Everything an amateur bowyer does to turn a log into a bow throughout the year.
Making bows, longbows and primitive bows with all the tips, tricks and problems.
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).
The 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)