Friday, 6 December 2013

Yew Stick Bow and Draw Weight to Brace a Bow.

The Yew Stick bow has come back to 28" at it's not going to make 90# but it gives me room to reduce the length by an inch or two and fine tune the tiller for 70# at 28" for someone who has been waiting for a bow from me for a while. (pics tomorrow)
Maybe the other fatter Churchyard stick stave will make a 90# Warbow.

I've been in conversation online with a fellow bowyer about draw weight, long strings, string tension etc and it's all a bit confusing.
The upshot is I think I encapsulated it by saying the draw weight on a long sting, short string and string tension are 3 very different things. So 50 pounds isn't necessarily the same as 50 pounds! It depends how you measure it.
As an example the draw weight of a bow at brace height is zero.
(A force draw curve pretty much shows the draw weight going to zero at brace height).
Even if you argue that it can't be measured 'cos you aren't drawing the bow, we can say, ok! Then measure the draw weight at 1" of draw and it will be very little.
However consider the tension on the string at brace height, it's probably 40 pounds or so! Anyone who as struggled to string a bow will vouch for that.

Can you get a bow tillered almost to final draw weight by just pulling it to brace height on a long string, if you know how the long sting draw weight converts to a final fully braced draw weight?
Yes and no!
Say we know that a 80# bow needs 60# on a long string to pull the tips back 6" to brace height. Theoretically we could simply do that ... BUT of course the bow has never been draw beyond 6" tip deflection!
So,  assuming we are using a real world stave with knots kinks, dips etc.
When we put our short string, brace it and pull it back to 28" draw will the tiller look right?
Of course not!
It will be out of kilter and may even be trying to twist or bend sideways. Maybe the draw weight will be spot on... But we will need to adjust the tiller and when we do so it will loose weight and we will end up with a 70# bow in stead of 80#.
That's the difference between a nice even laminated stave, where you can probably get a lot closer to final weight a lot earlier.

What is the point of all that?
Well, it illustrates why, if I'm making a 80#bow, I will use the full 80# on a long string to get it back to brace height. It may be a bit over weight and I'll still have a good bit of wood to remove, BUT I'll have a chance to get the tiller right!
Won't it over strain the wood?
I don't think so as:-
a) The bow is going to take 80# anyway when it's finished.
b) 80# on a long string is less strain on the bow than 80# on a short string, due to string angles etc. In the same way that at brace there is substantial tension on the sting but no draw weight.
c) The defection on the bow is a lot less than on the fully drawn bow.
d) Any belly wood that was stressed will probably get removed during tillering anyway.
e) How much weight and stress do you think the wood was supporting when it was still  in the tree in a howling Winter gale?

So there you go! I hope I haven't bored you all rigid with this diatribe, but maybe it answers some fundamental question... conversely you may disagree with my analysis.

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