Friday, 11 May 2012

Shooting the Hazel D Bow & the Archer's Paradox

I made a decent string for the bow, using just 6 strands of Angel Majesty. That's the thinnest string I've made as I wanted it as light as for maximum performance. I bulked up the nocks and centre serving with a few extra strands, but kept the serving down to a minimum. I've kept the brace height fairly low for maximum length of power stroke. Some find this rather counter-intuitive, thinking that a higher brace height and thus higher tension at brace will give give more power, yes it increases the early draw weight a little but reduces the length of actual draw (e.g 28"- 5.5" brace is 22.5" power stroke whereas  28"- 7" brace is 21" power stroke) .
To be honest I haven't done actual tests to prove this and I'm really just quoting from the Traditional Bowyer's Bible, but my experience tends to indicate that cranking up the brace height in an attempt to raise draw weight and power doesn't achieve what one might expect.
In fact scrabbling round trying to gain extra draw weight is fraught with problems, the two most effective ways are heat treating the belly or shortening the bow.
Shooting into the garage at about 10 paces I could see the white scrap of paper I was shooting at but couldn't see where the arrows were landing as it was rather dark in there and bright sunshine outside (yes it's finally stopped raining!) It shows the arrows are a bit stiff for the bow as it's only about 35-40# draw weight and is very wide at the grip. They are all kicking off left, due to the archer's paradox but the group is ok-ish considering I haven't shot it before. There is a little bit of wrist slap too (due to the low brace height).
Those are my lightest arrows (70grain points on a 5/16" shaft).
With bows everything is inter-related, so I may actually twist the string up a bit to raise the brace height, but I'd do it to hopefully reduce the wrist slap and it might also reduce the kick left as the arrow will leave the string a little sooner and won't be exerting so much bending force.
I'll be getting all my stuff together to go to the Primitive Archery meet tomorrow, and this little bow will get some serious shooting, so I'll spend a little time trying to tune it up a bit. (I'll post some pics when I get back).

Archers Paradox:-
videoThe paradox is, why doesn't the arrow shoot off to the left when it can be seen that it points that way at brace height?
You can see at the start of the video the arrow is pointing to a place vertically above the knuckle of my left hand, but by the time I have let the string down to brace height it's pointing way off left. Assuming there is a target in the bushes, it's now pointing to a spot about a yard left.
So why doesn't the arrow go left?
Because it flexes around the bow, during the early part of the release it is going fairly fast and straight and the relatively heavy point has inertia which tries to keep it going straight. The back end of the arrow is also going fairly straight, but there is some flexibility in the string and some oscillation as the string slips off the fingers, this starts the arrow flexing and the laws of physics do the rest. So an arrow which is too stiff for the bow will kick a little left, one which is too flexible can be seen waggling in flight.

I get inordinately irritated by people who shoot compounds and modern target bows talking about archers paradox when their bows are 'centre shot', e.g The arrow points straight at the target at brace height.
The effect they are discussing are arrow tuning, and matching the arrow to bow and the method of release. As the string slips off your fingers it is deflected sideways and causes oscillations and flexing in the arrow/string and even the bow limb. That's not a paradox.
If you took the same two pics with a modern target bow with a cut away sight window, the arrow would be pointing the same way in both.
There are some good slo-mo videos on YouTube showing arrows flexing in flight. http://www.youtube.com/watch?v=WzWrcpzuAp8
I'm sure some people would wish to argue with the above, but Ha! That's the beauty of a blog I can indoctrinate you all into making primitive bows and put my view unimpeded. (exit stage left cackling wildly and rubbing hands in glee)

5 comments:

  1. I knew of arrow oscillation, and had a 'feel' for the paradox; but this is the best articulation I've heard.

    I just knew my early instruction was to focus on the target, not the arrow, until I met the compound people who had everything but gunsights. Which I found boring.

    So while my bow is certainly not a primitive and does have a slight inset for the arrow rest; I continue to shot with the Zenlike instruction to 'be the target'.

    I was going to dust off my bow and the fat, fat arrows it uses, but my rare local park that tolerates bows has an upper limit on draw weight well below my bow. But shooting is calling so I may have to look around.
    Emmett

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  2. Thanks for that, it's one of those things that is hard to explain, and I've often ended up feeling like banging my head on a wall!
    Like most things it's consitency that matters, so I'll soon accomodate the slightkick left. I tried my arrows with the 100gn point, I think they kicked a bit less, but it could just have been me getting used to the bow and my 'inner computer' correcting automatically.

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  3. What was the final draw weight of the hazel bow?
    Just out of curiosity (and because I'm re-reading the chapter on tillering according to the mass principle in TBB 4): what is the physical weight of the bow?

    Regarding the brace height vs efficiency:
    I have had the same difficulties explaining this to people, nd here's a method that has worked frequently.
    *State that a bow can only store a given amount of energy. Most ppl don't have a problem with this.
    *State that a bow cannot use any energy stored below its brace height (as the string can't move any further and thus can't push the arrow. (Most ppl don't have a problem with this either)
    *Now ask them if they think that a bow that can store "100 units of energy" (an arbitrary figure but effective as it moves away from the field of physics and most ppl will breathe a sigh of relief) will deliver more energy to the arrow if the bracing of the bow uses up 10 (aka 90 units available to the arrow) or 30 units (aka 70 units availabe to the arrow)?


    regarding the arrows:
    do they clear the grip/bow without hitting it with both the 70gn and 100 grain points?
    I'd have thought that the difference in static spine (40+% from 70 to 100 gn) would be too much for both types to clear the bow without one of the two setups striking the grip in flight.

    /Håkan

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  4. I think the final draw weight is about 35#, but I haven't measured it with the decent string.
    Going from 70grain to 100 grain isn't 40%, as the point is only a fraction of the total arrow weight.
    All the spine figures are only really a guide anyway, it depends on arrow length, weight of arrow, weight of point length of arrow etc. If the arrow is long and overhangs the grip this gives the point more leverage to try and keep flying straight and effectively softens the spine.
    Bothe arrows rub the grip but don't actually 'strike' it. A bad loose or a missmatched arrow will sometimes start flexing and the back end will actually strike the bow with a horrible clattering noise.
    The width of the bow at the grip is the main factor determining how sensitive to spine the bow will be. Generally a long bow will need an arrow about 10# softer than a recurve and this Hazel bow probably needs softer still. Having said that as long as the arrows are reasonably consistent the archer can correct for the performance. The following Youtube vid is rather good.
    http://www.youtube.com/watch?v=WzWrcpzuAp8

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  5. What I should have written was "the difference in dynamic spine (40+% increase in point weight from 70 to 100 gn)". My bad. A change in point weight will not affect static spine at all, only the dynamic spine.
    /H

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