Primitive Archer
Main Discussion Area => Bows => Topic started by: stuckinthemud on July 14, 2020, 05:30:13 am
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OK, so after 4 failures in a row my confidence is a little battered and I want to run over the basics. I am switching from ash (crysals) via apple (clumsy tillering/flawed timber?) to yew. I have had to go shorter, from 33 inches to 29 and there is a bit more character in there than I would like. Draw remains at 8.5 inches so there is still some extra length at triple draw length plus a bit.
Reducing the length by 3 inches should add somewhere around 10 to 15 lb to the draw weight?
By switching to yew, there is about a 10 percent increase in draw weight?
Width controls draw weight, thickness controls bend in both shape and amount. This is up to a point, I have always tillered with a bit of one and a bit of the other, bit of a balancing act.
Thicker is faster - huh? never made sense to me if you go with width controls weight - would love some comments on the theory behind this one, seems to be two camps, flat and wide versus narrow and thick.
Thanks for looking
Andrew
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Thicker stores more energy per mass. Twice as wide is twice as strong, twice as thick is much stronger.
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To add while being narrower to reduce mass.
On a regular lengthed bow,If this is a cross bow it does'nt matter then.
Some wood can perform well flat or D sectioned.Some can't.That's why osage is king and yew is queen of the bow woods.
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Thicker is faster - huh? never made sense to me if you go with width controls weight - would love some comments on the theory behind this one, seems to be two camps, flat and wide versus narrow and thick.
Thicker is faster because it causes higher stresses in the wood, so is working closer to the material limit and storing more energy per unit mass (as PatM says). This assumes that the wood is up to the job in the thicker bow. If the higher stresses cause a lot of set then a thinner, wider version would probably be faster in the end because it doesn't take the same amount of set. You always have to work within the limitations of your wood, you can't just decide to go thicker if the wood is not capable of surviving the stresses it will see.
Thicker stores more energy per mass. Twice as wide is twice as strong, twice as thick is much stronger.
Thicker is stiffer, not stronger. It results in higher draw weight but also causes higher stresses in the wood, which can lead to excessive set or other failure modes.
Mark
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"Width controls draw weight, thickness controls bend in both shape and amount"
I'd say that's wrong.
Width gives stability (reduced twist) and some draw weight (draw weight directly proportional width. E.G twice the width=twice the draw weight)
Thickness gives draw weight proportional to the cube of the thickness (eg twice the thickness=8 times the draw weight)...
Which is why it is also critical to the bend as small thickness errors can show up as poor tiller ('cos the error is cubed).
Del
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:-K
I have had to go shorter, from 33 inches to 29 and there is a bit more character in there than I would like. Draw remains at 8.5 inches so there is still some extra length at triple draw length plus a bit.
Andrew
Did I miss something? Are you making a crossbow? Or are you talking about draw length?
Thickness is the limiting factor for degree of curvature, as well as the strength info posted above.
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twice the width=twice the draw weight
Thickness gives draw weight proportional to the cube of the thickness (eg twice the thickness=8 times the draw weight)...
yes
you can't just decide to go thicker if the wood is not capable of surviving the stresses it will see.
yes, stress must be considered if you do not want too much set or a broken stave.
By switching to yew, there is about a 10 percent increase in draw weight?
switching from what? a white wood of the same density?
Width controls draw weight, thickness controls bend in both shape and amount. This is up to a point, I have always tillered with a bit of one and a bit of the other, bit of a balancing act.
you balance shape and amount?
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[quote author=mmattockx link=topic=68500.msg961484#msg961484
Thicker is stiffer, not stronger. It results in higher draw weight but also causes higher stresses in the wood, which can lead to excessive set or other failure modes.
Mark
[/quote]
Of course. I was using the term as understood in a poundage sense, not as a material.
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Imagine building some bridges over a river. One bridge is made of stone, one wood, one steel.
All these materials can make a bridge that will span the river.
However due to the constraints of the materials used the bridges will look very different :)
Just the same with bows and woods used.
for the same drawlength and weight they are going to look quite different if made optimally.