Advantage with heavy points?

[sambow]

Originally Posted by geoffretired

I wonder of changing bracing height can change the pattern of the string vibrations enough to give a better match with the arrow in use?

It will certainly effect where the bends occur relative to the riser. When tuning my recurve bow I dry powder test for clearance and adjust the brace height. On one occassion increasing the brace height by a quarter of an inch meant the tail of the arrow was consistently coming in and striking the riser.

[phil_r_58]

Originally Posted by geoffretired

Great explanation, Phil. So the string and the arrow have their own oscillation patterns, but while they are joined, they interfere with one another, yes?
I wonder of changing bracing height can change the pattern of the string vibrations enough to give a better match with the arrow in use?

I wish that tech site, was it Joe tapley/tarpley or something was still on line. He went into a lot of archery physics.

But yes, we have two motions. Brace height won't affect it though IMHO. The bend, and subsequent oscillation by moment of inertia begins on release.

The string path oscillation also begins as the string releases from the fingers on a re-curve. There is a small amount of oscillation from a release aid, but not so pronounced, as no jaw or rope can be totally 100% in line.

So on parting from the string, the string path oscillations are already well established, and should not be affected by the brace height or point of separation from the string. The bend, or archers paradox is well documented, and can be adjusted, so the point at which the bend is the greatest can clear the bow. Rather like the arrow is "limbo dancing" to clear the bow, is the explanation I use to beginners. A little oversimplified, but it gets the idea across to them.

I know we are off thread now, but this is a whole field of possible research that is only showing it's effect as we make bows, and arrows more efficient, and are reaching new levels of potential accuracy. I know of one PhD mathematician who due to retirement is bored and is doing some research into archery physics. His little brother is an archer, and a friend of mine.

Some of our pub conversations are getting very interesting, as we delve into a number of possibilities. And as I have said before, don't mock some of OB's ideas, some of his blog observations are tying up with some of the maths being done in the background. If only he knew how close he is.

[Random_guy]

Erm, not really no, have a look at high speed footage of compound arrows being shot (loads on youtube, including stuff posted by watchman of archerymum) the arrows hardly oscillate side to side at all.

That's because the oscillations are caused by a disturbance left/ right by the sideways movement of the string. Because its physically impossible to concentrate all force *exactly* down the centre axis of the arrow shaft (and infinitely thin line) you will always get some oscillation.

With a compound there is more up/ down oscillation, which i'd speculate is due to the reaction from the arrow rest on the arrow (I hated mechanics and so won't be attempting to come up with even a simple model any time soon!).

Ultimately, from my basic understanding of it, when tuning a *recurve* you are attempting to alter the *FREQUENCY* of the oscillations so that the arrow is straight when it leaves the bow, and gets maximum clearance from the rest/ sight window. I'd imagine the frequency of oscillations is a function of the force acting on the nock end of the arrow and the stiffness of the arrow shaft.

I don't remember anything about inertia so couldn't comment on point weight's effect on the frequency of oscillation of an arrow, but again i'd imagine it makes more of an impact of the amplitude of oscillation, rather than the frequency, i.e. the example of putting a really heavy weight on the front of an arrow and having it snap, this would be due to the *amplitude* of the oscillation being to great, nothing to do with the frequency.

In terms of compound shooting, since we can alter the nocking point height to get clearance on the rest, the frequency of the oscillations (up and down in compound) isn't so important, i would have thought and also the amplitude will be much lower in the first place due to the disturbance being smaller.

Now i'll go and get back to my combinatorics, while a physicist comes and corrects the poor mathematicians horrendously inaccurate mechanics!

[Random_guy]

Sorry, that was aimed at the comment that a compound arrow would still oscillate side to side!

[geoffretired]

My thinking on changing brace height was that the string is under different tension, so like a stringed musical instrument, will have a different frequency of vibration. Could that not produce some better or worse matching between arrow and string/bow?
I think there are probably several "archery tales" that will be clarified when high speed video becomes more common.
The whole idea of archer's paradox must have been taken into account by bowmen long before they had much understanding of any of the science behind it, or am I wrong?

[sambow]

Originally Posted by Random_guy

Erm, not really no, have a look at high speed footage of compound arrows being shot (loads on youtube, including stuff posted by watchman of archerymum) the arrows hardly oscillate side to side at all.

If it is the video I think you are referring to that's on Utube it is pretty difficult to detect which way the oscillations occur because the camera angle is from behind at an angle, so depending upon how you want to look at it, it could look like up down or it could look like left right. the only real way to be certain would be to take one shot from above and one from the side.

The key thing is the shaft will bend because you are pushing a weight (the pile) from the end of a long hollow tube. If it didn't bend we could all shoot nice thin 1500 spine cartel triples with nice heavy 150gn points and have great aerodynamics and nice weight stability. But the reality is we can't because they would bend too much and become either instable or snap. That bending motion won't be caused by the rest but the inertia of the point acting against the acceleration of the string.

My understanding has always been that the knock constrains the direction that the arrow is allowed to bend in and that the direction it is constrained to is left right, that stacks up with my arrows hitting a hard target face where they appear to shake left and right. But I am happy to concede that the direction of flex could be up down until there is concrete proof which way it is.

[phil_r_58]

Originally Posted by geoffretired

My thinking on changing brace height was that the string is under different tension, so like a stringed musical instrument, will have a different frequency of vibration. Could that not produce some better or worse matching between arrow and string/bow?
I think there are probably several "archery tales" that will be clarified when high speed video becomes more common.
The whole idea of archer's paradox must have been taken into account by bowmen long before they had much understanding of any of the science behind it, or am I wrong?

Of course they did Geoff, just like thousands of years ago they noticed that willow leaves were good for headaches, they contain salicylic acid, aspirin is acetylsalicylic acid I believe, honey was good for wounds etc etc etc.

& yes, if you read the Easton tuning guides, they suggest that using a compound and platform rest, the paradox actually occurs in the vertical plane.

We may use more advanced materials, and have the scientific abilities to explain why things happen, but don't ever discount the accumulated wisdom of the ages. Look at OB for example.

[Random_guy]

Sambow, look at this video. Its particularly obvious when the arrow hits the target in what direction its bending. As i said, there will be *some* left right bending, but this will be minor by comparison, else the shoot through compound could not exist!

Also, as i said, heavier point weights will effect the AMPLITUDE of the oscillations, and if these are too large the arrow will be unstable, as you quite rightly said, but i couldn't speculate on how they would effect the FREQUENCY, which is, after all, the important bit (especially on recurves).

Also, what constitutes a *heavy* point weight depends on the stiffness of the shaft (hence easton's different recommendations). So i guess it's safe to assume that the effect of point weight on AMPLITUDE is reduced the stiffer the arrow (as makes intuitive sense).

Hence there is no such thing as an arrow that is truly *too stiff* for compound, as in general its only the AMPLITUDE that compound archers need to worry about, rather than FREQUENCY, which is the major concern for recurves, and a stiffer arrow with the same point weight will probably oscillate with a smaller amplitude.

I have no idea as to whether point weight changes the frequency of oscillation, and is thus a valid method for recurve tuning. I wouldn't even like to speculate, as I don't know enough about the mechanics involved.

[Cimbian]

Originally Posted by geoffretired

The whole idea of archer's paradox must have been taken into account by bowmen long before they had much understanding of any of the science behind it, or am I wrong?

I suspect this was on a suck-it-and-see basis as opposed to today's approach of listen-to-me-and-take-it-as-gospel.

[Clive]

[quote=phil_r_58;238683]I wish that tech site, was it Joe tapley/tarpley or something was still on line. He went into a lot of archery physics.

TOPICS ON ARCHERY MECHANICS

I think the Joe Tapley site has just moved, not gone away. Link above should work

Clive