of the parameters that sets the arrow vibration frequency, and nothing more than that.
What do you think?

[Marcus]

Basically I agree with you on tis one. (still don't sgree with your back tension theory, but that's another story)
Because changng the weight distribution and length of the arrow will change the stiffness of the arrow then what you call vibration frequency. Where the error may lie is in the spine testers. They can not accuratly tell you the spine of the arrow because spine has more variables than just the deflection caused by bending the shaft with a weight. All a spine tester can accuratly tell you is the spine relative to each other.
This is really what lies at the heart of years of frustration in finding the correct arrows to shoot in a bow. Because arrow charts try to predict the spine for your bow they can not take into all the variables which would includes factors such as cam placement, limb deflection, brace height, arrow rest interferance etc etc. This would give you a theorectical spine and an actual spine.
Cam placemnet would be one of the biggest. Depending on how your cam/s rollover is setup would depend on the draw curve of the bow and thus change the amount of energy delivered to the arrow during the shot.

Actually perhaps not. :-?

How do you measure oscillation? If the oscillation is slower wouldn't that make the arrow 'stiffer' as less oscillation would suggest less flex? Yet a heavy pointed arrow flexes more.
Spine determines the amount of movement in the arrow before the arrow is able to move the point. A light spined arrow from a heavy bow will flex more than it should before it is able to move the point and thus it's mass. Wouldn't that make stiffness the important factor? And thus while th spine is the same the stiffness changes because stiffness is relative to the bow weight and point weight.

stiffness = spine/(bow weight+arrow point weight)

Not claiming this is correct, just a possible. (ie both concepts have merit to me right now. )

[mike]

Frequency is simply the number of full oscillations that take place in a given time. The 'stiffer' or more competent the spine, the high will be the frequency. This is because the higher competency arrow can return the oscillatory energy faster and more efficiantly than the low competency spine.

As you go to a more powerfull bow, you need a more competent arrow to ensure that the power of the bow does not deflect the arrow to the extent that it snaps. That is, you need an arrow that can absorb the energy as linear kinetic energy (in line with the shot) rather than wave energy (which causes the oscillation).

The arrow starts oscillating basically from the moment of release, therefore the main consideration (after you have tuned it for the most efficiant flight possible, which often causes that 'bullet hole' tear but has other variations) is where are parts of the arrow as they go past the bow and the rest. If you happen to time the part of the oscillation that causes the fletch end to go down so that this happens as it passes the rest, you will always get shocking collision. So, you have a few options, stiffer spine, which will at least reduce the problem, but may if you are luckey completely change the oscillation pattern so that the arrow misses the rest on the way through. OR do things like change point weight which may acheive the same effect.

This is why there is not one spine for each bow set up. I think Jim has found that there are two optimal spines for his bow, one very stiff, one quite soft by comparison. In this case you may go for the soft spine to reduce arrow diametre and reduce weight thus increasing speed ( for the ballistic reasons that Accurate Sights will demonstrate).

The upshot of all this is clearly that spine stiffeness is really just a means to an end -- the right tune and oscilation pattern, which stems from the arrow's natural frequency as set up ready to shoot.

Must get coffee...brain hurts..... :-?

[Kuru]

Agree, adding weight to the end of an arrow isn't going to change the stiffness, now if Easton was to make an arrow like an ace which had interchangeable lenghts of alloy, ie take one out and put a thicker walled piece in then that would change the spine. I'm sure arrows like that will be on their way in the near future !

[James Park]

I have measured the vibration frequency of arrows. It is typically 40 to 50 Hertz. This matches quite well with the time it takes for an arrow to leave the bow (about 14 miliseconds). Hence, I think I can now model the arrow coming out of the bow quite well, which should ultimately let me predict the tuning a bit better.

[jm]

I don't have any difficulty with the concept of "dynamic spine" and can only surmise that different people think differentlyabout it.

An arrow shaft cetainly has a fixed "static spine" but with a string imparting energy to the shaft, then whatever is in front of the shaft (i.e. the point) becomes a factor: The shaft wants to move forward but is inhibited by the inertia of weigt at the point. The greater the weight of the resistance at the forward end of the shaft, the greater the stress on the shaft experiencing the impetus of the string at one end and the inertia of whatever is at the front end. Something has to give according to the laws of conservation of mass and energy; the shaft will bend more according to how much resistance it has to overcome in order to get moving.

The "vibration" concept, or how the energy dissipates in flight is an interesting one. Maybe it explains why some arrow set-ups which we think should work in theory just don't work out in practice.

It certainly gives me (for one) yet another excuse for cruddy shooting!

jm.

[mike]

What is interesting about this thread is that it seems to suggest that the determining factor in "tune" is the arrow itself, not where the rest is and the nocking point and the cams etc.

It is apparent that once you set certain variables such as the amount of energy the bow can impart into the arrow, the draw length etc (all of which are things that are difficult to change substatially on a bow without changing how nice it is to shoot) then the only way to truly tune a bow is to select the right arrow/point weight combination.

Its funny that because I remember someone telling me about recurves once and how for a given recurve bow there is usually just one arrow setup that will really work well. It seems that yet again the traditional (scientific) concepts win out. :roll:

[GaryH]

Important: A wrong arrow (an arrow with the wrong spine) cannot be compensated by any plunger!
Borrowed from the Beiter web site http://www.wernerbeiter.com/english/produkte/plunger.html

Given it's an issue for Recurve, is the button only really there to compensate for a finger release?? Exactly how much work does it actually do?? I noted somewhere in the vastness of this forum, someone whilst tuning replaces their button spring with a match to bare shaft tune, then replaces the spring then winds in the adjustment until it shoots the same!?! :-?

How much does a button effect the "vibration"??

[James Park]

The pressure button does not affect the frequency of arrow vibration at all. I agree with the Beiter web site: you can not fix a poor choice of arrow by adjusting the plunger button. Similarly, you cannot fix a poor choice of arrow for a compound bow by adjusting the launcher - the frequency of oscillation is the frequency of oscillation (that is, it is a fundamental property of the arrow and point weight), it is not different simply because the pressure button or launcher is set differently.
What the pressure button or launcher does change is the orientation of the arrow as it leaves the bow, and this is important.
I absolutely agree with tuning a recurve bow with the pressure button done up firm (with a match in it if necessary).

[GaryH]

Thanks James,

The question still lingers then, why does the pressure button have a spring??? Once tuned why would you need to adjust other than a change in arrow?? When talking of orientation are we talking centre shot??

[James Park]

I normally tune a recurve bow with a rock hard plunger set so that the arrow passes through exact centreshot. I adjust the bow weight until the bare shaft groups with the fletched shafts - the bow weight is by far the most sensitive thing you can change, things like point weight only make a minor difference.
Then I put a bit of give in the pressure button spring, and wind the button out until the arrows hit the same point on the target as they did with the hard button.
I think the point of the spring is that it can make the bow a little more tolerant of a poor release.
Note that if you change the spring tension, you will also have to change the centreshot, otherwise you will have the arrow taking a different line out of the bow.
Consequently, I agree that once you have it properly adjusted you should not have to change it, unless something moves on the bow.

[DavidA]

Jim, as you have said, adding weight to the point of an arrow decreases the frequancy and reduces the 'dynamic' stiffness. What of the McKinney weight system, which adds a small amount off weight to the nock end to stiffen the arrow, this appears to go against your theory.
In practice adding weight to the arrows tail has more effect than a equivelent weight added to the pile.

[James Park]

Adding weight to the tail of an arrow will make no difference for a compound, because while the arrow is on the string the rear node will be at the nock.
For a recurve the situation is different: there the node is not at the nock but about 150mm from the end, so adding weight at the rear will result in a lower resonant frequency. I agree that it will have more effect (on a recurve) than the same amount added at the front because at the rear it is further away from the node than at the front.

[mike]

Just to clarify (for compound), wouldn't adding mass to the tail of the arrow have the same effect as adding mass to the string at the nocking point? That is to say it would slow the string down (it increases the string's "virtual mass"), creating a similar effect as winding the poundage down, which has a dramatic effect on the behavior of the arrow? This of course would only apply whilst the arrow is still on the string, once it has left the string I would have thought that mass on the tail would definitely change the arrow's dynamic spine characteristics.

My brain will start hurting soon, must stop considering this too far!!! :o

[James Park]

Yes, I agree with Mike's comments.
Adding weight at the nock is equivalent to adding weight to the string and it does slow the bow down slightly. What the weight at the nock does not do is change the resonant frequency of the arrow while it is still on the string - this resonant frequency is the key element of matching the arrow to the bow. As soon as the arrow leaves the string the nodes about which the arrow ascillates shift and then the added weight at the nock does of course affect the frequency (it lowers it).

[Joe]

As one of the 2 people who voted for stiffness in this poll here's my spin on it I took 'stiffness' to mean the stiffness of the arrow not just the stiffness of the shaft (so it includes pile weight etc.)

While there is a close correlation between the vibration frequency of an arrow and how well it 'matches' the bow (which is why computerised arrow selection programs can produce acceptable results using a vibrational model) when an arrow is shot it is not vibrating in the sense most people take it. There are no nodal points and so on. When an arrow is shot you get a propagating stress wave with the max. stress point starting towards the rear of the arrow and travelling forward (and decreasing). It's the buckling characteristics of the arrow determined by its mass, stiffness and the forces on it (string and button) which ultimately determine the arrow match.

[Stash]

Several different items being addressed here....

Stiffness does not necessarily correspond directly to frequency. A steel bar with the same "spine" deflection as a carbon arrow won't vibrate nearly as quickly as the arrow. Static spine is only a starting point in arrow selection.

It's the vibration frequency that has to match the speed of the arrow as it passes the bow that is the important issue. Seems to me that arrows should be matched more to bow speeds than bow weights. Maybe arrow manufactureres should wake up and start doing their calculations based on IBO ratings instead of bow weights. I see Easton is starting to get that idea going.

As for the plunger with fingers....Nothing will make a good shot better, but a properly set-up plunger will make the bad shots less bad, just as a car's shock absorbers make the ride less bouncy.

There's no way to condense all the variables of arrow spine/frequency and bow/string/fingers/centershot and all the other factors into one general theory, but the prefered method is to get the best matched arrow to your setup (found by bare shaft testing with no give to the plunger), and then tune for best groups by varying center-shot and spring tension.

What group-tuning does is finds the least variation in groups among all your less-than-perfectly-shot arrows. If you shoot absloutely perfectly, without ANY error, you don't need a plunger. But nobody does, so everyone (finger shooters) needs an adjustable pressure point to minimize errors.

[James Park]

ttaass,
Agree with most of what you say. Very much agree that it is matching the arrow speed and the vibration frequency that matters, not the spine.
However, I think you can get a general theory of how it all works (although it is complex).